Retatrutide (GLP-3): what the triple agonist is, what the trials show, and what cannot be verified

This article does not provide sourcing, dosing, reconstitution or injection instructions, and none should be inferred from it. Retatrutide (development code LY3437943) is an investigational drug from Eli Lilly that acts on three hormone receptors at once: GIP, GLP-1 and glucagon. In hybrid and endurance circles it goes by "GLP-3" or simply "Reta". As of October 2026 it is not approved by the FDA, the EMA or any other regulator, which has not stopped it from circulating widely as a "research peptide".

Three generations of incretin drugs

Incretin drugs mimic hormones the gut releases after a meal. Each generation adds a receptor target, and each has moved the weight-loss ceiling.

• GLP-1 receptor agonists (semaglutide, sold as Ozempic and Wegovy). GLP-1 slows gastric emptying, increases satiety and supports glucose-dependent insulin release. In STEP 1, adults with obesity on weekly semaglutide lost a mean 14.9% of body weight at 68 weeks, against 2.4% on placebo (Wilding et al., 2021).
• Dual GIP/GLP-1 agonists (tirzepatide, sold as Mounjaro and Zepbound). Adding the GIP receptor, a second incretin with effects on insulin secretion and fat metabolism, raised the mean loss in SURMOUNT-1 to 20.9% at the highest dose at 72 weeks, against 3.1% on placebo (Jastreboff et al., 2022).
• Triple GIP/GLP-1/glucagon agonists (retatrutide). Glucagon is usually described as the hormone that raises blood sugar. In this combination the intent is to add energy expenditure and liver-fat oxidation on top of the appetite and insulin effects of the other two receptors.

"GLP-3" is a misleading nickname. There is no third glucagon-like peptide hormone. The label is shorthand for "three receptors" or "third generation", and it is used mostly in marketing and on forums rather than in the literature.

What the trials show

Retatrutide has a full clinical program behind it, which separates it from most compounds sold as research peptides.

• Phase 2, obesity (338 adults, 48 weeks): mean weight loss of 24.2% in the highest-dose group, against 2.1% on placebo. At that dose 83% of participants lost at least 15% of their body weight (Jastreboff et al., 2023).
• Phase 2, type 2 diabetes: improvements in HbA1c and body weight against both placebo and an active comparator (Rosenstock et al., 2023).
• Phase 3, TRIUMPH-1 (2,339 adults with obesity, 80 weeks): mean weight change of -17.6% to -25.0% across the three dose arms, against -3.9% on placebo, under the intention-to-treat estimand. Knee osteoarthritis pain and sleep apnea events also fell in the trial's subgroups (Jastreboff et al., 2026).
• Phase 3, TRIUMPH-2 and TRIUMPH-3 (type 2 diabetes; severe obesity with established cardiovascular disease, both 80 weeks): mean weight loss of 12.7% to 22.6% across dose arms, against 3% to 4% on placebo, per Lilly's topline results from July 2026 (PharmExec). Full peer-reviewed publications for these two trials were not yet available at the time of writing.

Every one of those numbers comes from supervised trials: pharmaceutical-grade drug, a fixed dose-escalation schedule, and regular clinical follow-up. None of those conditions carry over to a vial bought online.

Side effects reported in trials

The adverse-event profile resembles that of the earlier incretin drugs, with one newer signal. In TRIUMPH-1, rates across the retatrutide arms against placebo were (Eli Lilly, May 2026):

• Nausea: 28.6% to 42.4%, against 14.8%
• Diarrhea: 25.2% to 34.1%, against 13.5%
• Constipation: 23.8% to 26.1%, against 10.9%
• Vomiting: 10.6% to 25.3%, against 4.8%
• Dysesthesia (abnormal skin sensations such as tingling, burning or touch sensitivity): 5.1% to 12.5%, against 0.9%. Lilly described these events as mostly mild to moderate, and most resolved during treatment.
• Discontinuation for adverse events: 4.1% to 11.3%, against 4.9%, rising with dose.

The phase 2 trials also reported a dose-related increase in resting heart rate. For athletes there is a further open question the trials were not built to answer: how much of the weight lost is lean mass, and what that means for strength, power and training capacity. Weight loss of this size without deliberate resistance training and adequate protein will include muscle.

Regulatory and anti-doping status

Retatrutide is not approved for any indication anywhere. Outside clinical trials it circulates as a freeze-dried powder labelled "for research use only, not for human consumption". That label exists precisely because the product has not cleared the review required for human use. It does not authorize self-administration, and buying, importing or possessing such products for personal use is treated differently from one jurisdiction to the next. Where this needs independent verification: the current, dated position of the regulator and customs authority in the reader's own country.

Athletes in tested sport should check the current WADA Prohibited List and their federation's rules directly. Peptide and metabolic-modulator categories change from season to season, and this article does not substitute for that check.

Why a research peptide cannot be verified by its buyer

The distinction between a trial drug and a gray-market vial is not the molecule's name on the label. It is everything the buyer cannot see.

People choosing a vendor tend to weigh what they can observe: delivery speed, how quickly messages get answered, price, how professional the website looks, and how many reviews they can find. Those are reasonable proxies for customer service. None of them measures whether a vial contains the stated compound, at the stated amount, without contaminants or degradation products. Reviews for unregulated products are easy to manufacture, and genuine ones cannot speak to purity either.

The only real check is analytical: independent testing of identity and purity, repeated for each batch, because one clean result says nothing about the next shipment. Even that addresses a single risk, product quality. It does not turn an unapproved drug into an approved one, and it does not replace medical supervision.

The product format adds practical hazards of its own. A lyophilized (freeze-dried) peptide has to be reconstituted into solution before use, and every subsequent measurement depends on getting that step right. The consumer-facing guidance online is inconsistent, equipment that looks adequate often is not suited to the volumes involved, and a calculation error translates directly into a wrong dose. This is the step where clinical oversight matters most, and it is the step most gray-market users handle alone.

How to evaluate what you read about retatrutide

Search results for retatrutide are dominated by vendors, affiliate "review" sites and pages that look like clinical resources but end in a buy button. A workable filter: if a claim cannot be traced to a peer-reviewed paper, a registered trial on ClinicalTrials.gov (TRIUMPH-1 is NCT05929066), or the manufacturer's own disclosures, treat it as marketing. Forums are useful for learning which questions people ask and unreliable as a source of answers. A medical-sounding domain name is not a credential.

What a conservative reader should take from this

Retatrutide has strong, consistent trial data and a side-effect profile that is broadly familiar from the incretin class, plus a dysesthesia signal and a heart-rate effect worth watching. It is also unapproved, and nothing bought as a research peptide comes with the dose, purity or supervision that produced those trial numbers. Semaglutide and tirzepatide are approved and can be prescribed. Anyone considering a metabolic drug should start with a physician, and anyone who competes under a testing authority should check the prohibited list first.

A first-person account of starting retatrutide, by this site's editor, is on prommer.net.

FAQ

Is retatrutide approved?

No. As of October 2026 retatrutide is an investigational drug. Eli Lilly has reported phase 3 results from its TRIUMPH program, but no regulator, including the FDA and the EMA, has approved it for any use.

What is GLP-3?

GLP-3 is a nickname for retatrutide, not the name of a hormone. It refers to the drug acting on three receptors (GIP, GLP-1 and glucagon), or to it being the third generation after GLP-1 drugs such as semaglutide and dual agonists such as tirzepatide.

Retatrutide vs tirzepatide: what is the difference?

Tirzepatide acts on two receptors (GIP and GLP-1) and is approved for obesity and type 2 diabetes. Retatrutide adds the glucagon receptor and produced larger mean weight loss in trials, but it is not approved, has a shorter safety record, and showed a dysesthesia signal that is not prominent with tirzepatide.

What are the side effects of retatrutide?

In the trials the most common side effects were gastrointestinal: nausea, diarrhea, constipation and vomiting, more frequent at higher doses and mostly mild to moderate. Dysesthesia (abnormal skin sensations), a dose-related rise in heart rate, and discontinuation for adverse events of up to about 11% at the highest dose were also reported.