Peptide glossary

These are two sections of US federal law that define who can legally compound drugs. A 503A pharmacy makes patient-specific preparations from a prescription and is mostly overseen by state pharmacy boards. A 503B outsourcing facility can produce larger batches without individual prescriptions but has to register with the FDA and follow manufacturing standards closer to those of a drug company. Whether a given peptide can be compounded at all depends on separate FDA eligibility lists.

Synthetic peptides are usually supplied as a salt, meaning the peptide is paired with a small counter-ion left over from purification. Acetate is the preferred form because it is well tolerated; trifluoroacetate (TFA) is cheaper to leave in but is more irritating and is something a good certificate of analysis will quantify. The salt also affects the true peptide content of a vial, since some of the labeled weight is counter-ion and water rather than peptide.

An agonist is a molecule that binds to a receptor and switches it on, producing the same kind of signal the body's own messenger would. Semaglutide is a GLP-1 receptor agonist because it activates the same receptor as natural GLP-1, just for much longer. Agonists can be full or partial depending on how strongly they activate the receptor.

A small air bubble in a syringe is mostly a dosing accuracy problem rather than a safety problem for subcutaneous injections. The bubble occupies volume that should be liquid, so the dose drawn is smaller than it looks. Tapping the barrel so the bubble rises and then pushing it out before final measurement is the usual fix. The danger of air in a vein applies to intravenous use, not to a tiny bubble under the skin.

An aliquot is a smaller portion split off from a larger batch, for example dividing a reconstituted vial into several syringes or smaller vials. People do this to reduce the number of times a vial is punctured or to pre-load doses for travel. It adds contamination risk every time the liquid is transferred, so it should be done with clean technique or not at all.

Alpha-MSH is a short natural peptide cut from a larger precursor protein called POMC. It activates melanocortin receptors and is best known for driving pigment production in skin, but it also plays roles in appetite, inflammation, and sexual arousal. Melanotan 1 and 2 and PT-141 are synthetic analogs built to mimic parts of what alpha-MSH does.

Amino acids are the building blocks that peptides and proteins are made from. There are 20 standard ones in human biology, each with a shared backbone and a different side chain that gives it its character. When chemists describe a peptide they often list its amino acids in order, and a single amino acid inside a chain is called a residue.

AMPK is an enzyme that acts as the cell's low-fuel sensor. When energy runs short it switches on, pushing the cell to burn fat, take up glucose, and build more mitochondria while turning down expensive building projects. Exercise and fasting both activate it, and several peptides marketed for metabolism or longevity claim to work through it, though most of that evidence comes from cells and rodents.

Amylin is a hormone released by the pancreas alongside insulin after a meal. It slows stomach emptying, dampens glucagon, and signals fullness to the brain. Cagrilintide is a long-acting amylin analog being paired with semaglutide in trials because the two hormones reduce appetite through different pathways.

Angiogenesis is the growth of new blood vessels from existing ones. It is essential for healing because damaged tissue needs fresh blood supply to bring in oxygen and cells. Healing peptides such as BPC-157 and TB-500 are often credited with promoting it in animal wound models. The same process also feeds tumors, which is why anyone with an active cancer should be cautious with compounds that boost it.

An antagonist binds to a receptor without activating it, blocking the natural messenger from getting in. It is the opposite of an agonist. Some compounds act as agonists at one receptor and antagonists at another, and a few peptides reduce a signal indirectly rather than by direct blocking, so the label is not always simple.

An anti-inflammatory compound reduces some part of the inflammatory response, whether by lowering inflammatory cytokines, calming immune cells, or blocking the signals that recruit them. Many peptides in this space, like KPV or ghk-Cu, show this effect in cell and animal studies. Reducing inflammation is not always good, since inflammation is also how the body clears infection and starts repair, so context matters.

Antimicrobial peptides are short peptides the immune system produces to kill bacteria, fungi, and some viruses directly, usually by punching holes in their membranes. LL-37 is the best-known human example. Because they attack membranes rather than a specific enzyme, resistance develops slowly, which makes them interesting to researchers, but that same bluntness can also damage human cells at high concentration.

Appetite is controlled by a conversation between the gut, fat tissue, and the brain, mostly in the hypothalamus. Hormones like GLP-1, amylin, and leptin say stop eating; ghrelin says start. Peptides used for weight management mostly work by amplifying the stop signals, which is why they reduce hunger rather than directly burning fat.

Aspiration means pulling back slightly on the plunger after inserting the needle to check whether blood enters the syringe, which would suggest the tip is in a vessel. It was standard teaching for intramuscular injections for decades. Current guidance for subcutaneous injections with short, thin needles generally says it is unnecessary, and most people skip it for that route.

Autophagy is the process by which a cell breaks down and recycles its own damaged parts, from misfolded proteins to worn-out mitochondria. It is turned up by fasting and exercise and turned down when mTOR is active. It is a popular target in longevity research, though claims that a specific peptide meaningfully raises autophagy in humans are mostly extrapolated from cell and animal data.

Bacteriostatic water is sterile water with 0.9 percent benzyl alcohol added to stop bacteria from growing in it. It is the standard liquid for reconstituting lyophilized peptides because a vial gets punctured many times over weeks and the preservative keeps it from becoming a culture dish. An opened vial of bacteriostatic water is usually considered good for about 28 days.

BDNF is a protein that supports the survival of neurons and helps them form new connections. Exercise, sleep, and learning all raise it, and low levels are associated with depression and cognitive decline. Semax, Dihexa, and several other cognitive peptides are described as raising BDNF, with most of the direct evidence coming from rodent studies.

Bioavailability is the fraction of a dose that actually reaches the bloodstream in active form. Injected peptides score high, while oral peptides are mostly destroyed by stomach acid and digestive enzymes and score near zero unless specially formulated. This is why an oral version of a peptide usually needs a far larger dose, or does not work at all, compared with an injection.

Bioregulator peptides are very short peptides, often two to four amino acids, developed by Russian researchers and claimed to tune gene expression in a specific organ, such as the thymus, pineal gland, or pancreas. Examples include epithalon, thymalin, and pinealon. Nearly all of the published work comes from one research group, much of it in Russian-language journals, so independent replication is thin.

A blend is a single vial containing two or more peptides mixed together, for example CJC-1295 with ipamorelin or BPC-157 with TB-500. Blends are convenient but lock you into a fixed ratio, and the label needs to state how much of each compound is present. A certificate of analysis for a blend should show each peptide separately.

Blood glucose is the amount of sugar circulating in the blood, tightly regulated by insulin, glucagon, and gut hormones. Several peptide classes move it: GLP-1 drugs lower it, growth hormone and its secretagogues can nudge it upward, and stacking both can pull in opposite directions. Anyone using peptides that touch this system should know their fasting numbers.

Cardiolipin is a fat molecule found almost exclusively in the inner membrane of mitochondria, where it holds the energy-producing machinery in the right shape. It is easily damaged by oxidative stress, and that damage is linked to aging and heart disease. SS-31 is a peptide designed to bind cardiolipin and protect it, which is the basis of its mitochondrial claims.

A certificate of analysis is a lab report stating what a specific batch of peptide contains. A good one shows the purity by HPLC, confirms the identity by mass spectrometry, names the lot number, and comes from a lab that is independent of the seller. A COA with no lot number, no lab name, or a suspiciously round purity figure is not worth much.

The circadian rhythm is the roughly 24 hour cycle that times sleep, hormone release, body temperature, and appetite. Growth hormone peaks in the first deep sleep of the night, cortisol peaks near waking, and melatonin rises after dark. Timing peptides around these peaks, such as taking a GH secretagogue before bed, is an attempt to work with the clock rather than against it.

Cold chain means keeping a temperature-sensitive product refrigerated or frozen at every step from manufacturer to end user. Lyophilized peptides are fairly forgiving and can survive days at room temperature in transit. Reconstituted peptides and some pre-mixed products are not, and a warm shipment can quietly degrade them without any visible change.

Collagen is the main structural protein in skin, tendon, ligament, bone, and cartilage. It is made by fibroblasts, and its production slows with age and speeds up during wound repair. Peptides marketed for skin and joint health, like ghk-Cu, are usually claiming to stimulate fibroblasts to make more of it, mostly on the basis of cell culture and small human skin studies.

A compounding pharmacy prepares customized medications, such as a specific strength or a combination not sold commercially, under a prescription. During drug shortages and for eligible bulk substances, compounders have supplied peptides like semaglutide and tirzepatide. What they can legally make changes as FDA lists and shortage status change, so availability of a compounded peptide is not permanent.

Concentration is how much peptide is dissolved in each milliliter of liquid, set by how much bacteriostatic water you add to a vial. A 5 mg vial with 2 mL of water is 2.5 mg/mL, which is 2500 mcg per mL, or 25 mcg per unit on a U-100 insulin syringe. Every dose calculation starts here, and most dosing mistakes come from skipping this step.

Cortisol is the body's main stress hormone, released by the adrenal glands with a strong morning peak. It raises blood glucose, suppresses inflammation, and at chronically high levels breaks down muscle and promotes belly fat. Some older growth hormone secretagogues, especially GHRP-6 and GHRP-2, raise cortisol and prolactin as a side effect, while ipamorelin was developed specifically to avoid that.

A cycle is a planned period of use followed by a break. People cycle peptides to limit receptor desensitization, to see whether the effect holds once the compound is stopped, and to give the body's own signaling a chance to reset. Typical community cycles for healing and GH peptides run four to twelve weeks, while GLP-1 medications are generally used continuously.

Cytokines are small signaling proteins that immune cells use to talk to each other and to the rest of the body. Some raise inflammation, some lower it, and many do both depending on context. When a peptide is said to modulate the immune system, it usually means it shifts the balance of cytokines that cells release.

A drug affinity complex is a chemical tag added to a peptide that makes it latch onto albumin, the most abundant protein in blood. Bound to albumin, the peptide escapes rapid clearance and can last about a week instead of minutes. CJC-1295 with DAC is the well-known example, and the tradeoff is that it produces a steady elevation of growth hormone rather than the natural pulses.

Dead space is the small volume of liquid that stays trapped in the needle and hub after the plunger is fully pressed. Insulin syringes with a fixed, built-in needle have very little of it, which is one reason they are preferred for peptides. Syringes with a detachable needle can hold several units of liquid in the hub, which matters when your whole dose is only ten units.

Delta sleep is the deepest stage of non-REM sleep, named for the slow delta brain waves seen on an EEG. It is when most of the night's growth hormone is released and when the body does much of its physical repair. DSIP, delta sleep-inducing peptide, was named for its supposed ability to promote this stage, although human data on it is old and mixed.

Desensitization is when a receptor responds less and less to the same stimulus over time, often because the cell pulls receptors off its surface. It is the main reason people cycle secretagogues and take breaks. Constant stimulation causes it faster than pulsed stimulation, which is a core argument for short-acting peptides over long-acting ones.

A dual agonist is a single molecule engineered to activate two different receptors. Tirzepatide is the leading example, hitting both the GLP-1 and GIP receptors, which appears to produce more weight loss than GLP-1 alone. Survodutide and mazdutide instead pair GLP-1 with the glucagon receptor to add an energy-burning component.

Endogenous means produced inside the body. Endogenous growth hormone is what your pituitary releases on its own, as opposed to injected hGH. The distinction matters because many peptides work by nudging endogenous production rather than replacing it, which keeps the body's feedback loops partly intact.

Endotoxins are fragments of bacterial cell walls that can remain in a product even after the bacteria are dead and the solution is sterile. Injected endotoxin causes fever, chills, and inflammation. Pharmaceutical injectables are tested for it, and a thorough certificate of analysis for a peptide will include an endotoxin result from an LAL test.

An excipient is an inactive ingredient added to a product to help it hold together, dissolve, or stay stable. Peptide vials often contain mannitol, which is why a 5 mg vial can look like a much bigger cake of powder than 5 mg of pure peptide would. Excipients are normal, but a vial with no listed excipient and a large visible cake is worth a question to the seller.

Exogenous means coming from outside the body. Injected testosterone, hGH, or IGF-1 is exogenous, and the body typically responds by cutting its own endogenous production. This feedback is why long courses of exogenous hormones can leave the natural system suppressed after stopping.

An expiration date is set by the manufacturer and applies to the unopened, lyophilized product stored as directed. A beyond-use date is the shorter window that starts once the vial is reconstituted or first punctured. For most peptides mixed with bacteriostatic water, the practical beyond-use date is around four weeks in the fridge, regardless of how far away the printed expiration is.

The FDA sorts substances nominated for compounding into categories. Category 1 substances can be compounded while the agency reviews them; Category 2 substances have raised significant safety concerns and are not supposed to be compounded. In late 2023 the FDA placed a number of popular peptides, including BPC-157, CJC-1295, ipamorelin, and several others, into Category 2, which removed them from legitimate compounding channels in the US.

Fibroblasts are the cells in connective tissue that produce collagen and the rest of the structural matrix. They are the main workers in wound repair and scar formation. Many healing and skin peptides are described as activating or recruiting fibroblasts, which is easy to show in a dish and harder to prove in a person.

Flushing is a warm, red feeling in the face and upper body caused by blood vessels widening. It is a common reaction to melanotan 2, PT-141, and some GH secretagogues, and it usually passes within an hour. Persistent or severe flushing with a drop in blood pressure is a different matter and worth stopping for.

Follistatin is a natural protein that binds and neutralizes myostatin and related signals that limit muscle growth. Animals with high follistatin grow visibly more muscle, which is why follistatin-344 is sold on the research market. Human evidence is very limited, the product is fragile, and what is actually inside many vials sold under this name is questionable.

FSH is a pituitary hormone that drives sperm production in men and follicle development in women. It is released in response to GnRH from the hypothalamus, alongside LH. Peptides such as gonadorelin and kisspeptin work upstream by stimulating that release, while exogenous testosterone tends to suppress it.

Gastric emptying is the rate at which the stomach passes food into the small intestine. GLP-1 and amylin both slow it down, which keeps you feeling full longer but also causes the nausea, bloating, and sulfur burps that come with these drugs. Slowed emptying also affects how quickly oral medications are absorbed, which is worth knowing if you take other pills.

Ghrelin is a hormone made mostly by the stomach that rises before meals and drives hunger. Its receptor in the pituitary also triggers growth hormone release, which is the pathway GHRPs use. This shared receptor is why GHRP-6 makes people intensely hungry while ipamorelin, which activates it more selectively, mostly does not.

GHRH is the hypothalamic hormone that tells the pituitary to release growth hormone. Sermorelin, tesamorelin, and CJC-1295 are all modified versions of it, differing mainly in how long they last. A GHRH analog works only if the pituitary is still responsive, which is why it is often paired with a GHRP that acts through a separate receptor.

GHRPs are synthetic peptides that trigger growth hormone release by mimicking ghrelin at its receptor in the pituitary. The family includes GHRP-2, GHRP-6, hexarelin, and ipamorelin. They differ in potency and in how much they also raise hunger, cortisol, and prolactin, with ipamorelin considered the most selective.

GIP is a gut hormone released after eating that boosts insulin secretion and influences how fat tissue handles nutrients. On its own it was long considered a weak target, but combining it with GLP-1 in tirzepatide produced stronger weight loss than GLP-1 alone in head to head trials. Why the combination works better is still being worked out.

GLP-1 is a gut hormone released after a meal that increases insulin, suppresses glucagon, slows stomach emptying, and tells the brain you are full. Natural GLP-1 lasts only minutes, so drugs like semaglutide are altered versions that survive for days. GLP-1 receptor agonists are the most thoroughly studied peptides on this site, with large human trials behind them.

The glucagon receptor responds to glucagon, the hormone that raises blood sugar by telling the liver to release glucose. Activating it also increases energy expenditure and fat burning in the liver. Newer weight loss peptides like survodutide and retatrutide deliberately hit this receptor alongside GLP-1 to add a calorie-burning effect, with the insulin-boosting side keeping blood glucose in check.

The grey market is the network of vendors selling peptides labeled for research use to individuals who will use them on themselves. It is not illegal in the way controlled substances are, but the products are not regulated as drugs, so quality varies from excellent to fake. Third-party testing and vendor reputation are the only real quality controls a buyer has.

Growth hormone is a pituitary hormone released in pulses, mostly during deep sleep and after exercise, that drives growth in children and supports muscle, bone, and fat metabolism in adults. Most of its effects are carried out by IGF-1, which the liver produces in response. Synthetic hGH is a prescription drug; secretagogue peptides try to raise the body's own output instead.

Gut permeability describes how easily substances pass from the inside of the intestine into the bloodstream through the spaces between cells. Some leakiness is normal, but increased permeability is linked to inflammation, celiac disease, and possibly other conditions. Larazotide and several gut peptides are studied for tightening this barrier, with larazotide having reached human trials.

Half-life is the time it takes for the amount of a compound in the body to fall by half. It decides how often you need to dose and how long an effect lingers after stopping. Peptides range from minutes, like natural GHRH, to about a week for semaglutide, and the difference comes down to how easily enzymes can chew them up and how they are modified to resist that.

The HPG axis is the chain of command for sex hormones. The hypothalamus releases GnRH in pulses, the pituitary responds with LH and FSH, and the testes or ovaries respond to those with testosterone or estrogen. Rising sex hormones then feed back to quiet the top of the chain, which is why adding exogenous testosterone shuts down natural production.

HPLC is the lab method used to measure peptide purity. The sample is pushed through a column that separates components by their chemistry, and each shows up as a peak on a chart. The purity figure on a COA is the size of the main peak relative to everything else. HPLC alone does not prove the main peak is the correct peptide, which is what mass spectrometry is for.

An HPTA restart is an attempt to bring natural testosterone production back online after it has been suppressed by exogenous hormones. Protocols typically use hCG or gonadorelin to wake the testes, sometimes with clomiphene or enclomiphene to push the pituitary. Recovery time depends heavily on how long the suppression lasted, and some people do not fully recover.

Hypogonadism is a condition where the testes or ovaries produce too little sex hormone. Primary hypogonadism is a problem in the gland itself; secondary hypogonadism is a problem with the pituitary or hypothalamus signaling. The distinction decides whether peptides that stimulate the pituitary, like kisspeptin or gonadorelin, can help at all.

IGF-1 is a hormone the liver makes in response to growth hormone, and it carries out most of what GH gets credit for: tissue growth, muscle repair, and cell survival. It is also the standard blood marker for checking whether a GH secretagogue is doing anything. Injected IGF-1 variants like LR3 and DES bypass the GH step entirely and can cause low blood sugar.

An immunomodulator is anything that adjusts how the immune system responds, whether up, down, or sideways. Thymosin alpha-1 is the clearest peptide example, with approved use in some countries for hepatitis and as a vaccine adjuvant. The term is vague on purpose, and marketing tends to use it when the exact mechanism is not well defined.

Inflammatory cytokines are the signaling proteins that ramp up inflammation, with TNF-alpha, IL-6, and IL-1 beta being the ones most often measured. They are essential for fighting infection but drive tissue damage when they stay elevated. Peptides described as anti-inflammatory are usually shown to lower one or more of these in cell or animal experiments.

Injection site rotation means moving each injection to a different spot rather than reusing the same patch of skin. It prevents lumps, bruising, and lipohypertrophy, and keeps absorption consistent. A common approach is to work around the abdomen in a grid, staying an inch or two from the navel and at least an inch from the last site.

Insulin sensitivity is how well the body's cells respond to insulin's signal to take up glucose. High sensitivity is healthy; low sensitivity, called insulin resistance, leads toward type 2 diabetes. GLP-1 drugs and weight loss improve it, while high doses of growth hormone or IGF-1 can worsen it, which is one reason GH stacks sometimes push fasting glucose up.

An insulin syringe is a small syringe with a fixed, thin needle marked in units rather than milliliters. U-100 syringes are the standard, where 100 units equals 1 mL, and they come in 0.3, 0.5, and 1 mL sizes. Smaller barrels have finer markings, which makes measuring tiny peptide doses easier and more accurate.

Intramuscular injection delivers a compound into muscle tissue, usually the shoulder, thigh, or glute, with a longer needle than subcutaneous use. Absorption is somewhat faster than under the skin. Some people inject healing peptides intramuscularly near an injury on the theory of local effect, but the evidence that this beats a subcutaneous shot elsewhere is thin.

Intranasal delivery sends a peptide through the lining of the nose, which is rich in blood vessels and avoids both needles and the stomach. Bioavailability varies a lot between peptides and is usually lower than injection, so nasal doses are higher. Semax, selank, and oxytocin are commonly used this way, and some cognitive peptides may reach the brain more directly through the nasal route.

An IU is an international unit, a measure of biological activity rather than weight, used for hGH, hCG, insulin, and some vitamins. The conversion to weight depends on the substance: for hGH, roughly 3 IU equals 1 mg. IU has nothing to do with the unit markings on an insulin syringe, and confusing the two is a common and dangerous mistake.

Khavinson peptides are the short bioregulator peptides developed by Vladimir Khavinson and colleagues at the St. Petersburg Institute of Bioregulation and Gerontology. Epithalon, thymalin, pinealon, cortagen, and pancragen are all from this line. The group has published for decades, but most of the studies are small, old, or not independently replicated, so the evidence should be read with that in mind.

The LAL test detects bacterial endotoxin using an extract from horseshoe crab blood, which clots in its presence. It is the standard pharmaceutical test for injectables and is reported as endotoxin units per milliliter or per milligram. Most grey market COAs skip it, which means sterility and endotoxin levels of those products are simply unknown.

LH is the pituitary hormone that tells the testes to make testosterone and triggers ovulation in women. It is released in pulses in response to GnRH. hCG works because it mimics LH at the testes, while gonadorelin and kisspeptin work by prompting the pituitary to release more of the real thing.

Lipohypertrophy is a rubbery, raised lump of fat tissue that forms under the skin at sites that have been injected too often. It is well documented in insulin users and happens with any repeated subcutaneous injection. Compounds injected into these lumps absorb erratically, and the fix is rotating sites and leaving affected areas alone until they soften.

Lipolysis is the breakdown of stored fat into fatty acids that can be burned for energy. Growth hormone, adrenaline, and glucagon promote it; insulin shuts it off. Peptides marketed for fat loss, such as AOD-9604 or the GH secretagogues, generally claim to increase lipolysis, but mobilizing fat is not the same as burning it, and without a calorie deficit it goes back where it came from.

A loading dose is a larger or more frequent initial dose meant to bring levels up quickly before dropping to a maintenance dose. It is used with compounds that have long half-lives or that need to build up in tissue. TB-500 protocols commonly use a loading phase of several weeks, while GLP-1 drugs do the opposite and start low to limit side effects.

A lot number identifies the specific manufacturing batch a vial came from. It links the product on your desk to a particular certificate of analysis and lets a vendor trace a problem back to its source. A COA without a lot number, or one whose lot does not match your vial, is not evidence of what your vial contains.

Lyophilization is freeze-drying, where a frozen peptide solution has its water pulled off under vacuum, leaving a dry powder or cake in the vial. Peptides are shipped this way because the dry form is far more stable than a solution and can survive weeks at room temperature. Reconstitution reverses the process by adding water back.

Mass spectrometry measures the exact molecular weight of what is in a sample. For peptides it confirms identity: the measured mass should match the expected mass of the sequence. Paired with HPLC, which measures purity, it forms the core of a credible certificate of analysis. A COA with only HPLC could be showing a very pure sample of the wrong compound.

Mast cells are immune cells stationed in skin, gut, and airways that release histamine and other inflammatory chemicals when triggered. They are central to allergies and to conditions where they fire too easily. Several peptides, including KPV and some anti-inflammatory compounds, are studied for calming them, and people with mast cell disorders sometimes react unpredictably to injections of any kind.

MC4R is the melanocortin receptor in the hypothalamus that regulates appetite, energy use, and sexual arousal. Mutations in it are one of the more common genetic causes of severe obesity. PT-141 (bremelanotide) is an FDA-approved MC4R agonist for low sexual desire in premenopausal women, and it produces the same effect in men, which is where most off-label use happens.

MCAS is a condition in which mast cells release their contents too readily, causing flushing, hives, gut symptoms, and sometimes serious reactions to ordinary triggers. It is increasingly discussed in peptide communities because people with it are looking for anything that stabilizes mast cells. Anyone with MCAS should treat every new compound, and every preservative like benzyl alcohol, as a potential trigger.

A microgram is one millionth of a gram, or one thousandth of a milligram. Most peptide doses are measured in mcg, which is why a 5 mg vial can contain twenty or more doses. Always write mcg rather than the symbol, since µg is easy to misread as mg, and a thousandfold mistake in either direction is not something you want.

Mechano growth factor is a variant of IGF-1 that muscle produces locally in response to mechanical stress, like heavy lifting. It is thought to activate satellite cells that repair and grow muscle fibers. The synthetic versions sold, MGF and its PEGylated form, are based mainly on rodent work, and it is unclear whether an injected version reaches the muscle in a useful way.

Melanocortin receptors are a family of five receptors that respond to alpha-MSH and related hormones. MC1R in skin controls pigment, MC4R in the brain controls appetite and sexual function, and the others handle adrenal and inflammatory signaling. Melanotan 2 hits several of them at once, which explains why one compound can cause tanning, nausea, reduced appetite, and spontaneous erections.

Melanogenesis is the production of melanin, the pigment that darkens skin, by cells called melanocytes. Alpha-MSH triggers it, which is why melanotan 1 and 2 cause tanning without sun exposure. The pigment does provide some real UV protection, but the effect is not evenly distributed, and existing moles and freckles often darken more than the surrounding skin.

A milligram is one thousandth of a gram, and it is the unit vials are labeled in. A 5 mg vial holds 5000 mcg of peptide. Doses are usually in mcg, so the vial label tells you how much total product you have, not how much to take. The exception is GLP-1 class drugs, where doses are in mg because the amounts are larger.

Mitochondria are the structures inside cells that turn food and oxygen into usable energy. They have their own small genome and their function declines with age, which is why they are a central focus of longevity research. MOTS-c, humanin, and SS-31 are all peptides that act on or come from mitochondria.

Mitochondrial biogenesis is the process of making new mitochondria inside a cell. It is driven by exercise, cold, and fasting through signals like AMPK and PGC-1 alpha, and it is one reason trained muscle has more endurance. Several metabolic and longevity peptides claim to stimulate it, with evidence mostly from rodent and cell studies.

Mitochondrial-derived peptides are small peptides encoded in the mitochondria's own DNA rather than in the cell nucleus. Humanin and MOTS-c are the best studied, and SHLPs are a related family. They appear to act as signals that help the cell cope with metabolic and oxidative stress, and levels tend to fall with age. Human data on injecting them is still very early.

A nevus is the medical term for a mole. Melanotan 1 and 2 reliably darken existing moles and can bring out new ones, because they stimulate the same pigment cells that moles are made of. Whether they increase melanoma risk is not settled, but a darkening or changing mole is exactly what a dermatologist wants to see, so anyone using these compounds should track their skin.

mTOR is a protein complex that acts as the cell's growth switch. When nutrients, insulin, and growth factors are plentiful, it turns on protein synthesis and cell growth and turns off autophagy. It is the pathway muscle-building signals like IGF-1 flow through, and also the pathway longevity researchers want to dial down, which is the basic tension between building muscle and slowing aging.

Myostatin is a protein that limits how much muscle the body will build. Animals and rare humans born without it are dramatically muscular. Blocking it is an obvious target, and follistatin and various antibodies have been tried, but human trials have been disappointing so far and the research market products claiming to inhibit it are poorly characterized.

NAD+ is a molecule every cell uses to move electrons around during energy production and to fuel enzymes like sirtuins and PARPs. Levels decline with age, and raising them is a major longevity theme. NAD+ is not a peptide, though it is sold and injected alongside them. Injections and IV drips deliver it directly, while oral precursors like NMN and NR rely on the body to convert them.

Nausea is the most common side effect of GLP-1 and related peptides, driven by slowed gastric emptying and direct signaling to the brain's nausea center. It is worst in the first weeks and after each dose increase, and usually fades as the body adapts. Smaller meals, avoiding fatty food, and slower titration are the standard ways to manage it.

Gauge is the thickness of a needle, and the number runs backward: a higher gauge means a thinner needle. Insulin syringes come in 29 to 31 gauge, which is thin enough that most subcutaneous injections are barely felt. Intramuscular injections use thicker 23 to 25 gauge needles that are longer, and drawing thick liquids through a very fine needle is slow.

Neuropeptides are peptides that nerve cells use as chemical messengers, either alongside or instead of classic neurotransmitters. Oxytocin, vasopressin, and the natural forms of substance P and endorphins are all neuropeptides. Synthetic cognitive peptides like semax and selank are modeled on this class and are usually taken intranasally because the peptide must reach the brain.

Neuroplasticity is the brain's ability to rewire itself by forming, strengthening, or pruning connections between neurons. It underlies learning, recovery from injury, and the effects of many antidepressants. BDNF is one of its key drivers, and peptides promoted for cognition or mood are almost always claiming to increase it.

A nootropic is any substance taken to improve memory, focus, or mental clarity. The term covers everything from caffeine to prescription stimulants to research peptides like semax, selank, and noopept. The peptide nootropics generally have small human studies at best, and the effects people report are subtle and hard to separate from expectation.

Oxidative stress is the damage that reactive oxygen molecules cause to proteins, fats, and DNA when they outpace the body's antioxidant defenses. Mitochondria are the main source and also the main victim. It is implicated in aging and nearly every chronic disease, which is why so many peptides claim antioxidant effects, though most antioxidant supplements have not lived up to that logic in human trials.

PEGylation attaches chains of polyethylene glycol to a peptide to make it bigger and harder for the kidneys and enzymes to clear. The result is a much longer half-life, often days instead of minutes. PEG-MGF is the common example on the research market. PEG is generally well tolerated, though some people develop antibodies to it.

A peptide is a short chain of amino acids linked by peptide bonds, generally fewer than 50, though the cutoff is fuzzy. Many of the body's hormones and signaling molecules are peptides, including insulin, GLP-1, and oxytocin. Because peptides are digested like food, most therapeutic ones have to be injected or sprayed rather than swallowed. Not everything sold as a peptide is one; some products in this space are small molecules.

A peptide bond is the chemical link that joins one amino acid to the next in a chain. It forms when the acid group of one amino acid reacts with the amine group of the next, releasing a water molecule. Digestive enzymes are built to break exactly this bond, which is why swallowed peptides mostly do not survive the gut.

The sequence is the specific order of amino acids in a peptide, usually written as a string of one or three letter codes. It is what defines the peptide: change one amino acid and you have a different compound. Synthetic analogs like semaglutide are natural sequences with a few deliberate swaps and additions that make them last longer in the body.

Priming means pushing a small amount of liquid through the needle before injecting to clear air and confirm flow. It matters most with pen injectors, where the manufacturer's instructions usually call for a priming shot before each use. With a fixed-needle insulin syringe it is mostly about expelling the air bubble, and the small amount of liquid lost should be accounted for in the dose.

Prolactin is a pituitary hormone best known for milk production, but it also affects libido, mood, and testosterone. Elevated prolactin in men can cause low sex drive, erectile problems, and in rare cases breast tissue growth. GHRP-6, GHRP-2, and hexarelin can raise it, which is a reason many people favor ipamorelin, and it is worth checking on bloodwork if symptoms appear.

A protein is a long chain of amino acids, usually more than 50, folded into a specific three-dimensional shape that gives it its function. Enzymes, antibodies, collagen, and growth hormone are all proteins. The line between a large peptide and a small protein is a matter of convention, and hGH at 191 amino acids sits on the protein side.

Pulsatile release means a hormone is secreted in bursts rather than steadily. Growth hormone, GnRH, and LH all work this way, and the pulses matter: the body reads a pulse differently from a constant level and tends to shut down receptors that are stimulated without pause. Short-acting secretagogues preserve pulses while long-acting versions like CJC-1295 DAC flatten them into a plateau.

Purity is the percentage of a sample that is the intended peptide as opposed to fragments, impurities, or leftover synthesis chemicals, measured by HPLC. Reputable research peptides typically show 98 percent or higher. Purity does not tell you how much peptide is in the vial by weight, since water, salt, and excipient are separate, and it does not cover sterility or endotoxin.

A receptor is a protein, usually on the surface of a cell, that recognizes a specific signaling molecule and triggers a response inside the cell when that molecule binds. Hormones, neurotransmitters, and most peptide drugs work by activating or blocking receptors. Which tissues carry a given receptor decides where a peptide can act.

Reconstitution is adding bacteriostatic or sterile water to a lyophilized peptide to turn it back into a liquid you can draw up and inject. The water should run slowly down the inside wall of the vial rather than being sprayed onto the powder, and the vial should be swirled gently rather than shaken. The amount of water you add sets the concentration, and that number drives every dose calculation afterward.

Research use only is the label vendors put on peptides to sell them without going through drug regulation. Legally, the product is sold as a laboratory reagent and the buyer is responsible for what happens next. In practice it means no one has verified the product is sterile, correctly dosed, or safe for injection unless the buyer or an independent lab checks.

SASP is the cocktail of inflammatory cytokines, enzymes, and growth factors that senescent cells pump out. It is how a small number of aged cells can inflame surrounding tissue and spread dysfunction. Clearing senescent cells with senolytics is meant to shut off this output, and reducing SASP is a common endpoint in aging research.

Satellite cells are stem cells that sit along muscle fibers and stay dormant until the muscle is damaged or stressed. When activated they multiply and fuse into the fiber, donating new nuclei that allow it to repair and grow. IGF-1, mechano growth factor, and the removal of myostatin all push satellite cells to activate.

A secretagogue is anything that causes the body to secrete a hormone. In peptide use the term almost always means growth hormone secretagogues, covering both GHRH analogs and GHRPs. Because they raise the body's own output rather than replacing it, they preserve feedback control, but they also depend on a pituitary that can still respond.

Senescence is a state in which a cell stops dividing permanently but does not die, often after damage or too many divisions. Senescent cells accumulate with age and secrete inflammatory signals that harm their neighbors. They are a leading target in aging research, and FOXO4-DRI is a peptide designed to push them into self-destruction, so far mainly in mice.

A senolytic is a compound that selectively kills senescent cells. The best-studied ones are the drug dasatinib combined with quercetin, and fisetin. FOXO4-DRI is the peptide example, which showed striking results in aged mice but has almost no human data and is expensive and unstable to produce.

Sirtuins are a family of seven enzymes that regulate metabolism, DNA repair, and stress responses, and they need NAD+ to work. SIRT1 in the nucleus and SIRT3 in mitochondria get the most attention in longevity research. Much of the interest in NAD+ boosters comes from the idea that more NAD+ means more active sirtuins, though the human evidence remains modest.

Edema is swelling from fluid trapped in tissue, and it usually follows the body holding onto sodium. Growth hormone and its secretagogues are the peptides most likely to cause it, producing puffy hands, ankles, and face and sometimes tingling fingers from fluid pressing on nerves. It typically resolves with a lower dose, and it is not the same as gaining muscle.

Somatostatin is the hypothalamic hormone that shuts growth hormone release off, acting as the brake to GHRH's accelerator. Its rise and fall helps create the natural GH pulses. GHRPs partly work by suppressing somatostatin, which is why a GHRH analog plus a GHRP produces more GH than either alone.

Stacking is using two or more peptides at the same time, either for effects that reinforce each other or to cover different goals at once. The classic examples are a GHRH analog with a GHRP, or BPC-157 with TB-500. Stacks add variables, so if something goes wrong it is harder to know which compound caused it, and the usual advice is to start each compound alone first.

Sterile technique is the set of habits that keep bacteria out of your vial and out of your body: washing hands, wiping the vial stopper and skin with alcohol and letting it dry, never touching the needle, and using a fresh needle for every draw and injection. It matters more with peptides than with pharmacy drugs because the vial is punctured many times over weeks.

Sterile water is water that has been sterilized but contains no preservative. It is fine for reconstituting a vial that will be used in a single dose, but once opened it has no defense against contamination, so multi-dose vials should be mixed with bacteriostatic water instead. Some peptides that are sensitive to benzyl alcohol are the exception.

Subcutaneous injection delivers a compound into the fat layer just under the skin, most often in the abdomen or thigh, using a short thin needle. It is the standard route for nearly all injectable peptides because it is easy, low-pain, and absorbs at a steady rate. Pinching a fold of skin and inserting at 45 to 90 degrees is the usual method.

Sulfur burps are belches that smell like rotten eggs, a well-known side effect of GLP-1 drugs. They happen because food sits in a slowed stomach long enough for bacteria to produce hydrogen sulfide. Smaller meals, less fat and protein at once, and staying upright after eating reduce them, and they tend to ease as the dose stabilizes.

T cells are white blood cells that coordinate and carry out the adaptive immune response, recognizing infected or abnormal cells and either killing them or directing other cells to. They mature in the thymus, which is where the T comes from. Thymic peptides like thymosin alpha-1 and thymalin are claimed to support T cell development and function.

Tachyphylaxis is a rapid drop in response to a compound after repeated doses over a short period, sometimes within hours or days. It is faster and more abrupt than the gradual tolerance most people picture. It is a practical concern with some secretagogues and nasal peptides, and it is one of the arguments for spacing doses and taking breaks.

Telomerase is the enzyme that rebuilds telomeres, the protective caps on chromosomes. Most adult cells keep it switched off, which is part of why they can only divide so many times. Epithalon is claimed to activate telomerase, based mostly on old cell culture work from the Khavinson group. Since cancer cells also rely on telomerase, activating it is not an unambiguous good.

Telomeres are repeating DNA sequences at the ends of chromosomes that protect the genetic material during cell division. They shorten a little each time a cell divides, and when they get too short the cell stops dividing and becomes senescent. Telomere length is a popular but noisy marker of biological age.

Testosterone is the main male sex hormone, produced by the testes in response to LH and in smaller amounts by the ovaries and adrenals. It drives muscle, bone density, libido, and mood. Peptides in the reproductive space either prompt the body to make more of it, such as kisspeptin and hCG, or try to restore production after it has been suppressed by outside hormones.

Thermogenesis is the body burning calories to produce heat, mostly in brown fat and muscle. Cold exposure and certain hormones switch it on, and it is one of the few ways to raise energy expenditure without moving. Glucagon receptor agonists and some experimental compounds like SLU-PP-332 are studied for boosting it, though the human effect sizes are usually small.

Third-party testing means a lab with no financial ties to the vendor analyzes the product. It is the only real check on a research peptide, since the seller's own claims are not verified by anyone. Some vendors publish independent results per lot, some community members send samples to labs themselves, and some vendors publish nothing at all, which tells you something.

The thymus is a small organ behind the breastbone where T cells mature. It is largest in childhood and shrinks steadily through adult life, which is one reason immune function declines with age. Thymosin alpha-1, thymalin, and thymulin are all peptides originally isolated from or modeled on thymus tissue.

Tight junctions are the protein seals between neighboring cells in the gut lining and other barriers. They control what can slip between cells, and when they loosen, gut permeability increases. Zonulin is the main signal that opens them, and larazotide works by blocking that signal.

Titration is starting at a low dose and stepping up gradually until you reach the target dose or the side effects say stop. It lets the body adapt and reveals your own tolerance before you commit to a full dose. GLP-1 drugs have formal titration schedules, typically stepping up every four weeks, and most people find skipping steps is what causes bad nausea.

A triple agonist activates three receptors with one molecule. Retatrutide is the leading example, targeting GLP-1, GIP, and glucagon receptors, and it produced larger weight loss in phase 2 trials than any earlier drug in the class. It is still investigational, and the glucagon component brings its own questions about heart rate and long-term effects.

The units on an insulin syringe are marks of volume, not of drug. On a U-100 syringe, 100 units is 1 mL, so each unit is 0.01 mL. How much peptide is in one unit depends entirely on how you reconstituted the vial, so saying you take 10 units means nothing without the concentration. Units on a syringe are also unrelated to IU, which is a measure of biological activity.

VEGF is the main signal that tells the body to grow new blood vessels. It rises in tissue that is short of oxygen, such as a healing wound or a growing tumor. Healing peptides are often shown to raise VEGF in animal injury models, which is a plausible route for their effects and also the reason for caution in anyone with cancer or diabetic eye disease.

A vial is the small glass bottle with a rubber stopper and metal cap that peptides come in. The label states the total peptide by weight, such as 5 mg, which is not a dose. Peptide vials are multi-dose, meaning you draw from the same vial for weeks, so the stopper should be wiped with alcohol before every draw and the vial kept refrigerated once mixed.

Water retention is the body holding extra fluid, felt as puffiness, tight rings, or a few pounds gained overnight. It is common with growth hormone, secretagogues, and IGF-1, and it is often mistaken for fat gain or, in the other direction, for muscle. It generally settles within a couple of weeks at a steady dose or after reducing it.

Zonulin is a protein the gut releases that loosens tight junctions and increases intestinal permeability. Gluten and certain bacteria trigger its release, and it is elevated in celiac disease. Larazotide was developed as a zonulin antagonist to keep the junctions closed, making it one of the few gut peptides with human trial data behind it.