Key facts
- Class: synthetic 39-amino-acid peptide; agonist at the GIP, GLP-1, and glucagon receptors (development code LY3437943)
- Developer: Eli Lilly and Company
- Approval status: investigational — not approved by the FDA, EMA, MHRA, or any other regulator, anywhere
- Route in trials: once-weekly subcutaneous injection with stepwise dose escalation
- Evidence level: three published randomized phase 2 trials (obesity, type 2 diabetes, liver fat) plus company-announced phase 3 toplines not yet peer-reviewed
- Reported side effects: nausea, diarrhea, vomiting, constipation, decreased appetite, dysesthesia, transient heart-rate increase
- Legal status: not a lawful consumer product; the FDA has warned about unapproved retatrutide sold online and has stated it cannot be used in compounding
What is retatrutide?
Retatrutide (LY3437943) is a synthetic peptide built on the backbone of GIP, engineered so that a single molecule binds and activates three separate hormone receptors: glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon. It is given as a weekly subcutaneous injection in trials. Nothing else in the obesity pipeline has produced comparable average weight loss.
The naming convention gives away the design. Semaglutide is a single agonist — GLP-1 only. Tirzepatide is a dual agonist — GIP plus GLP-1. Retatrutide adds a third arm, and the "tri" in the stem name signals it. Each generation has produced a larger average weight reduction than the last, which is why retatrutide has attracted a level of public attention wildly out of proportion to its actual availability.
It is worth being precise about status. Retatrutide has three published, peer-reviewed randomized phase 2 trials behind it. It also has two positive phase 3 toplines announced by press release. It has zero regulatory approvals, zero pharmacy availability, and no published phase 3 manuscript. Everything below is separated on that basis.
How does retatrutide work?
The three receptor targets do different jobs, and the theory is that hitting all three produces effects that no single one delivers alone.
GLP-1 is the best-understood of the three. Released from intestinal L-cells after eating, it stimulates glucose-dependent insulin secretion, suppresses glucagon, slows gastric emptying, and acts in the hypothalamus and brainstem to reduce food intake — the mechanism reviewed in detail in the physiology of glucagon-like peptide 1. Every drug in the GLP-1 receptor agonist class leans on this.
GIP is the other main incretin. Its role in appetite is more contested — both agonism and antagonism have shown weight benefits in different programs — but adding GIP activity to GLP-1 activity measurably improved outcomes when tirzepatide was tested against GLP-1-only comparators.
Glucagon is the genuinely novel arm, and the counterintuitive one. Glucagon raises blood glucose, which sounds like the last thing you want in a diabetes drug. But it also increases energy expenditure and drives hepatic fat oxidation. The bet is that when glucagon's glucose-raising effect is offset by simultaneous GLP-1 and GIP action, what remains is the metabolic-rate and liver-fat benefit. The liver-fat data discussed below is the clearest signal that this bet may be paying off.
For background on why gut hormones became the dominant approach to obesity pharmacology in the first place, see the review of peptide hormones discovered in the 21st century in the regulation of appetite and metabolism, this primer on how peptide hormones move from precursor proteins to biological function, and our overview of peptides studied for weight loss.
Does retatrutide actually work for weight loss?
In controlled trials, yes — and by a margin that surprised the field.
The pivotal published evidence is the phase 2 obesity trial published in the New England Journal of Medicine in 2023. It randomized 338 adults with a BMI of 30 or higher, or 27–29.9 with a weight-related condition, to weekly retatrutide at 1, 4, 8, or 12 mg, or placebo, for 48 weeks. At 24 weeks the mean weight reduction was 7.2% on 1 mg, 12.9% on the combined 4 mg groups, and 17.3% on the combined 8 mg groups, against 1.6% on placebo. At 48 weeks the 12 mg group had lost 24.2% of baseline body weight versus 2.1% on placebo.
Two details matter more than the headline number. First, the response rates: by week 48, every single participant on 12 mg had lost at least 5% of body weight, 93% had lost at least 10%, and 83% had lost at least 15%. Non-response, a chronic problem with earlier obesity drugs, essentially disappeared at the top dose. Second, the weight-loss curve had not flattened at 48 weeks — it was still descending when the trial ended, which is why the phase 3 program was designed to run to 80 and 104 weeks.
What no retatrutide trial has yet answered is what happens after the injections stop. The relevant precedent comes from semaglutide, where the STEP 1 extension study of weight regain after withdrawal found participants regained roughly two thirds of their lost weight within a year of stopping, with cardiometabolic improvements reverting alongside it. Nothing about a triple agonist's pharmacology suggests it would behave differently. These are drugs studied as chronic therapy, not courses.
Retatrutide phase 3: the TRIUMPH program
TRIUMPH is Lilly's phase 3 program for retatrutide, spanning obesity, obesity with type 2 diabetes, knee osteoarthritis, obstructive sleep apnea, cardiovascular disease, and chronic low back pain. Two readouts have been announced so far, both by press release rather than peer-reviewed publication.
TRIUMPH-4 (announced December 2025) enrolled 445 adults with overweight or obesity plus knee osteoarthritis, randomized to 9 mg, 12 mg, or placebo for 68 weeks. Lilly reported 26.4% and 28.7% average weight reduction on the two doses against 2.1% on placebo by the efficacy estimand, alongside greater improvement in WOMAC osteoarthritis pain scores than placebo.
TRIUMPH-1 (announced May 2026) is the pivotal obesity trial: 2,339 adults without diabetes, randomized 1:1:1:1 to 4 mg, 9 mg, 12 mg, or placebo for 80 weeks, with a prespecified 104-week extension in 532 participants with a BMI of 35 or above. Reported average weight reductions at 80 weeks were 19.0%, 25.9%, and 28.3% versus 2.2% on placebo. In the extension, the 12 mg group reached 30.3% at 104 weeks.
One technical point that separates careful reading from headline reading: those figures come from the efficacy estimand, which models what happens in people who stay on treatment as assigned. The treatment-regimen estimand, which counts everyone regardless of whether they stopped, is consistently lower — 17.6%, 23.7%, and 25.0% in TRIUMPH-1. Both are legitimate. Marketing copy quotes the first; a regulator reads both. Expect the approved label, if approval comes, to reflect the more conservative number.
Retatrutide for type 2 diabetes
The phase 2 type 2 diabetes trial published in The Lancet enrolled 281 adults in the US with baseline HbA1c between 7.0% and 10.5% and BMI between 25 and 50. It compared several retatrutide dose arms against placebo and against dulaglutide 1.5 mg as an active comparator.
At 24 weeks, HbA1c fell by 0.43% in the lowest retatrutide arm up to 2.02% in the 12 mg escalation arm, versus 1.41% with dulaglutide and 0.01% with placebo. At 36 weeks, body weight fell by up to 16.94% in the 12 mg arm, versus 2.02% with dulaglutide and 3.00% with placebo. Gastrointestinal adverse events occurred in about 35% of retatrutide recipients. No severe hypoglycemia and no deaths were reported.
The glucagon-receptor arm made the diabetes result the one to watch, since glucagon agonism could in principle worsen glycemic control. It did not. The pattern instead matched what would be expected from a potent incretin agent, consistent with the class behavior described in this updated review of head-to-head GLP-1 receptor agonist studies.
Context helps here. Weight loss is reliably smaller in people with type 2 diabetes than in people without it, across every drug in this class. In SURMOUNT-2, tirzepatide once weekly for obesity in people with type 2 diabetes, top-dose weight reduction was about 15.7% at 72 weeks, against 20.9% in the non-diabetic SURMOUNT-1 population. Retatrutide's 16.94% at 36 weeks in a diabetes population should be read against that benchmark, not against its own 24.2% obesity figure.
Retatrutide and liver fat (MASLD)
Some of the most striking retatrutide data has nothing to do with the scale. A phase 2a substudy published in Nature Medicine in 2024 followed 98 participants from the obesity trial who had metabolic dysfunction-associated steatotic liver disease (MASLD), measuring liver fat by MRI.
At 24 weeks, mean relative liver-fat reduction was 42.9% on 1 mg, 57.0% on 4 mg, 81.4% on 8 mg, and 82.4% on 12 mg, against a 0.3% increase on placebo. Normal liver fat, defined as under 5%, was reached by 27%, 52%, 79%, and 86% of those groups respectively, and by nobody on placebo. Liver-fat reductions tracked with weight loss, abdominal fat loss, and improved insulin sensitivity markers.
The caveat: this was imaging in 98 people over 24 weeks. It is not biopsy-confirmed resolution of steatohepatitis, and it is not fibrosis regression. Those are the endpoints regulators care about in liver disease, and they have not been tested here. The signal is genuinely large; the evidence tier is still phase 2a.
Retatrutide side effects reported in trials
The adverse-event profile is recognizably incretin-class, with a couple of additions.
Gastrointestinal effects
Nausea, diarrhea, vomiting, constipation, and decreased appetite were the dominant adverse events across all three phase 2 trials and both phase 3 toplines. They were dose-dependent, mostly mild to moderate, and concentrated during dose escalation rather than at steady state. This is the same profile documented across the class and one of the main reasons trials use stepwise escalation schedules at all — a design constraint discussed in the literature on strategies to improve the efficiency of GLP-1 receptor agonists.
Heart rate
The phase 2 obesity trial recorded dose-dependent increases in resting heart rate. The increase peaked around week 24 and then declined toward baseline through week 48. Heart-rate elevation is a known incretin-class effect; what is not yet known for retatrutide is whether it carries any long-term cardiovascular consequence, because no completed cardiovascular outcomes trial has reported.
Dysesthesia
Both phase 3 toplines flagged dysesthesia — abnormal skin sensations such as tingling or prickling — at rates well above placebo. Lilly reported it in up to 20.9% of the 12 mg group in TRIUMPH-4 versus 0.7% on placebo, and in 5.1% to 12.5% across doses in TRIUMPH-1 versus 0.9% on placebo, described as generally mild. This is not a signal that appeared prominently with earlier incretin drugs and it will be scrutinized closely in the eventual label.
Discontinuation
In TRIUMPH-1, discontinuation due to adverse events was 4.1% on 4 mg, 6.9% on 9 mg, and 11.3% on 12 mg, versus 4.9% on placebo. In TRIUMPH-4 the figures were 12.2% on 9 mg and 18.2% on 12 mg versus 4.0% on placebo — with some discontinuations attributed to weight loss perceived as excessive. Efficacy at the top dose comes with a real tolerability cost.
Retatrutide vs tirzepatide vs semaglutide
No head-to-head trial has ever compared retatrutide with tirzepatide or semaglutide. Cross-trial comparison is the only option available, and it is unreliable — populations, durations, escalation schedules, and analysis methods all differ. The table below is a reference point, not a ranking.
| Drug | Trial | Duration | Top-dose weight change | Status |
|---|---|---|---|---|
| Retatrutide | Phase 2 obesity, n=338 | 48 weeks | −24.2% vs −2.1% placebo | Peer-reviewed |
| Retatrutide | TRIUMPH-1, n=2,339 | 80 weeks | −28.3% vs −2.2% placebo | Topline only |
| Retatrutide | TRIUMPH-1 extension (BMI 35+), n=532 | 104 weeks | −30.3% | Topline only |
| Retatrutide | TRIUMPH-4 (knee OA), n=445 | 68 weeks | −28.7% vs −2.1% placebo | Topline only |
| Tirzepatide | SURMOUNT-1, n=2,539 | 72 weeks | −20.9% vs −3.1% placebo | FDA approved |
| Tirzepatide | SURMOUNT-5 vs semaglutide, n=751 | 72 weeks | −20.2% vs −13.7% semaglutide | FDA approved |
| Semaglutide 2.4 mg | STEP 1, n=1,961 | 68 weeks | −14.9% vs −2.4% placebo | FDA approved |
Retatrutide figures are efficacy-estimand values. SURMOUNT-1 is documented in Tirzepatide Once Weekly for the Treatment of Obesity, the only approved-drug head-to-head in Tirzepatide as Compared with Semaglutide for the Treatment of Obesity, and STEP 1 in Once-Weekly Semaglutide in Adults with Overweight or Obesity. For a fuller treatment of the mechanistic differences, see our retatrutide vs tirzepatide comparison.
The single most important line in that table is the right-hand column. Two of these drugs can be prescribed by a doctor and dispensed by a pharmacy under a label the FDA reviewed. One cannot.
Is retatrutide FDA approved?
No. As of July 2026 retatrutide holds no marketing authorization from the FDA, the EMA, the MHRA, or any other regulator in the world. There is no approved label, no approved indication, no approved dose, and no pharmacy that can lawfully dispense it. The only lawful way to receive retatrutide is as an enrolled participant in a Lilly clinical trial.
The remaining path is long even in the best case. Lilly must complete the TRIUMPH readouts it has not yet reported, assemble the full safety database, file a New Drug Application, and wait through FDA review. Each step can add or subtract months, and positive topline results do not guarantee approval — regulators review the full dataset, not the press release.
Two claims circulating online are simply false. Retatrutide has not been "approved in another country" — it has not been approved anywhere. And it is not available through legitimate compounding: the FDA has stated that retatrutide cannot be used in compounding under federal law because it is not a component of any approved drug, and issued warning letters to sellers on that basis.
The grey-market retatrutide problem
Because the trial numbers are extraordinary and the drug is unobtainable, a large online market has appeared selling vials labeled "retatrutide, for research use only, not for human consumption." That disclaimer is a legal shield, not a description of how the product is used, and the practical consequences are worth stating plainly.
- Nobody has verified what is in the vial. These products are not manufactured under FDA-inspected GMP conditions. Identity, purity, peptide content, and sterility are attested by the seller alone. Third-party certificates of analysis circulated by vendors are not regulatory verification and are trivially faked.
- There is no approved dose to follow. Escalation schedules in the TRIUMPH trials were designed by the sponsor, supervised by investigators, and paired with monitoring and stopping rules. None of that exists outside a trial.
- There is no adverse-event safety net. Trial participants who developed problems had a protocol, an investigator, and a route to unblinding. A person injecting an unregulated vial has none.
- No legal recourse. A product sold as "not for human consumption" carries no consumer protection when something goes wrong.
The FDA has publicly warned consumers about unapproved GLP-1 products sold outside the regulated supply chain, citing counterfeit ingredients, microbial contamination, and inaccurate concentrations. The regulatory picture around research-labeled peptides generally is covered in our guide to whether peptides are legal. The honest position on retatrutide in mid-2026 is that the clinical data is remarkable and the consumer product does not exist.
Frequently asked questions
Is retatrutide FDA approved?
No. As of July 2026 retatrutide is not approved by the FDA or by any other regulator anywhere in the world. It is an investigational drug still moving through Eli Lilly's phase 3 TRIUMPH program. The only lawful way to receive it is as an enrolled participant in one of those clinical trials.
How much weight can you lose on retatrutide?
In the published phase 2 trial, adults on the 12 mg dose lost an average of 24.2 percent of body weight at 48 weeks versus 2.1 percent on placebo. Lilly's phase 3 TRIUMPH-1 topline reported 28.3 percent at 80 weeks and 30.3 percent at 104 weeks in a higher-BMI extension group. Individual results in trials varied widely.
Is retatrutide better than tirzepatide?
No head-to-head trial has compared them, so the honest answer is that nobody knows. Across separate trials retatrutide's average weight loss numbers are larger than tirzepatide's, but the studies used different populations, durations, and analysis methods. Tirzepatide is approved and available; retatrutide is not.
When will retatrutide be available?
There is no confirmed launch date. Two pivotal phase 3 trials, TRIUMPH-4 and TRIUMPH-1, reported positive topline results in December 2025 and May 2026, and further TRIUMPH readouts are still outstanding. A regulatory submission has to be filed, reviewed, and approved before any pharmacy can dispense it, and none of that has happened yet.
What are the most common retatrutide side effects?
Gastrointestinal effects dominate: nausea, diarrhea, vomiting, constipation, and reduced appetite. They were dose-dependent, mostly mild to moderate, and clustered around dose escalation. Trials also reported dysesthesia, an odd skin sensation, plus transient increases in resting heart rate. Discontinuation due to adverse events reached 11.3 percent on the highest dose in TRIUMPH-1.
Does retatrutide raise heart rate?
Yes, modestly. The phase 2 obesity trial recorded dose-dependent increases in resting heart rate that peaked around week 24 and then declined toward baseline over the rest of the 48-week study. This mirrors what has been seen with other incretin drugs. Long-term cardiovascular outcome data for retatrutide are not yet published.
Is it legal to buy retatrutide online?
No. Retatrutide is not an approved drug, so it cannot legally be sold as a consumer medicine, and the FDA has stated it cannot be used in compounding either. Vials marketed online as research chemicals sit entirely outside the regulated supply chain: purity, sterility, and actual peptide content are unverified by anyone.
Does retatrutide help fatty liver disease?
A phase 2a substudy in 98 people with steatotic liver disease found large reductions in liver fat by MRI at 24 weeks: 81.4 percent on 8 mg and 82.4 percent on 12 mg versus a 0.3 percent increase on placebo. Normal liver fat below 5 percent was reached by 86 percent of the 12 mg group. That is imaging, not biopsy-confirmed fibrosis improvement.
Do you regain weight after stopping retatrutide?
No withdrawal study of retatrutide has been published. With semaglutide, the closest evidence available, participants regained roughly two thirds of their lost weight within a year of stopping, and cardiometabolic improvements reverted with it. There is no reason to expect a triple agonist behaves differently once treatment ends.
Sources
Peer-reviewed literature behind this page, all indexed on PubMed. Phase 3 TRIUMPH figures are company-announced topline results from Eli Lilly press releases dated 11 December 2025 (TRIUMPH-4) and 21 May 2026 (TRIUMPH-1) and have not yet been published in a peer-reviewed journal — they are labeled as such throughout the article.
- Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial — PubMed 37366315
- Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a phase 2 trial — PubMed 37385280
- Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial — PubMed 38858523
- Tirzepatide Once Weekly for the Treatment of Obesity — PubMed 35658024
- Tirzepatide once weekly for obesity in people with type 2 diabetes — PubMed 37385275
- Tirzepatide as Compared with Semaglutide for the Treatment of Obesity — PubMed 40353578
- Once-Weekly Semaglutide in Adults with Overweight or Obesity — PubMed 33567185
- Weight regain and cardiometabolic effects after withdrawal of semaglutide — PubMed 35441470
- GLP-1 receptor agonists: an updated review of head-to-head clinical studies — PubMed 33767808
- The physiology of glucagon-like peptide 1 — PubMed 17928588
- Glucagon-Like Peptide-1 Receptor Agonists and Strategies To Improve Their Efficiency — PubMed 31050435
- The Role of Peptide Hormones Discovered in the 21st Century in the Regulation of Appetite and Metabolism — PubMed 34067710
- Understanding peptide hormones: from precursor proteins to biological function — PubMed 40234176