Tirzepatide
Also known as Mounjaro, Zepbound, LY3298176
Tirzepatide is a once-weekly peptide that activates both the GIP and GLP-1 receptors, producing average weight loss of about 20 percent at the top dose in trials. It is sold as Mounjaro for diabetes and Zepbound for weight management and sleep apnea.
Overview
Tirzepatide is a single 39 amino acid peptide built on the GIP hormone backbone and tuned to activate both the GIP and GLP-1 receptors. Eli Lilly developed it with a fatty diacid side chain for albumin binding, which gives it a half-life of about five days and once-weekly dosing. It reached the market as Mounjaro for type 2 diabetes in 2022 and as Zepbound for chronic weight management in late 2023, later adding an indication for obstructive sleep apnea in people with obesity.
The headline numbers come from SURMOUNT-1: adults with obesity but not diabetes lost an average of about 15, 19.5 and 20.9 percent of body weight at the 5, 10 and 15 mg doses over 72 weeks, against about 3 percent on placebo. In the head-to-head SURMOUNT-5 trial against semaglutide 2.4 mg, tirzepatide 15 mg produced roughly 20 percent loss versus about 14 percent, which makes it the strongest approved option as of now.
As with semaglutide, the vials sold through research suppliers are not the branded product. Lilly does sell single-dose Zepbound vials through its own direct-to-consumer channel in the US, and those are legitimate. Grey-market lyophilized tirzepatide is a separate thing with no guaranteed purity, and underdosed or mislabeled vials are a documented problem in that market. If you are going to inject it, batch testing is worth the cost.
How it works
Tirzepatide hits two incretin receptors at once. The GLP-1 side does what semaglutide does: it boosts glucose-dependent insulin release, slows gastric emptying and dampens hunger signaling in the brain. The GIP side is less well understood but appears to add to appetite suppression, improve how fat tissue handles lipids, and may soften some of the nausea that GLP-1 activation alone produces. The molecule is actually biased toward the GIP receptor and is a weaker GLP-1 agonist than semaglutide molecule for molecule, yet the combination produces larger weight loss, which is the main argument that dual agonism is doing something real rather than just being a stronger GLP-1 drug.
What the research shows
- human RCTUp to 20.9 percent mean weight loss at 15 mg weekly over 72 weeks in adults with obesity (SURMOUNT-1).
- human RCTBeat semaglutide 2.4 mg head to head, about 20 percent versus 14 percent weight loss at 72 weeks (SURMOUNT-5).
- human RCTGreater HbA1c reduction than semaglutide 1 mg in type 2 diabetes (SURPASS-2).
- clinical (approved drug)Cut the apnea-hypopnea index by roughly 25 to 30 events per hour in obesity-related sleep apnea (SURMOUNT-OSA), leading to an FDA indication.
- human RCTReduced progression from prediabetes to type 2 diabetes by over 90 percent across three years of treatment (SURMOUNT-1 extension).
Evidence labels: rodent and in vitro mean no human data for that finding. Human pilot means small, often uncontrolled. Human RCT means randomized and controlled. Clinical means an approved drug with regulatory data.
Dose calculator
U-100 insulin syringeUnits are a volume on the syringe, not an amount of peptide. Concentration changes every time you change the water volume. The standalone calculator explains the math.
Dosing and protocol
Start 2.5 mg weekly for 4 weeks, then increase by 2.5 mg every 4 weeks as tolerated. 5, 10 and 15 mg are the maintenance doses studied; plenty of people hold at 5 or 7.5 mg.
Ongoing. Weight regain after stopping was substantial in the withdrawal trial, so treat it as long-term.
Same day each week, any time, with or without food. Rotate sites between abdomen, thigh and upper arm.
| Vial | Suggested water | Concentration | Low dose draw |
|---|---|---|---|
| 10 mg | 1 mL | 10 mg/mL | 25 units |
| 15 mg | 1.5 mL | 10 mg/mL | 25 units |
| 30 mg | 3 mL | 10 mg/mL | 25 units |
| 60 mg | 6 mL | 10 mg/mL | 25 units |
Lyophilized: fridge 2-8C away from light, freezer for long-term. Reconstituted: fridge 2-8C, use within 4 weeks. Branded pens and vials: fridge, may sit at room temperature up to 21 days.
Cautions
- ▲GI effects (nausea, vomiting, diarrhea, constipation) are the main reason people quit. Slow escalation and smaller meals fix most of it.
- ▲Same rodent thyroid C-cell tumor warning as other GLP-1 drugs. Avoid with medullary thyroid cancer or MEN2 history.
- ▲Hypoglycemia is a real risk when combined with insulin or sulfonylureas. Not an issue for most non-diabetic users.
- ▲Rapid loss can mean hair shedding, gallstones and lean mass loss. Protein, lifting and a sane rate of loss help.
- ▲Delayed gastric emptying matters for anesthesia and can change absorption of oral medications, including birth control pills during titration.
Commonly stacked with
Frequently asked
Tirzepatide or semaglutide?
Tirzepatide produced more weight loss in the head-to-head trial, about 20 percent versus 14 percent at 72 weeks, and many users report milder nausea. Semaglutide has the cardiovascular outcomes data and a longer safety track record. Cost and availability often decide it in practice.
Why does the nausea come back after each dose increase?
Each step up raises the blood level and the gut has to re-adapt to slower emptying. It usually settles within a week or two. If it does not, staying at the current dose for another month before increasing is the standard fix.
Do I have to reach 15 mg?
No. The trials show a dose-response, but 5 mg still produced 15 percent average loss. The right dose is the lowest one that keeps appetite under control with tolerable side effects.
What is the difference between a Zepbound vial and research tirzepatide?
Zepbound vials are Lilly's own product, sold through their direct channel, already in solution and lot tested. Research tirzepatide is lyophilized powder from an unregulated supplier that you reconstitute yourself. The molecule is meant to be the same; the quality assurance is not.
Does it work for people who are not diabetic?
Yes, SURMOUNT-1 was run specifically in people without diabetes and produced the largest weight loss numbers. Blood sugar effects in non-diabetics are minimal because insulin release is glucose-dependent.
References
- Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). New England Journal of Medicine, 2022
- Tirzepatide versus semaglutide once weekly in patients with type 2 diabetes (SURPASS-2). New England Journal of Medicine, 2021
- Tirzepatide versus semaglutide for weight loss in adults with obesity without diabetes (SURMOUNT-5). New England Journal of Medicine, 2025
- Tirzepatide for the treatment of obstructive sleep apnea and obesity (SURMOUNT-OSA). New England Journal of Medicine, 2024