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Peptide drug (FDA approved)Human trialsFDA approved

Tirzepatide

Tirzepatide

Last reviewed October 27, 2026 · 13 sources · Written for general readers · Compare with another compound →

The bottom line

Tirzepatide is an FDA approved drug sold as Mounjaro for type 2 diabetes and as Zepbound for weight loss and sleep apnea. In large trials people lost about 15% to 21% of their weight, more than with semaglutide, but much of it came back after stopping. Stomach side effects are common, and copies sold online or as research products are not the approved drug.

Where the evidence stands

What it is

Tirzepatide is a prescription drug made by Eli Lilly. It is a single molecule that acts on receptors for two gut hormones, GLP-1 and GIP, which help control appetite and blood sugar.[1]

It is sold as Mounjaro, approved to improve blood sugar control in adults with type 2 diabetes, and as Zepbound, approved for weight loss and keeping weight off in adults with obesity, or overweight plus a weight related health problem.[2] Zepbound is also approved for moderate to severe obstructive sleep apnea (repeated breathing pauses during sleep) in adults with obesity.[3]

Chemically, it is a large modified peptide. PubChem lists its formula as C225H348N48O68.[4]

Name
Tirzepatide (Tirzepatide)
Other names
Mounjaro, Zepbound
Type
Peptide drug (FDA approved)
CAS number
2023788-19-2
Formula
C225H348N48O68
Molecular weight
About 4813 g/mol

How it works

Tirzepatide switches on the receptors for two natural gut hormones at once, GLP-1 and GIP.[1]

  • Activating these receptors lowers appetite and food intake.[1]
  • In animal studies, it reaches and switches on nerve cells in brain areas that control appetite.[3]
  • Lab studies suggest the added GIP action may help control food intake beyond what GLP-1 action does alone.[3]
  • It is given as a shot once a week.[1]

The two receptor action is well established. How much each receptor adds to the effect in people is still being studied.

What the research shows

We keep animal and human evidence apart. A result in animals is a reason to study something in people, not proof that it works in people.

Animal and lab studies

Animal data mainly flag thyroid and pregnancy risks

  • In a two year rat study, tirzepatide caused thyroid C cell tumors in males and females, and combined benign and cancerous tumors rose at every dose tested.[3]
  • In a six month study in mice bred to be prone to tumors, it did not cause tumors.[3]
  • In pregnant rats it was linked to birth defects and smaller fetuses, and in pregnant rabbits stomach effects caused a few mothers to die or miscarry.[3]
  • Female rats had fewer eggs released and fewer embryos, which the label links to lower food intake and weight. Male fertility was not affected.[3]

Limit: Whether the thyroid finding applies to people is unknown, which is why the label carries a boxed warning.

Human studies

Strong evidence from large, placebo controlled trials

  • In SURMOUNT-1, 2,539 adults without diabetes took a weekly shot or placebo for 72 weeks. Average weight loss was 15.0%, 19.5% and 20.9% across three doses versus 3.1% on placebo, and 85% to 91% on the drug lost at least 5%.[3][5]
  • In SURMOUNT-2, 938 adults with type 2 diabetes and excess weight lost 12.8% to 14.7% over 72 weeks versus 3.2% on placebo.[3]
  • In SURMOUNT-3, 579 adults first followed a 12 week diet and exercise program. Over the next 72 weeks, the drug group lost another 18.4% while the placebo group gained 2.5%.[3]
  • In SURMOUNT-4, 670 adults lost about 21% in the first 36 weeks. People switched to placebo regained 14.0% over the next year, while those who stayed on the drug lost another 5.5%. At the end, 89.5% of those who stayed on kept at least 80% of their earlier loss versus 16.6% on placebo.[7]
  • A body scan substudy of 160 SURMOUNT-1 participants found fat mass fell 33.9% and lean mass fell 10.9% over 72 weeks, with about a quarter of total weight lost coming from lean mass.[8]
  • In two 52 week trials in about 470 adults with sleep apnea and obesity, breathing pauses fell by about 25 to 29 per hour on the drug versus about 5 on placebo, and weight fell about 18% to 20%.[3]
  • In SURMOUNT-5, a head to head trial of 751 adults with obesity over 72 weeks, average weight loss was 20.2% on tirzepatide versus 13.7% on semaglutide. Fewer people stopped tirzepatide because of stomach side effects, 2.7% versus 5.6%.[6]

Limit: Trials were run by the maker and lasted one to two years, and weight returns when the drug stops, so long term use is needed to keep the benefit.[7]

Claims check

The most common claims about this compound, and what studies actually found. Verdicts rate the evidence, not whether you should use it.

Causes major weight loss

Shown in people

In SURMOUNT-1, 2,539 adults with obesity or overweight and no diabetes took a weekly shot or placebo for 72 weeks. Average weight loss was 15.0% to 20.9% depending on dose, versus 3.1% on placebo, and up to 57% of people on the drug lost a fifth of their weight.[3][5]

Works better than semaglutide

Shown in people

In SURMOUNT-5, 751 adults with obesity were assigned to tirzepatide or semaglutide for 72 weeks. Weight loss averaged 20.2% versus 13.7%, and fewer people stopped tirzepatide because of stomach side effects.[6]

The weight stays off after you stop

No benefit in trials so far

In SURMOUNT-4, 670 adults first lost about 21% on the drug over 36 weeks. Those switched to placebo regained 14.0% of their weight over the next year, while those who stayed on it lost another 5.5%.[7]

You lose fat, not muscle

Mixed results

In a body scan substudy of 160 SURMOUNT-1 participants, fat mass fell 33.9% and lean mass (muscle, organs, water and bone) fell 10.9% over 72 weeks. About three quarters of the weight lost was fat and a quarter was lean mass, the same split seen on placebo.[8] So most loss is fat, but some lean mass is lost too, and effects on strength were not measured.

Treats sleep apnea

Shown in people

In two 52 week trials in about 470 adults with obesity and moderate to severe sleep apnea, breathing pauses fell by about 25 to 29 per hour on the drug versus about 5 on placebo. Up to half reached remission or mild disease that caused no symptoms.[3]

Helps people with type 2 diabetes lose weight

Shown in people

In SURMOUNT-2, 938 adults with type 2 diabetes and excess weight lost 12.8% to 14.7% over 72 weeks, versus 3.2% on placebo.[3] Mounjaro is separately approved to lower blood sugar in type 2 diabetes.[2]

Known risks and side effects

  • Boxed warning for thyroid tumors. People with a personal or family history of medullary thyroid cancer or MEN 2 (an inherited tumor syndrome) should not use it. The warning is based on rat studies.[3]
  • Stomach and gut side effects are very common. In the weight trials, nausea hit 25% to 29% versus 8% on placebo, diarrhea 19% to 23% versus 8%, constipation 11% to 17% versus 5% and vomiting 8% to 13% versus 2%. About 5% to 7% stopped because of side effects versus 3.4% on placebo.[3]
  • Other common side effects. Injection site reactions (6% to 8% versus 2%), hair loss (4% to 5% versus 1%), tiredness, burping and heartburn were more common than with placebo. Resting heart rate rose by 1 to 3 beats per minute on average.[3]
  • Gallbladder, pancreas and kidney problems. Inflamed gallbladder was reported in 0.7% versus 0.2% on placebo, and pancreatitis in 0.2% of both groups in the weight trials. The label also warns about kidney injury from fluid loss and food entering the lungs during anesthesia or deep sedation.[3]
  • Compounded and research only products. FDA had more than 730 adverse event reports for compounded tirzepatide as of May 31, 2026, and has warned online sellers whose tirzepatide was labeled research use only.[9][10]

Regulatory status

Today: Tirzepatide is FDA approved as Mounjaro and Zepbound.[2] FDA declared the shortage over in December 2024, and its leeway for pharmacies and outsourcing facilities to make copies ended in March 2025.[2][11]

  1. FDA approved Zepbound for chronic weight management.[1]
  2. FDA first said the tirzepatide shortage was over, and a court later sent that decision back for review.[2]
  3. FDA issued a formal order confirming the shortage was resolved.[2]
  4. FDA approved Zepbound as the first medicine for obstructive sleep apnea in adults with obesity.[12]
  5. Leeway to copy tirzepatide ended for state licensed pharmacies after a court ruling on March 5 and for outsourcing facilities on March 19.[11]
  6. FDA launched a Green List import alert to keep GLP-1 ingredients from unverified foreign sources out of the US.[13]
  7. FDA sent a warning letter to an online seller of tirzepatide and retatrutide labeled research use only.[10]

Sources

  1. U.S. Food and Drug Administration. FDA Approves New Medication for Chronic Weight Management. November 8, 2023. FDA news release
  2. U.S. Food and Drug Administration. Declaratory Order: Resolution of Shortages of Tirzepatide Injection Products (Mounjaro and Zepbound). December 19, 2024. FDA declaratory order
  3. Eli Lilly and Company. ZEPBOUND (tirzepatide) injection: prescribing information. Revised 02/2026. FDA label
  4. National Center for Biotechnology Information. PubChem Compound Summary for CID 156588324, Tirzepatide. PubChem CID 156588324
  5. Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387(3):205-216. doi 10.1056/NEJMoa2206038
  6. Aronne LJ, et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity. N Engl J Med. 2025;393(1):26-36. doi 10.1056/NEJMoa2416394
  7. Aronne LJ, Sattar N, Horn DB, et al. Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial. JAMA. 2024;331(1):38-48. doi 10.1001/jama.2023.24945
  8. Look M, Dunn JP, Kushner RF, et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes Obes Metab. 2025;27(5):2720-2729. doi 10.1111/dom.16275
  9. U.S. Food and Drug Administration. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. Content current as of October 1, 2026. FDA
  10. U.S. Food and Drug Administration. Warning letter 721806 to an online peptide seller. March 31, 2026. FDA warning letter
  11. U.S. Food and Drug Administration. FDA clarifies policies for compounders as national GLP-1 supply begins to stabilize. Content current as of April 1, 2026. FDA
  12. U.S. Food and Drug Administration. FDA Approves First Medication for Obstructive Sleep Apnea. December 20, 2024. FDA news release
  13. U.S. Food and Drug Administration. FDA Launches Green List to Protect Americans from Illegal Imported GLP-1 Drug Ingredients. September 5, 2025. FDA news release
Medical disclaimer

This page is educational information only. It is not medical advice and does not replace care from a licensed medical provider. Do not use this page to decide whether to take any substance. Talk to a licensed medical provider first.

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