At a glance: Tirzepatide is a once-weekly injection from Eli Lilly that activates two gut-hormone receptors, GIP and GLP-1. It is FDA-approved as Mounjaro for type 2 diabetes and as Zepbound for weight management and obesity-related sleep apnea. In its main obesity trial, people on the highest dose lost an average of about 21% of body weight over 72 weeks. Weight tends to return after stopping. It carries a boxed warning about thyroid tumors, and FDA ended the shortage-era allowance for compounded copies in 2025.
What is tirzepatide?
Tirzepatide is a lab-made peptide, 39 amino acids long, with a fatty side chain that lets it last about a week in the body. It is the first approved drug to act on both GIP and GLP-1 receptors, which is why it is sometimes called a "dual agonist" or "twincretin."
FDA approved it as Mounjaro for type 2 diabetes in 2022 and as Zepbound for chronic weight management in 2023. Zepbound was approved for moderate to severe obstructive sleep apnea in adults with obesity in December 2024, and Mounjaro was approved to lower heart attack, stroke and cardiovascular death risk in high-risk adults with type 2 diabetes in August 2026.
How does tirzepatide work?
- GLP-1 receptor: reduces appetite, slows stomach emptying and boosts insulin release after meals
- GIP receptor: a second gut hormone that also boosts insulin and appears to add to appetite and fat-metabolism effects
- Blood sugar: insulin is released mainly when glucose is high, and the liver makes less glucose
- Weight: people feel full sooner and eat less, which drives most of the weight loss
What the research shows
Tirzepatide has been tested in tens of thousands of people in large, randomized trials.
| Trial | Who was studied | Main finding |
|---|---|---|
| SURMOUNT-1 (NEJM, 2022) | 2,539 adults with obesity or overweight, no diabetes, 72 weeks | Average weight loss of 15%, 19.5% and 20.9% across three doses vs 3.1% on placebo; about half on the top two doses lost 20% or more |
| SURMOUNT-4 | Adults who first took tirzepatide for 36 weeks | Those switched to placebo regained about 14% of body weight over a year; those who continued lost a further 5.5% |
| SURMOUNT-5 | Adults with obesity, no diabetes, 72 weeks; head-to-head vs semaglutide | About 20% weight loss with tirzepatide vs about 14% with semaglutide |
| SURPASS-2 | Adults with type 2 diabetes, 40 weeks; vs semaglutide | Lowered A1C more than the diabetes dose of semaglutide |
| SURPASS-CVOT | 13,299 adults with type 2 diabetes and heart disease, about 4 years; vs dulaglutide | 8% fewer heart attacks, strokes and cardiovascular deaths; met its goal of being at least as good, but not proven better |
Weight loss depends on staying on treatment; much of it returns within a year of stopping.
Why it gets attention
- Large weight loss: results approach those of some bariatric surgery for many patients
- Multiple approved uses: diabetes, obesity, sleep apnea and cardiovascular risk reduction
- Cost and shortages: high prices and 2022 to 2024 shortages drove demand for compounded copies
- Gray market: "research" tirzepatide is widely sold online, outside any medical oversight
How it is used
Tirzepatide is a prescription injection under the skin, once a week, using a pen or vial. Doctors start low and increase the dose gradually to limit stomach side effects. Dosing follows the FDA label and is set by the prescriber; this page does not provide dosing guidance.
What about side effects?
From the FDA label, trial reports and Mayo Clinic's drug information:
- Common: nausea, diarrhea, constipation, vomiting, stomach pain, belching and reduced appetite, mostly mild to moderate and worst while the dose is being increased; injection-site reactions; hair loss in some people
- Boxed warning, thyroid tumors: tirzepatide caused thyroid C-cell tumors in rats. It should not be used by people with a personal or family history of medullary thyroid cancer or MEN 2. A neck lump, trouble swallowing, hoarseness or shortness of breath should be reported
- Pancreatitis and gallbladder problems, including gallstones
- Dehydration and kidney injury, usually from vomiting or diarrhea
- Low blood sugar, mainly when combined with insulin or sulfonylureas
- Diabetic eye changes, serious allergic reactions, and a risk of inhaling stomach contents during anesthesia or sedation
- Birth control pills: may work less well after starting and after each dose increase, so a backup method is advised for 4 weeks
- Pregnancy: should be stopped before planned pregnancy; it may harm an unborn baby
In SURMOUNT-1, 4% to 7% of people stopped tirzepatide because of side effects, vs about 3% on placebo.
Regulatory status
- FDA-approved as Mounjaro (type 2 diabetes, including children 10 and older, and cardiovascular risk reduction) and Zepbound (chronic weight management and obesity-related sleep apnea).
- During the 2022 to 2024 shortage, pharmacies were temporarily allowed to make copies. FDA declared the shortage resolved, and the allowance ended in early 2025 after a federal court upheld the decision.
- Products sold online as "research" tirzepatide are not FDA-approved and may be mislabeled or contaminated.
- Prescription is required. Off-label use, such as purely cosmetic weight loss, is a decision for a prescriber.
Tirzepatide vs other incretin drugs
| Semaglutide (Ozempic, Wegovy) | Tirzepatide (Mounjaro, Zepbound) | Retatrutide | |
|---|---|---|---|
| Hormone targets | GLP-1 | GIP + GLP-1 | GIP + GLP-1 + glucagon |
| Typical trial weight loss | About 14% to 15% | About 20% to 21% | Up to about 28% |
| Status | FDA-approved | FDA-approved | Investigational |
The bottom line
Tirzepatide is one of the best-studied drugs on this site: large trials show substantial weight loss, better blood sugar control and fewer cardiovascular events than an older GLP-1 drug in people with diabetes. It also has real side effects, a thyroid warning, and benefits that fade when it is stopped. It should be used under a prescriber's care with an FDA-approved product, not a compounded or "research" version.
Frequently asked questions
What is Tirzepatide?
A peptide that acts on two hormone receptors, GIP and GLP-1, which affect blood sugar and appetite.
How is Tirzepatide taken?
Weekly subcutaneous injection.
Is Tirzepatide FDA approved?
FDA-approved (prescription).
What has research shown about Tirzepatide?
- Blood sugar control in type 2 diabetes.
- Weight loss in large clinical trials.
- Obstructive sleep apnea in adults with obesity.
What side effects have been reported?
- Nausea
- Diarrhoea
- Vomiting
- Constipation
- Decreased appetite
- Indigestion
- Injection site reactions
Serious or less common risks are listed in the Side Effects section above.
What is still unknown about its safety?
Side effects that are rare, or that only appear after many years of use, are still being tracked through post-marketing surveillance. Talk to a prescriber about your own risk.
References
- Mayo Clinic: Tirzepatide (subcutaneous route), description, precautions and side effects. View source
- American College of Cardiology: SURMOUNT-1 trial summary (Jastreboff et al., NEJM 2022). View source
- HCPLive: Tirzepatide gets FDA approval to cut cardiovascular risk in type 2 diabetes (SURPASS-CVOT, August 2026). View source
- Acibadem International: Tirzepatide vs semaglutide (SURMOUNT-4, SURMOUNT-5, SURPASS-2 and sleep apnea approval). View source
- NCPA: FDA ends compounding discretion for tirzepatide (March 2025). View source