Peptides for weight loss: what the evidence shows
In 2000, a growth hormone fragment made obese mice lean without changing what they ate. Five hundred people later took it for 24 weeks, and the weight stayed on. The story of why tells you which weight-loss peptides are worth your attention.
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The answer, in short
The peptides with the strongest weight-loss evidence are the GLP-1 medicines semaglutide and tirzepatide. In trials of thousands of adults, average weight fell by about 15 to 21 percent over about 16 months. Peptides aimed at the fat cell, such as AOD-9604 and Fragment 176-191, worked in rodents, and AOD-9604 failed its human trial.
Medically reviewed by Robert Lufkin, MD (opens in a new tab)
Chief Medical Officer, Vitals Vault · Updated · 27 sources
Why the scale fights back
Most people who come looking for a weight-loss peptide have already lost weight. Usually more than once. They know the arc by heart: a strong first few months, a plateau, a hunger that seems louder than before, and a slow drift back. It is tempting to read that as a failure of character. The physiology says something more interesting.
Body weight behaves like a regulated variable. Fat tissue, the gut and the muscles report to the brain all day, through hormones released after meals and signals that scale with how much fat you carry, and the brain adjusts hunger and energy use in reply. When weight drops, appetite tends to rise to meet it. That is a defended system working as designed, and it explains why so many diets end the same way.
The clearest demonstration comes, oddly, from the drugs that work best. In the STEP 4 trial, adults who had lost about 10.6 percent of their weight on semaglutide were split into two groups. Those who kept taking it lost a further 7.9 percent over 48 weeks. Those switched to placebo regained 6.9 percent. A year after the STEP 1 trial ended, people who stopped had regained two-thirds of the weight they lost. Whatever these medicines do, the body pushes back the moment they stop.
That is the frame for everything below. A weight-loss peptide can act on one of three places: the brain’s appetite circuits, the fat cell itself, or the way muscle and mitochondria burn fuel. Only one of those places has produced large, lasting weight loss in controlled human trials.34
Three doors into the same system
The first door is appetite. GLP-1, glucagon-like peptide-1, is a hormone the gut releases after you eat. It tells the brain that food has arrived, slows how fast the stomach empties and helps the pancreas release insulin in step with blood sugar. Semaglutide is a long-lasting version of that signal. Tirzepatide acts on the GLP-1 receptor and on the receptor for a second gut hormone, GIP. Both work mainly by making people feel full sooner and hungry less often.
The second door is the fat cell. Growth hormone breaks down stored fat, and in the 1970s and 1990s researchers found that the tail end of the growth hormone molecule carried much of that fat effect. AOD-9604 and Fragment 176-191 are copies of that tail, built to keep the fat effect and drop the growth effects. In obese mice, AOD-9604 raised fat burning and cut weight without binding the growth hormone receptor.
The third door is fuel use. MOTS-c is a small peptide coded in mitochondrial DNA that switches on AMPK, the cell’s energy sensor, in muscle. 5-Amino-1MQ, often sold alongside peptides, blocks an enzyme in fat cells called NNMT and raised two energy and methylation molecules, NAD+ and SAM, in mouse fat. Both made mice on high-fat diets gain less weight. Neither has a published human treatment trial.12111418
Weight-loss peptides compared
Here is each option with the best evidence it has. The gap between the first two rows and the rest is the most important thing on this page.
| Peptide | How it works | Best evidence | Evidence level |
|---|---|---|---|
| Semaglutide | GLP-1 signal: fullness, slower stomach emptying | STEP 1: 14.9% average weight loss over 68 weeks in 1,961 adults | FDA-approved |
| Tirzepatide | GLP-1 and GIP signals | SURMOUNT-1: up to 20.9% average weight loss over 72 weeks in 2,539 adults | FDA-approved |
| AOD-9604 | Growth hormone tail; fat breakdown in rodents | 24-week trial, 502 randomized; missed its weight-loss goal | Human trial, failed |
| Fragment 176-191 | Growth hormone tail, one amino acid from AOD-9604 | Rat and tissue studies; first study raised blood sugar | Early research |
| MOTS-c | Mitochondrial peptide; switches on AMPK in muscle | Prevented diet-induced obesity in mice; exercise raises it in people | Early research |
| 5-Amino-1MQ | Small molecule (not a peptide); blocks NNMT in fat | Less weight and fat gain in obese mice | Early research |
The GLP-1 medicines: where the evidence is
In STEP 1, published in 2021, 1,961 adults with overweight or obesity and no diabetes took weekly semaglutide or placebo for 68 weeks, alongside lifestyle advice. Average weight fell 14.9 percent on semaglutide and 2.4 percent on placebo. Half the people on the drug lost 15 percent or more of their body weight, against about 1 in 20 on placebo.
SURMOUNT-1 tested tirzepatide in 2,539 adults with obesity for 72 weeks. Average weight fell 15.0 percent at the lowest dose and 20.9 percent at the highest, against 3.1 percent on placebo. More than half of the people on the two higher doses lost at least a fifth of their weight. For context, that is a range that used to belong only to surgery.
Weight is a means to an end, and the SELECT trial asked about the end. It followed 17,604 adults with heart disease and overweight but no diabetes for about three years. A heart attack, stroke or cardiovascular death occurred in 6.5 percent of people on semaglutide and 8.0 percent on placebo, a 20 percent lower relative risk.
The costs are real and mostly gastrointestinal. Nausea, diarrhea, constipation and vomiting were the common complaints, usually mild to moderate and easing with time. In SELECT, 16.6 percent of people on semaglutide stopped because of side effects, twice the placebo rate. And as STEP 4 and SURMOUNT-4 showed, the weight tends to return when the medicine stops. In SURMOUNT-4, people switched to placebo after 36 weeks on tirzepatide regained 14 percent of their weight over the next year, while those who continued lost a further 5.5 percent.12563
AOD-9604 and Fragment 176-191: the mouse that roared
The fat-cell peptides begin in 1978, when researchers gave rats single doses of pieces from the tail of human growth hormone. The 176 to 191 piece, the one sold today as Fragment 176-191, raised blood sugar briefly and insulin for longer. That is a strange first result for a product now marketed as a clean fat burner.
Through the 1990s, an Australian group refined the idea. A 177 to 191 piece blocked fat production in rat fat tissue as strongly as whole growth hormone. AOD-9604, the same tail with one swapped amino acid, slowed weight gain in obese mice from day 16 without changing their food intake, and raised fat breakdown in human fat tissue in a dish. It did not bind the growth hormone receptor. On paper, it was a growth hormone that only did the useful part.
Then came people. A 24-week phase 2b trial enrolled 534 adults and randomized 502. It missed its primary weight-loss endpoint, side effects were mild, and the developer ended the program. The trial was reported by the sponsor and never published as a full peer-reviewed paper.
One mouse study hints at why. In mice lacking beta-3 adrenergic receptors, a fat-burning pathway prominent in rodent fat, AOD-9604 had no effect on weight at all. That pathway matters far less in adult humans, which may be part of why a drug that worked so cleanly in mice did nothing measurable in people. Fragment 176-191 has no published human trials of its own, and its closest relative is the one that failed.78910111213
MOTS-c and 5-Amino-1MQ: the fuel-use bets
MOTS-c is one of the stranger discoveries of the last decade. Mitochondria, the power plants inside cells, carry their own small genome, and in 2015 researchers found a 16-amino-acid peptide hidden in one of its genes. In mice on a high-fat diet, MOTS-c activated AMPK in muscle and prevented both diet-induced obesity and insulin resistance. Later work showed it can enter the cell nucleus and switch on stress and antioxidant genes.
The human evidence is real but indirect. Exercise raises MOTS-c in human muscle and blood. In a genetic analysis of 27,527 people, a less active natural variant of MOTS-c, found in East Asian populations, was linked to type 2 diabetes in men, mainly in inactive men. That points toward MOTS-c as part of how exercise protects metabolism. No published trial has given it to people as a treatment.
5-Amino-1MQ is a small molecule, with no amino acids at all, that blocks the enzyme NNMT. In diet-induced obese mice it reduced weight and fat with no change in food intake, and in a 28-day study it improved insulin sensitivity and liver fat. The enzyme itself is a mixed story in people. Fat-tissue NNMT tracks insulin resistance and falls with exercise and bariatric surgery, yet it rose during weight loss in another study, and mice bred without it showed sex- and diet-specific results. It may be a marker as much as a cause. Most of the treatment work comes from one group and its company.1415161718192021
Freeing fat is the easy part
Look again at the AOD-9604 data. In human fat tissue in a dish, it raised fat breakdown. In people, weight did not move. Those two results fit together once you remember where released fat goes. A fat molecule pushed out of a fat cell enters the blood as fuel. If nothing else changes, the body burns it in place of the next meal’s fuel or stores it again. Mobilizing fat and losing it are different events, and the second depends on the energy balance the brain is defending.
That gives a pattern worth repeating to anyone shopping for a fat-loss peptide. Every peptide that has produced large, lasting weight loss in controlled human trials works on appetite, the brain’s side of the ledger. The peptides aimed at the fat cell or at fuel burning have worked in mice and, so far, have nothing comparable in people.
Tesamorelin sharpens the point. It is an approved growth hormone releasing factor analog, and in a 26-week trial of 412 adults with HIV it cut visceral fat, the deep fat around the organs, by 15.2 percent while it rose 5.0 percent on placebo. Its label says it is not indicated for weight loss. Growth hormone signaling can change where fat sits. Changing how much you weigh has so far required changing how much you eat.9121222
What else moves the same system, and what to measure
Exercise shows up inside this story more than once. It raises MOTS-c in human muscle, and in one study fat-tissue NNMT fell with an exercise program. Movement is one of the few levers that touches the brain’s side and the fuel-use side at the same time. During weight loss on any medicine, resistance training and enough protein are how you ask the body to give up fat and keep muscle.
Blood tests make the metabolic picture concrete. HbA1c shows average blood sugar over about three months, and fasting insulin shows insulin resistance earlier than glucose does. Triglycerides often fall early as diet and weight change. ALT, a liver enzyme, is worth a baseline because fatty liver travels with abdominal weight. In the 5-Amino-1MQ mouse work, liver fat improved while ALT and AST stayed normal, and these are the same markers a clinician would watch in a person.
This is how Vitals Vault approaches weight care. A blood panel comes first, a licensed clinician reviews your results and history, and the same markers are retested to show what changed beyond the scale.162019
Where these stand in 2026
Semaglutide and tirzepatide are FDA-approved prescription medicines for chronic weight management. AOD-9604, Fragment 176-191 and 5-Amino-1MQ are not approved, and FDA’s compounding safety page lists AOD-9604 among nominated substances with immunogenicity and impurity concerns. Since October 29, 2026, 503A pharmacies may compound MOTS-c with a prescription under enforcement discretion while a final rule is pending. MOTS-c and the growth hormone fragments are prohibited in tested sport.2324252627
What this means for you
If your goal is meaningful, lasting weight loss, the evidence points in one direction. The GLP-1 medicines have the largest trials, the largest effects and a cardiovascular outcome trial behind them, along with real side effects and a body that pushes back when they stop. The fat-cell and fuel-use peptides are interesting biology with mouse-sized evidence, and the one that reached a proper human trial failed it.
The most useful question to bring to a clinician is which side of your energy balance is fighting you. If hunger and the slow regain are the problem, an appetite-acting medicine, paired with training and protein to protect muscle, is where the human data live. Start with a blood panel, so you and your clinician can see the metabolic picture before and after, beyond the number on the scale.12512
Start with your labs
A blood test before you start and a retest after shows what actually changed for you. It is how Vitals Vault approaches every treatment.
- TestA baseline panel at Quest Diagnostics.
- DecideA licensed clinician reviews your history and results.
- RetestThe same markers again, side by side.
Questions
What is the best peptide for weight loss?
On the evidence, the GLP-1 medicines. Semaglutide produced 14.9 percent average weight loss over 68 weeks in STEP 1, and tirzepatide up to 20.9 percent over 72 weeks in SURMOUNT-1. No other peptide has comparable human data. A clinician decides whether either suits you.
Does AOD-9604 work for weight loss?
It cut weight and fat in obese rodents. In a 24-week human trial with 502 people randomized, it missed its main weight-loss goal, and the developer ended the program. The trial was sponsor-reported and not published as a full paper.
Is Fragment 176-191 the same as AOD-9604?
They differ by one amino acid at position 176. Only AOD-9604 has been through human trials. The first study of the natural fragment found it raised blood sugar and insulin in rats.
Can MOTS-c help with weight loss?
In mice on a high-fat diet, MOTS-c prevented obesity and insulin resistance. In people, exercise raises it and a less active natural variant is linked to type 2 diabetes in inactive men. No human treatment trial has been published.
Is 5-Amino-1MQ a peptide?
No. It is a small molecule that blocks the enzyme NNMT and is often sold next to peptides. It limited weight and fat gain in obese mice, mostly in studies from one group and its company. No human trials exist.
Do you regain weight after stopping a GLP-1 medicine?
Often, yes. In STEP 4, people switched from semaglutide to placebo regained 6.9 percent of their weight over 48 weeks. One year after STEP 1 ended, people had regained about two-thirds of the weight they lost.
Do weight-loss peptides cause muscle loss?
Any large weight loss includes some lean tissue. Resistance training and enough protein are the usual ways to protect muscle while losing fat, whichever approach you use.
Are weight-loss peptides safe?
The GLP-1 medicines have large trials showing mostly gastrointestinal side effects. AOD-9604 and the other fat-loss peptides have little human safety data, and vials sold as research chemicals are not made or tested for people.
Peptides, prescribed and measured
The peptides with the strongest weight-loss evidence are the GLP-1 medicines. A licensed clinician reviews your history and labs and decides whether one fits.
- Prescription only
- Licensed US clinicians
- Quest Diagnostics labs
Keep reading
Full Peptide Guide- MOTS-cA peptide coded in your mitochondrial DNA, studied for insulin sensitivity, weight and exercise capacity.
- AOD-9604A fragment of human growth hormone designed to break down fat. It reached human obesity trials but did not beat placebo.
- Fragment 176-191The last 16 amino acids of human growth hormone, studied in animals for fat metabolism. It has no human efficacy trials.
- 5-Amino-1MQA small molecule that blocks the enzyme NNMT. It is sold next to peptides but is not one. Studied in obese mice.
- TesamorelinAn FDA-approved growth hormone releasing factor analog (Egrifta) for excess abdominal fat in adults with HIV-associated lipodystrophy.
- Peptides for muscle growthGrowth hormone releasers, IGF-1 variants and myostatin blockers: what each does to muscle, and what the human trials measured.
- FDA peptide update 2026What FDA’s October 2026 decision on six peptides means for you.
Sources
- 1. Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med. 2021 (opens in a new tab)
- 2. Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med. 2022 (opens in a new tab)
- 3. Rubino D, et al. Effect of continued weekly subcutaneous semaglutide vs placebo on weight loss maintenance (STEP 4). JAMA. 2021 (opens in a new tab)
- 4. Wilding JPH, et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: the STEP 1 trial extension. Diabetes Obes Metab. 2022 (opens in a new tab)
- 5. Lincoff AM, et al. Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT). N Engl J Med. 2023 (opens in a new tab)
- 6. Aronne LJ, et al. Continued treatment with tirzepatide for maintenance of weight reduction in adults with obesity (SURMOUNT-4). JAMA. 2024 (opens in a new tab)
- 7. Ng FM, et al. Hyperglycemic action of synthetic C-terminal fragments of human growth hormone. Am J Physiol. 1978 (opens in a new tab)
- 8. Wu Z, et al. Antilipogenic action of synthetic C-terminal sequence 177-191 of human growth hormone. Biochem Mol Biol Int. 1993 (opens in a new tab)
- 9. Heffernan MA, et al. Effects of oral administration of a synthetic fragment of human growth hormone on lipid metabolism. Am J Physiol Endocrinol Metab. 2000 (opens in a new tab)
- 10. Heffernan M, et al. The effects of human GH and its lipolytic fragment (AOD9604) on lipid metabolism following chronic treatment in obese mice and beta(3)-AR knock-out mice. Endocrinology. 2001 (opens in a new tab)
- 11. Heffernan MA, et al. Increase of fat oxidation and weight loss in obese mice caused by chronic treatment with human growth hormone or a modified C-terminal fragment. Int J Obes Relat Metab Disord. 2001 (opens in a new tab)
- 12. Dominikowski A, et al. The emerging landscape of performance-enhancing peptides modulating GH-IGF1 axis. Front Endocrinol (Lausanne). 2026 (opens in a new tab)
- 13. Dwaib HS, et al. Is the beta3-adrenoceptor a valid target for the treatment of obesity and/or type 2 diabetes? Biomolecules. 2023 (opens in a new tab)
- 14. Lee C, et al. The mitochondrial-derived peptide MOTS-c promotes metabolic homeostasis and reduces obesity and insulin resistance. Cell Metab. 2015 (opens in a new tab)
- 15. Kim KH, et al. The mitochondrial-encoded peptide MOTS-c translocates to the nucleus to regulate nuclear gene expression in response to metabolic stress. Cell Metab. 2018 (opens in a new tab)
- 16. Reynolds JC, et al. MOTS-c is an exercise-induced mitochondrial-encoded regulator of age-dependent physical decline. Nat Commun. 2021 (opens in a new tab)
- 17. Zempo H, et al. A pro-diabetogenic mtDNA polymorphism in the mitochondrial-derived peptide, MOTS-c. Aging (Albany NY). 2021 (opens in a new tab)
- 18. Neelakantan H, et al. Selective and membrane-permeable small molecule inhibitors of nicotinamide N-methyltransferase reverse high fat diet-induced obesity in mice. Biochem Pharmacol. 2018 (opens in a new tab)
- 19. Babula JJ, et al. Nicotinamide N-methyltransferase inhibition mitigates obesity-related metabolic dysfunction. Diabetes Obes Metab. 2024 (opens in a new tab)
- 20. Kannt A, et al. Association of nicotinamide-N-methyltransferase mRNA expression in human adipose tissue and the plasma concentration of its product, 1-methylnicotinamide, with insulin resistance. Diabetologia. 2015 (opens in a new tab)
- 21. Brachs S, et al. Genetic nicotinamide N-methyltransferase (Nnmt) deficiency in male mice improves insulin sensitivity in diet-induced obesity but does not affect glucose tolerance. Diabetes. 2019 (opens in a new tab)
- 22. Falutz J, et al. Metabolic effects of a growth hormone-releasing factor in patients with HIV. N Engl J Med. 2007 (opens in a new tab)
- 23. Drugs@FDA. Wegovy (semaglutide) injection, NDA 215256 (opens in a new tab)
- 24. Drugs@FDA. Zepbound (tirzepatide) injection, NDA 217806 (opens in a new tab)
- 25. FDA. Certain bulk drug substances for use in compounding that may present significant safety risks (current as of April 22, 2026) (opens in a new tab)
- 26. NCPA. FDA grants enforcement discretion for compounding six peptides. October 29, 2026 (opens in a new tab)
- 27. World Anti-Doping Agency. Prohibited List (opens in a new tab)