Peptides for Weight Loss: What the Trial Evidence Shows

The short version

The phrase “peptides for weight loss” covers two categories that share almost nothing. One is a small group of incretin peptides with large phase 3 trials, published results, and regulatory approvals: semaglutide, tirzepatide, liraglutide, and the investigational retatrutide. The other is a much longer list of peptides sold online and through wellness clinics, including AOD-9604, MOTS-c, CJC-1295, and ipamorelin, for which no phase 3 obesity trial exists and, in some cases, no efficacy results have ever been published at all.

The difference is not a matter of degree. It is the difference between a treatment effect measured in thousands of randomized patients and an absence of data.

The four peptides with real weight-loss trials

These are the numbers as the trial publications report them, not as they get repeated.

Semaglutide 2.4 mg (STEP 1). In 1,961 adults with overweight or obesity and without diabetes, mean weight change from baseline to week 68 was −14.9% with semaglutide and −2.4% with placebo, a treatment difference of −12.4 percentage points.^[1] Half the semaglutide group (50.5%) lost 15% or more of body weight, against 4.9% on placebo.

Tirzepatide (SURMOUNT-1). In 2,539 adults without diabetes, mean percentage weight change at week 72 was −15.0% at 5 mg, −19.5% at 10 mg, and −20.9% at 15 mg, against −3.1% on placebo.^[2] At the 15 mg dose, 57% of participants lost 20% or more of body weight.

Liraglutide 3.0 mg (SCALE). The older daily GLP-1 agonist produced a mean loss of 8.4 kg at week 56 versus 2.8 kg on placebo.^[4] It is the weakest of the approved incretins by a wide margin, which matters when a clinic quotes GLP-1 results generically.

Retatrutide. In the phase 2 trial, 338 adults, mean weight change at 48 weeks was −24.2% at 12 mg versus −2.1% on placebo.^[6]Lilly’s phase 3 TRIUMPH-1 topline, announced 21 May 2026 in 2,339 participants, reported −28.3% at 12 mg versus −2.2% on placebo at 80 weeks on the efficacy estimand.^[8] That topline has not yet been published in a peer-reviewed journal, and retatrutide is not approved. Treat the phase 3 figures as company-reported until the full paper appears.

Two structural notes before those numbers get used as a ranking. First, trials report weight change under more than one estimand, and the “efficacy” figure (what happens if you take the drug as assigned) always looks better than the treatment-regimen figure (what happens to everyone randomized, including those who quit). Second, cross-trial comparison is not evidence of superiority. Our guide to comparing GLP-1 medications sets out why the league table is the wrong instrument, and the trial design walkthrough covers how one trial can legitimately publish two different headline percentages.

The one head-to-head comparison that exists

SURMOUNT-5 randomized 751 adults with obesity and without diabetes to maximum tolerated tirzepatide or maximum tolerated semaglutide for 72 weeks.^[3] Least-squares mean weight change was −20.2% with tirzepatide and −13.7% with semaglutide, with a parallel difference in waist circumference (−18.4 cm versus −13.0 cm).

The important caveat is in the design: SURMOUNT-5 was open-label and funded by the manufacturer of the winning drug. Open-label weight trials cannot rule out differential behaviour by participants who know which arm they are in. It is still the best direct comparison available, and it is the only one. Full trial-level detail for both agents sits in the STEP and SURMOUNT program pages.

What the weight numbers do not cover

Weight returns when treatment stops. The STEP 1 extension followed 327 participants for a year after withdrawal.^[7] Semaglutide participants regained 11.6 of the 17.3 percentage points they had lost, roughly two-thirds, and most cardiometabolic improvements drifted back toward baseline. Any figure quoted from a 68-week or 72-week trial describes weight on treatment, not weight after it.

Only one of these drugs has hard outcome data. SELECT randomized 17,604 patients with pre-existing cardiovascular disease and overweight or obesity, without diabetes, to semaglutide 2.4 mg or placebo.^[5] Major adverse cardiovascular events occurred in 6.5% versus 8.0%, hazard ratio 0.80. That is a hard endpoint, not a surrogate. No comparable completed outcome trial exists for tirzepatide in obesity without diabetes, and none for retatrutide.

Discontinuation is not rare.Adverse events caused treatment discontinuation in 4.3% to 7.1% of tirzepatide arms in SURMOUNT-1, and in 16.6% of the semaglutide arm across SELECT’s mean 34 months of exposure, against 8.2% on placebo. Longer exposure surfaces more dropout than a 72-week efficacy trial does.

Body composition is under-reported. How much of the loss is fat and how much is lean mass is measured in substudies rather than in the primary endpoint of most pivotal trials, so the headline percentages say nothing about it directly.

The peptides marketed for weight loss without weight-loss evidence

AOD-9604. A synthetic fragment of human growth hormone, sold widely as a fat-loss peptide. The published human record is a 2013 safety review covering six placebo-controlled trials in roughly 900 adults between 2001 and 2006, including a 24-week phase 2b study in 502 obese adults at 0.25, 0.5 and 1 mg daily.^[11] That paper reports safety and tolerability indistinguishable from placebo and contains no efficacy results for weight. Secondary sources widely state that the phase 2b trial failed its weight endpoint and that development was terminated in 2007. We could not verify that specific outcome against a primary publication, because the efficacy results appear never to have been published. ClinicalTrials.gov currently returns no obesity trials of AOD-9604. The honest summary is that the human weight data for AOD-9604 is unpublished, not that it is positive.

MOTS-c. A mitochondrial-derived peptide with substantial preclinical metabolic literature. The registry currently lists one recruiting phase 2 study in adults with prediabetes and overweight, and its stated focus is insulin sensitivity rather than weight loss. There is no completed randomized weight-loss trial.

CJC-1295 and ipamorelin. Growth hormone secretagogues, usually sold as a pair. The registry lists a single phase 2 CJC-1295 study in HIV-associated visceral obesity, terminated, and no obesity trials of ipamorelin. Our pages on CJC-1295/ipamorelin mechanism and its safety profile cover what the preclinical and small human literature actually supports.

Tesamorelin. Worth separating out, because it is the one growth-hormone-axis peptide that is FDA-approved. The approval is narrow, and the label is explicit: Egrifta is indicated for reduction of excess abdominal fat in adults with HIV-associated lipodystrophy, and it is not indicated for weight loss management as it has a weight neutral effect.^[10] Visceral fat reduction and weight loss are different endpoints.

Regulatory status is part of the evidence picture

The FDA maintains a list of bulk drug substances nominated for pharmacy compounding that it considers to present significant safety risks. AOD-9604, BPC-157, MOTS-c, CJC-1295 and ipamorelin acetate all appear there.^[9] The stated concerns are consistent across entries: immunogenicity risk for certain routes, complexity of peptide-related impurities and API characterization, and, for several of them, an absence of human exposure data sufficient to judge whether the drug would cause harm. That is a regulator saying it lacks the information, which is a different claim from saying the substance is dangerous, and a very different claim from saying it works.

Vials labelled “for research use only” sit outside the approved-drug system entirely, with no requirement for the identity, purity or sterility standards that apply to a prescription product. See what “research peptides” actually means for how that designation works.