Peptide guide

Tirzepatide: What the Evidence Actually Shows

Also called LY3298176, Mounjaro, Zepbound, GIP/GLP-1 receptor agonist

Tirzepatide is the rare peptide on this site where the honest summary is that the evidence is genuinely large: a 2,539-participant phase 3 obesity trial, a 13,299-patient cardiovascular outcomes trial, a randomized head-to-head against semaglutide, and an FDA label. That does not make every claim attached to it true. The 20.9% figure people quote is the 15 mg arm — roughly 635 of those 2,539 people — not the trial average. The cardiovascular trial met noninferiority against an active comparator and missed superiority. And the label, as of August 2026, covers three indications, none of them cardiovascular risk reduction. Here is what has been measured, in whom, over how long, and where the published record stops.

By Grant Delaney, Research EditorAugust 2026

Highest phase reached

Approved

Largest trial

13,299randomized (SURPASS-CVOT)

Longest reported follow-up

176weeks on treatment

FDA-approved indications

3across two labels

Compound

What it is

Tirzepatide is a single peptide that activates two receptors at once: the glucose-dependent insulinotropic polypeptide receptor and the glucagon-like peptide 1 receptor. It reached late-stage testing in type 2 diabetes first, in a 1,879-participant open-label phase 3 trial that randomized it against semaglutide 1 mg, and then in obesity, in a 2,539-participant trial of adults with a body-mass index of 30 or more, or 27 or more with at least one weight-related complication, excluding diabetes. Eli Lilly funded both.^1,7

Mechanism

How it works

Two receptors, one molecule — where a selective GLP-1 receptor agonist engages one. The trials measure the consequence rather than the mechanism: against semaglutide 1 mg over 40 weeks in type 2 diabetes, tirzepatide 15 mg lowered glycated hemoglobin by 2.30 percentage points against 1.86, and produced a 5.5 kg larger weight reduction. What that weight is made of was measured in a dual-energy X-ray absorptiometry substudy of 160 of SURMOUNT-1's 2,539 participants: by week 72, about 75% of the weight lost was fat mass and about 25% was lean mass — and the same 75/25 split appeared in the placebo group.^1,7,9

Evidence

What the trials found

Randomized trials

Evidence strength: Randomized trials. Randomized controlled trials in humans, at scale.

SURMOUNT-1 randomized 2,539 adults to 5 mg, 10 mg, or 15 mg once weekly or placebo for 72 weeks, and reported mean weight changes of −15.0%, −19.5%, and −20.9% against −3.1% on placebo, with 57% of the 15 mg group losing at least 20% of body weight against 3% on placebo. The 1,032 participants in that trial who also had prediabetes continued to week 176, where the figures were −12.3%, −18.7%, and −19.7% against −1.3%, and 1.3% of the tirzepatide groups had received a type 2 diabetes diagnosis against 13.3% on placebo. SURMOUNT-5 randomized 751 adults with obesity and without diabetes to the maximum tolerated dose of tirzepatide or of semaglutide for 72 weeks: −20.2% against −13.7%. Two 52-week trials in adults with moderate-to-severe obstructive sleep apnea and obesity cut the apnea-hypopnea index by 25.3 and 29.3 events per hour against 5.3 and 5.5 on placebo. SURMOUNT-MAINTAIN, a 112-week trial, randomized 378 participants after a 60-week open-label weight-loss period and found week-112 reductions of 21.9% for those who continued the maximum tolerated dose, 16.6% for those reduced to 5 mg, and 9.9% for those switched to placebo.^1,2,3,5,8

20.9%

Weight loss, 15 mg — 72 wk

SURMOUNT-1's 15 mg arm — roughly 635 of 2,539 participants, not the trial average.

19.7%

Weight loss, 15 mg — 176 wk

The 1,032 SURMOUNT-1 participants who also had prediabetes.

0.92

Hazard ratio vs dulaglutide

SURPASS-CVOT: noninferior (95.3% CI 0.83–1.01), superiority not met (P = 0.09).

—

Placebo-controlled cardiovascular outcome

SURPASS-CVOT used dulaglutide as an active comparator, not placebo.

Weight change by dose and timepoint (%)

5 mg, 72 wk

15 %

10 mg, 72 wk

19.5 %

15 mg, 72 wk

20.9 %

15 mg, 176 wk

19.7 %

Placebo, 72 wk

3.1 %

Semaglutide MTD, 72 wk

13.7 %

Tirzepatide MTD, 72 wk

20.2 %

Trial size, participants randomized

SURMOUNT-MAINTAIN

378

SURMOUNT-4

670

SURMOUNT-5

751

SURPASS-2

1879

SURMOUNT-1

2539

SURPASS-CVOT

13299

How to read the evidence meter

Four steps, weakest to strongest. Most peptides sold for a goal light up one. We show the step the published human evidence actually reaches — not the step the seller implies.

AnecdotalUser reports and forum consensus. No controlled data.
Animal onlyRodent or cell studies. Nothing published in humans.
Early humanSmall, open-label, or phase 1/2 trials. Promising, unproven.
Randomized trialsRandomized controlled trials in humans, at scale.

Timeline

Development pipeline

Complete
Preclinical
Complete
Phase 1
Complete
Phase 2
Complete
Phase 3
Obesity, type 2 diabetes, obstructive sleep apnea, weight maintenance, cardiovascular outcomes
5 — Current step
FDA review & approval
Mounjaro approved May 2022; Zepbound approved November 2023

Summary

What the evidence does — and doesn't — support

Approved, but narrower than the programme

As of August 2026 the DailyMed labels for Zepbound and Mounjaro list three indications between them — weight reduction and maintenance, moderate-to-severe obstructive sleep apnea in adults with obesity, and glycemic control in type 2 diabetes. Cardiovascular risk reduction is not among them, and SURPASS-CVOT's superiority test against dulaglutide did not reach significance.

What the evidence supports

Large, dose-dependent weight reduction sustained to 176 weeks in the SURMOUNT-1 prediabetes subgroup
Superiority over semaglutide on weight in one randomized head-to-head, SURMOUNT-5
Reduced apnea-hypopnea index in two 52-week trials in moderate-to-severe obstructive sleep apnea
Lower progression to type 2 diabetes than placebo in participants with obesity and prediabetes
Noninferiority to dulaglutide on a cardiovascular composite in 13,299 patients

What it does not support

20.9% as a typical result — it is the 15 mg arm of one trial, roughly 635 of 2,539 people
A placebo-controlled cardiovascular benefit — SURPASS-CVOT's comparator was dulaglutide, and superiority was not met
Superiority over semaglutide at its 7.2 mg dose — no randomized trial cited here tested that comparison
Weight results past 176 weeks of treatment among the trials on this page
Use outside the enrollment criteria of these trials — obesity, overweight with a complication, type 2 diabetes, or sleep apnea with obesity

Reality check

The catch

The number that travels — 20.9% — is a dose arm, not a trial. On SURMOUNT-1's published 1:1:1:1 randomization, the 15 mg group is roughly 635 of 2,539 participants; the 5 mg group in the same trial reached 15.0%. Second, the cardiovascular question is narrower than it sounds. SURPASS-CVOT enrolled 13,299 patients with type 2 diabetes and established atherosclerotic cardiovascular disease and compared tirzepatide against dulaglutide, an active comparator rather than placebo; the primary composite occurred in 12.2% on tirzepatide and 13.1% on dulaglutide, which met the prespecified noninferiority margin and did not meet superiority (P = 0.09). That is a real and useful result, and it is not the same finding as a placebo-controlled reduction in cardiovascular events. Third, the label is narrower than the trial programme. As of August 2026 the DailyMed labels list weight reduction and long-term weight maintenance plus moderate-to-severe obstructive sleep apnea in adults with obesity for Zepbound, and glycemic control in type 2 diabetes for Mounjaro — three indications, none of them cardiovascular risk reduction, and none of them covering people without obesity, overweight with a complication, or diabetes. Fourth, duration: among the trials cited on this page, the longest on-treatment follow-up reported is 176 weeks, and that came from the 1,032-participant prediabetes subgroup of SURMOUNT-1 rather than from the full cohort.

Dosing

Dosing evidence

Each trial ran its own assignment scheme, and they are not interchangeable. SURMOUNT-1 assigned 5 mg, 10 mg, or 15 mg once weekly after a 20-week dose-escalation period. SURMOUNT-5 and the two obstructive sleep apnea trials did not assign a fixed dose at all — they used the maximum tolerated dose of 10 mg or 15 mg, which is a different design question from a fixed-dose comparison. SURMOUNT-4 ran a 36-week open-label lead-in at the maximum tolerated dose of 10 or 15 mg before randomizing 670 of its 783 participants to continue or switch to placebo. SURPASS-2 in type 2 diabetes assigned 5 mg, 10 mg, or 15 mg against semaglutide 1 mg over 40 weeks. What none of these establishes is a dose for an individual reader: a trial assigns a dose to a randomized group to answer a question about the group.^1,3,5,6,7

Already have a dose in mind? Our reconstitution calculator converts it to syringe units. It does not suggest one.

Safety

Safety and side effects

Gastrointestinal events were the most commonly reported adverse events in SURMOUNT-1, SURPASS-2, and SURMOUNT-4, mostly mild to moderate, and in SURMOUNT-1 they occurred primarily during dose escalation. In SURMOUNT-1, adverse events caused treatment discontinuation in 4.3%, 7.1%, and 6.2% of the 5 mg, 10 mg, and 15 mg groups against 2.6% on placebo — so the highest discontinuation rate was not at the highest dose. In SURPASS-2, serious adverse events were reported in 5 to 7% of tirzepatide participants against 3% on semaglutide, and hypoglycemia below 54 mg per deciliter in 0.6%, 0.2%, and 1.7% of the 5 mg, 10 mg, and 15 mg groups against 0.4% on semaglutide. The largest safety dataset is SURPASS-CVOT, where 6,586 patients received tirzepatide and 6,579 dulaglutide; the incidence of adverse events appeared similar between groups, with more gastrointestinal events on tirzepatide.^1,4,6,7

Approved for this use and available by prescription.

What the approvals actually cover

Two applications, two labels. A drugsFDA search returns Mounjaro under NDA 215866, approved May 13, 2022, and Zepbound under NDA 217806, approved November 8, 2023, both sponsored by Eli Lilly. As of August 2026 the DailyMed label for Mounjaro reads as an adjunct to diet and exercise to improve glycemic control in adults and pediatric patients 10 years of age and older with type 2 diabetes. The Zepbound label reads as an adjunct to a reduced-calorie diet and increased physical activity to reduce excess body weight and maintain weight reduction long term in adults with obesity or overweight with at least one weight-related comorbid condition, and to treat moderate-to-severe obstructive sleep apnea in adults with obesity. That is the full approved scope. Everything else tirzepatide is discussed for — including the diabetes-prevention result at 176 weeks — sits outside it.

Stopping is a measured outcome, twice

Two trials answered the question people actually ask, which is what happens when you stop. In SURMOUNT-4, 670 participants who had already lost a mean 20.9% during a 36-week open-label lead-in were randomized to continue or to switch to placebo; over the next 52 weeks the continuing group lost a further 5.5% while the placebo group regained 14.0% ⁶. At week 88, 89.5% of those still on tirzepatide had held at least 80% of their lead-in weight loss, against 16.6% of those switched to placebo ⁶. SURMOUNT-MAINTAIN then tested a middle option — dropping to 5 mg rather than stopping — and reported that 11 of 138 participants on the maximum tolerated dose, 35 of 142 on 5 mg, and 60 of 90 on placebo went on to need rescue therapy for regaining at least half their lost weight ⁸. Those are 8%, 25%, and 67% of the respective observed denominators. Read together, the two trials say the same thing in different registers: the effect is maintained while the drug is, and a lower dose is a partial hedge rather than an exit.

The head-to-head, and what it did not test

SURMOUNT-5 is the direct comparison: 751 adults with obesity and without diabetes, randomized 1:1, open-label, 72 weeks, maximum tolerated dose of tirzepatide (10 or 15 mg) against maximum tolerated dose of semaglutide (1.7 or 2.4 mg) ³. Tirzepatide won on weight, −20.2% against −13.7%, and on waist circumference, −18.4 cm against −13.0 cm ³. The comparison it did not run is against semaglutide's higher 7.2 mg dose, which was tested against 2.4 mg in a separate trial and is not in SURMOUNT-5's protocol ³. Anyone extending SURMOUNT-5 into a claim about tirzepatide versus every semaglutide dose is extending it past what was randomized. Our semaglutide page covers that trial from the other side.

Where the programme has and has not gone

The published trials on this page enrolled adults with obesity, adults with overweight and a weight-related complication, adults with type 2 diabetes, and adults with moderate-to-severe obstructive sleep apnea and obesity. Populations outside those criteria — people at a normal body-mass index, people seeking body recomposition rather than weight reduction, adolescents in the obesity indication — are not populations these trials randomized, so the results on this page do not describe them. That is a statement about the enrollment criteria of the specific trials cited here, not a claim that no such trial exists anywhere.

How this page treats a dose

Tirzepatide has an FDA label, which is what separates it from a compound like retatrutide and from the grey market entirely. As of August 2026 the Zepbound label specifies a starting dosage of 2.5 mg once weekly for four weeks, increases in 2.5 mg increments after at least four weeks, maintenance dosages of 5 mg, 10 mg, or 15 mg for weight reduction and 10 mg or 15 mg for obstructive sleep apnea, and a maximum of 15 mg once weekly — and states explicitly that the 2.5 mg dose is for initiation and is not approved as a maintenance dosage. We report that because it is the published standard against which any other number should be read. We do not tell you which rung applies to you; that is a decision for the clinician who prescribes it and monitors what happens next.

Questions

Frequently asked questions

How much weight do people actually lose on tirzepatide?

In SURMOUNT-1, mean weight change at 72 weeks was −15.0% on 5 mg, −19.5% on 10 mg, and −20.9% on 15 mg, against −3.1% on placebo. The widely quoted 20.9% is the 15 mg arm, roughly 635 of 2,539 participants on the published 1:1:1:1 randomization. In the 1,032-participant prediabetes subgroup followed to 176 weeks, the same doses read −12.3%, −18.7%, and −19.7% against −1.3%.

Is tirzepatide better than semaglutide?

In the one randomized head-to-head in obesity, SURMOUNT-5, tirzepatide at its maximum tolerated dose produced −20.2% at 72 weeks against −13.7% for semaglutide at its maximum tolerated dose of 1.7 or 2.4 mg. In type 2 diabetes, SURPASS-2 found tirzepatide superior to semaglutide 1 mg on glycated hemoglobin at 40 weeks. Both comparisons used semaglutide doses below 7.2 mg, so neither answers how tirzepatide compares with semaglutide's higher dose.

Does tirzepatide reduce heart attacks and strokes?

SURPASS-CVOT, in 13,299 patients with type 2 diabetes and established atherosclerotic cardiovascular disease, compared tirzepatide against dulaglutide rather than placebo. The composite of cardiovascular death, myocardial infarction, or stroke occurred in 12.2% on tirzepatide and 13.1% on dulaglutide — noninferior by the prespecified margin, and not superior (P = 0.09). As of August 2026 neither the Mounjaro nor the Zepbound label carries a cardiovascular risk-reduction indication.

What happens if you stop tirzepatide?

It has been measured. In SURMOUNT-4, participants switched to placebo after a 36-week lead-in regained a mean 14.0% over the following 52 weeks while those who continued lost a further 5.5%. In SURMOUNT-MAINTAIN, 60 of 90 participants switched to placebo went on to need rescue therapy for regaining at least half their lost weight, against 11 of 138 who stayed on the maximum tolerated dose and 35 of 142 who dropped to 5 mg.

How much of the weight lost is muscle?

A dual-energy X-ray absorptiometry substudy of 160 of SURMOUNT-1's 2,539 participants found that of the weight lost by week 72, roughly 75% was fat mass and 25% was lean mass — and the placebo group showed the same proportion. That is a 160-person substudy, not the whole trial, and it measures lean mass rather than muscle function or strength.

What are tirzepatide's side effects?

Gastrointestinal events were the most commonly reported adverse events across the trials cited here, mostly mild to moderate and concentrated during dose escalation. In SURMOUNT-1 they caused discontinuation in 4.3%, 7.1%, and 6.2% of the 5 mg, 10 mg, and 15 mg groups against 2.6% on placebo. As of August 2026 the Zepbound label carries a boxed warning for thyroid C-cell tumors and warnings covering severe gastrointestinal reactions, acute kidney injury from volume depletion, acute gallbladder disease, acute pancreatitis, hypersensitivity reactions, hypoglycemia, diabetic retinopathy complications in type 2 diabetes, and pulmonary aspiration under anesthesia or deep sedation. The label is the authoritative list; this is a summary of it.

Glossary

Key terms

Dual agonist: A molecule that binds and activates two separate receptors — here GIP and GLP-1 — rather than one drug per receptor.
Noninferiority: A trial design asking whether a drug is not meaningfully worse than a comparator by a pre-set margin. Passing it is not the same as being better.
Active comparator: A trial control that receives another working drug rather than placebo. It answers 'better than this drug', not 'better than nothing'.
Maximum tolerated dose: A design where each participant escalates to the highest dose they can tolerate, so the group's dose varies rather than being fixed.
Randomized withdrawal: A design where everyone first takes the drug, then some are switched to placebo — the way regain after stopping gets measured.
Estimand: The precise definition of the treatment effect a trial reports, including how it handles people who stop early. Different estimands give different numbers from the same data.

Sources

References

Jastreboff AM, Aronne LJ, Ahmad NN, et al. (2022). Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine. https://pubmed.ncbi.nlm.nih.gov/35658024/
Jastreboff AM, le Roux CW, Stefanski A, et al. (2025). Tirzepatide for Obesity Treatment and Diabetes Prevention. New England Journal of Medicine. https://pubmed.ncbi.nlm.nih.gov/39536238/
Aronne LJ, Horn DB, le Roux CW, et al. (2025). Tirzepatide as Compared with Semaglutide for the Treatment of Obesity. New England Journal of Medicine. https://pubmed.ncbi.nlm.nih.gov/40353578/
Nicholls SJ, Pavo I, Bhatt DL, et al. (2025). Cardiovascular Outcomes with Tirzepatide versus Dulaglutide in Type 2 Diabetes. New England Journal of Medicine. https://pubmed.ncbi.nlm.nih.gov/41406444/
Malhotra A, Grunstein RR, Fietze I, et al. (2024). Tirzepatide for the Treatment of Obstructive Sleep Apnea and Obesity. New England Journal of Medicine. https://pubmed.ncbi.nlm.nih.gov/38912654/
Aronne LJ, Sattar N, Horn DB, et al. (2024). Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial. JAMA. https://pubmed.ncbi.nlm.nih.gov/38078870/
Frías JP, Davies MJ, Rosenstock J, et al. (2021). Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes. New England Journal of Medicine. https://pubmed.ncbi.nlm.nih.gov/34170647/
Horn DB, Aronne LJ, Wharton S, et al. (2026). Tirzepatide for maintenance of bodyweight reduction in people with obesity in the USA (SURMOUNT-MAINTAIN): a multicentre, double-blind, randomised, placebo-controlled trial. The Lancet. https://pubmed.ncbi.nlm.nih.gov/42119587/
Look M, Dunn JP, Kushner RF, et al. (2025). Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes, Obesity and Metabolism. https://pubmed.ncbi.nlm.nih.gov/39996356/

Medical disclaimer: This content is for general educational purposes only and is not medical advice, diagnosis, or treatment. Always consult a licensed healthcare professional before starting, stopping, or changing any treatment.