Peptide Graded

All peptides / Gut and weight

Tirzepatide

Dual GIP + GLP-1 receptor agonist

Also Mounjaro, Zepbound, Tirz, LY3298176, Compounded tirzepatide, GIP/GLP-1 dual agonist

Evidence
AStrong human evidence
FDA status
FDA approvedas of 2026-10-27
Route
Injection
Studies cited
8 human0 animal or cell

It is the weekly shot behind Mounjaro and Zepbound, used for diabetes and weight loss. Large trials show 15% to 21% weight loss.

Sold for

Tirzepatide is the weight-loss injection that outperformed semaglutide, cutting body weight by about a fifth in trials.

Studies say

Multiple large randomized trials show weight loss of 15 to 21 percent, better glucose control than other diabetes drugs, and improvement in sleep apnea, with weight regain after stopping, which is a clean A.

Why grade A. Tirzepatide's weight-loss use rests on the SURMOUNT program of randomized placebo-controlled trials, the largest being SURMOUNT-1 with 2,539 adults, plus trials in type 2 diabetes, sleep apnea, weight maintenance and diabetes prevention. Every approved indication was granted on randomized data, so it grades A. 281415

FDA. Tirzepatide is approved by the FDA as Mounjaro (2022) for type 2 diabetes in adults, extended to children 10 and older and to cardiovascular risk reduction in adults with type 2 diabetes and established cardiovascular disease, and as Zepbound (2023) for chronic weight management and for moderate to severe obstructive sleep apnea in adults with obesity. 1341011

Written from 17 cited sources. Fact-checked 2026-10-27: 30 claims checked, 5 corrected. Last refreshed 2026-10-27. Not physician reviewed.

What this means for you

What people take it for
Type 2 diabetes and losing weight
Can a US doctor prescribe it?
Yes, FDA approved for type 2 diabetes, weight loss and sleep apnea.
Properly tested in people?
Yes, in large controlled trials. The biggest weight trial had 2,539 people.

Evidence checklist

  • Randomised controlled trial SURMOUNT programme; diabetes and sleep apnoea trialsyes
  • Human study over 100 people SURMOUNT-1, 2,539 adultsyes
  • More than one research group Article does not say who ran the trialsno information
  • Clinical outcomes, not surrogate markers Weight, glucose, apnoea scores; outcomes not citedno
  • Human safety data beyond case reports Large trials; GI effects, boxed thyroid warningyes

Drawn from the cited studies below. A dash means the sources give no information.

What it is

Tirzepatide is a lab-made peptide of 39 amino acids. It switches on two gut-hormone receptors: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). The intestine releases both hormones after eating. They tell the pancreas to make insulin. A fatty acid chain attached to the peptide binds it to albumin in the blood. So one injection under the skin lasts a week. 15

Eli Lilly sells the same molecule under two names. Mounjaro is labeled for type 2 diabetes. Zepbound is labeled for weight management and obstructive sleep apnea. The FDA has warned that no one has evaluated compounded and online versions for safety or quality. 1313

How it works

GLP-1 receptor activation slows stomach emptying and reduces appetite through the brain. It raises insulin release only when glucose is high. GIP receptor activation adds to the insulin effect. It may improve how fat tissue handles lipids. In trials it also appears to reduce the nausea that GLP-1 drugs alone cause. That allows higher effective doses. The result is less eating and lower blood sugar. Triglycerides and LDL cholesterol also improve. 15

What human research shows

SURMOUNT-1 randomized 2,539 adults with obesity or overweight and without diabetes. They got three tirzepatide doses or placebo for 72 weeks. Mean weight change was 15 percent, 19.5 percent and 20.9 percent across the doses. On placebo it was 3.1 percent. Half or more of participants on the two higher doses (50 and 57 percent) lost at least 20 percent of body weight. A three-year extension of the same trial included participants with prediabetes. It found that 1.3 percent progressed to type 2 diabetes on tirzepatide. On placebo it was 13.3 percent. 215

SURMOUNT-2 enrolled 938 adults with obesity and type 2 diabetes. They lost 12.8 percent and 14.7 percent of body weight at the two doses. On placebo they lost 3.2 percent. A 2024 network meta-analysis covered 28 trials in type 2 diabetes. Tirzepatide lowered HbA1c (the three-month blood sugar average) by 1.6 to 2.0 percentage points against placebo, depending on dose. It lowered weight more than semaglutide at each comparable dose. 68

SURMOUNT-OSA was two randomized trials with 469 adults in total. All had obesity and moderate to severe sleep apnea. Tirzepatide cut the apnea-hypopnea index (breathing interruptions per hour) by 25 to 29 events. On placebo it fell by 5 to 6. This was the basis for the 2024 sleep apnea approval. In SURMOUNT-4, participants lost weight on tirzepatide for 36 weeks. They were then switched to placebo. Over the next year they regained 14 percent of body weight. Those who continued lost a further 5.5 percent. A 2025 meta-analysis pooled seven direct comparisons. Five were observational. Tirzepatide users had higher odds of reaching 10 percent weight loss than semaglutide users. 71416

StudyModelFinding
SURMOUNT-1 (Jastreboff 2022)Humans, n=2,539 with obesity and no diabetes, randomized placebo-controlled, 72 weeksWeight loss of 15 to 20.9 percent across doses versus 3.1 percent on placebo. 2
SURMOUNT-2 (Garvey 2023)Humans, n=938 with obesity and type 2 diabetes, randomized placebo-controlled, 72 weeksWeight loss of 12.8 and 14.7 percent versus 3.2 percent on placebo, with large HbA1c reductions. 8
SURMOUNT-OSA (Malhotra 2024)Humans, n=469 with obesity and sleep apnea, two randomized placebo-controlled trials, 52 weeksApnea-hypopnea index fell by 25 to 29 events per hour versus 5 to 6 on placebo. 14
SURMOUNT-4 (Aronne 2024)Humans, n=670, randomized withdrawal after 36-week lead-inSwitching to placebo led to 14 percent weight regain over 52 weeks; continuing tirzepatide produced further loss. 7
SURMOUNT-1 three-year extension (2024)Humans with prediabetes, n=1,032, randomized placebo-controlled, 176 weeksProgression to type 2 diabetes in 1.3 percent on tirzepatide versus 13.3 percent on placebo. 15

Safety

Stomach and gut effects dominate. These are nausea, diarrhea, vomiting, constipation and belly pain. They mostly occur while the dose is raised. In SURMOUNT-1 about 4 to 7 percent of tirzepatide participants stopped because of adverse events. On placebo it was 2.6 percent. Low blood sugar is uncommon unless the drug is combined with insulin or sulfonylureas. 12

The label carries a boxed warning for thyroid C-cell tumors, based on rodent studies. No one knows if this applies to humans. The drug is barred in people with a personal or family history of medullary thyroid cancer. Other warnings include pancreatitis and gallbladder disease. They also include acute kidney injury from dehydration and severe gut disease. People with diabetes face worse diabetic retinopathy. The label also notes that delayed stomach emptying can affect anesthesia. Lean mass loss accompanies fat loss. The effects on muscle over many years are not yet known. 13

A retrospective series from Siena followed 23 adults with bipolar disorder. They started tirzepatide while taking lithium. Average lithium levels rose slightly over the first 12 weeks. Two patients went above the usual upper limit. One stopped tirzepatide after symptomatic lithium toxicity. With 23 patients this is a signal, not an incidence estimate. The authors call for structured monitoring when doctors prescribe the two drugs together. 17

Questions for your clinician

  • What happens to my weight if I stop?
  • Does the thyroid tumour warning apply to my family history?
  • How do the side effects change as the amount is increased?

Questions people ask

How does tirzepatide differ from GLP-1 drugs like semaglutide?

Semaglutide activates only the GLP-1 receptor. Tirzepatide activates both GLP-1 and GIP receptors. The added GIP activity contributes to insulin release and appears to reduce nausea. That lets the drug reach higher effective exposure. In pooled comparisons, mostly observational, tirzepatide users were likelier to lose 10 percent of body weight. 1516

What did SURMOUNT-1 show?

2,539 adults with obesity and no diabetes took tirzepatide for 72 weeks. Mean weight loss was 15 to 20.9 percent depending on dose. On placebo it was 3.1 percent. Half or more of participants on the two higher doses lost at least a fifth of their body weight. 2

What is tirzepatide approved for?

As Mounjaro: type 2 diabetes in adults and children 10 and older. It is also approved to lower cardiovascular risk in adults with type 2 diabetes and heart disease. As Zepbound: chronic weight management in adults with obesity or overweight plus a weight-related condition. It is also approved for moderate to severe obstructive sleep apnea in adults with obesity. 1341011

Does tirzepatide help sleep apnea?

Yes. Two randomized trials included 469 adults with obesity and sleep apnea. Breathing interruptions per hour fell by 25 to 29 on tirzepatide. On placebo they fell by about 5. The FDA approved it for this use in December 2024. The benefit tracks weight loss. 414

What happens when you stop?

Weight comes back. In SURMOUNT-4, people were switched to placebo after 36 weeks. Over the next year they regained 14 percent of body weight. Those who kept taking tirzepatide lost more. 7

Citations

  1. FDA 2025: Mounjaro (tirzepatide) prescribing information accessdata.fda.gov
  2. Jastreboff 2022, NEJM: Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1) doi.org
  3. FDA 2025: Zepbound (tirzepatide) prescribing information accessdata.fda.gov
  4. FDA 2024: FDA approves first medication for obstructive sleep apnea fda.gov
  5. 2022, Int J Mol Sci: Tirzepatide, a dual GIP/GLP-1 receptor agonist, mechanism and metabolic effects doi.org
  6. 2024, Diabetologia: network meta-analysis of tirzepatide versus semaglutide in type 2 diabetes (28 RCTs) doi.org
  7. Aronne 2024, JAMA: Continued treatment with tirzepatide for maintenance of weight reduction (SURMOUNT-4) doi.org
  8. Garvey 2023, Lancet: Tirzepatide once weekly in adults with obesity and type 2 diabetes (SURMOUNT-2) doi.org
  9. FDA Drugs@FDA: Mounjaro NDA 215866 accessdata.fda.gov
  10. FDA 2026: Mounjaro label, pediatric type 2 diabetes accessdata.fda.gov
  11. FDA 2026: Mounjaro label, cardiovascular risk reduction accessdata.fda.gov
  12. FDA Drugs@FDA: Zepbound NDA 217806 accessdata.fda.gov
  13. FDA: Concerns with unapproved GLP-1 drugs used for weight loss fda.gov
  14. Malhotra 2024, NEJM: Tirzepatide for obstructive sleep apnea and obesity (SURMOUNT-OSA) doi.org
  15. Jastreboff 2024, NEJM: Tirzepatide for obesity treatment and diabetes prevention, three-year SURMOUNT-1 doi.org
  16. 2025, Cureus: tirzepatide versus semaglutide for weight loss, meta-analysis of seven direct comparative studies doi.org
  17. Rescalli 2026, Hum Psychopharmacol: tirzepatide initiation in lithium-treated bipolar disorder, renal and lithium safety over 12 months pubmed.ncbi.nlm.nih.gov

Updates

  • 2026-10-27 added 23-patient lithium co-prescription series with one symptomatic lithium toxicity (PMID 42803597) to safety.

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