Retatrutide clinical trials explained: what the published studies actually show

Retatrutide (LY3437943) is an investigational peptide that activates three receptors at once: GIP, GLP-1 and glucagon. In the peer-reviewed phase 2 obesity trial (Jastreboff et al., New England Journal of Medicine, 2023) the highest dose arm reported a mean 24.2% reduction in body weight at 48 weeks versus 2.1% on placebo, and in the phase 2 type 2 diabetes trial (Rosenstock et al., The Lancet, 2023) HbA1c fell by roughly 2 percentage points. The main adverse events were gastrointestinal, plus a dose-dependent rise in heart rate. The phase 3 TRIUMPH program is testing these findings at scale. As far as public regulatory information shows, retatrutide is not approved in Canada or the United States.

How does retatrutide work? The triple-agonist mechanism

Retatrutide was described in detail by Coskun and colleagues at Eli Lilly (Cell Metabolism, 2022). It is a single peptide chain built on a GIP-like backbone with a fatty diacid side chain that binds serum albumin and slows clearance. Relative to the native hormones, the paper reported that it is more potent than native GIP at the GIP receptor and less potent than native GLP-1 and glucagon at their receptors. That imbalance is deliberate.

• GLP-1 receptor: glucose-dependent insulin secretion, slowed gastric emptying, and reduced food intake through central pathways. This is the mechanism semaglutide relies on alone.
• GIP receptor: a second incretin pathway. Combined GIP and GLP-1 agonism is the tirzepatide approach.
• Glucagon receptor: the distinctive third arm. Glucagon signalling increases hepatic fat oxidation and energy expenditure. On its own it raises blood glucose, which is why it is paired with incretin activity that counteracts that effect.

The receptor-by-receptor comparison with tirzepatide is in retatrutide vs tirzepatide, with semaglutide in retatrutide vs semaglutide, and background on the class in retatrutide tri-agonist pharmacology.

Retatrutide half-life and phase 1 findings

Two phase 1 publications established the pharmacology in people. Coskun et al. (Cell Metabolism, 2022) included the first-in-human single ascending dose data, and Urva et al. (The Lancet, 2022) reported a phase 1b multiple-ascending-dose study in people with type 2 diabetes. Both reported a half-life of roughly 6 days, which supports once-weekly administration, and both found the expected dose-related gastrointestinal effects. The phase 1b study also explored dose escalation, since starting at a lower dose and stepping up is the standard way incretin trials reduce gastrointestinal events.

Retatrutide phase 2 obesity trial (Jastreboff et al., NEJM 2023)

This 48-week randomised, double-blind, placebo-controlled trial enrolled 338 adults with obesity, or overweight with at least one weight-related condition, without type 2 diabetes. Participants received placebo or once-weekly retatrutide at 1, 4, 8 or 12 mg, with some arms using dose escalation. The published mean percentage changes in body weight were:

Two details matter when reading those figures. First, weight in the higher-dose arms had not plateaued at 48 weeks, so the trial did not capture the full trajectory. Second, these are means within a phase 2 population of a few hundred people. Phase 3 trials, with thousands of participants and longer follow-up, are what establish whether a phase 2 signal holds.

A prespecified substudy of the same trial (Sanyal et al., Nature Medicine, 2024) measured liver fat by MRI in participants with metabolic dysfunction-associated steatotic liver disease. It reported large relative reductions in liver fat, with most participants in the higher-dose arms reaching normal liver fat levels by 48 weeks. That is consistent with the glucagon-receptor arm acting on hepatic lipid metabolism, although the substudy design cannot separate the glucagon effect from the effect of weight loss itself.

Retatrutide phase 2 type 2 diabetes trial (Rosenstock et al., Lancet 2023)

This 36-week trial enrolled 281 adults with type 2 diabetes and compared several retatrutide doses against placebo and against dulaglutide 1.5 mg, an approved GLP-1 receptor agonist. The primary endpoint was HbA1c at 24 weeks. At the higher retatrutide doses the reduction was about 2 percentage points, compared with essentially no change on placebo and roughly 1.4 points on dulaglutide. Body weight also fell dose-dependently, reaching approximately 17% at 36 weeks at the top dose. Including an active comparator makes this trial more informative than a placebo-only design, because it shows the third receptor adding effect beyond GLP-1 agonism alone.

Retatrutide side effects reported in trials

Across the phase 1 and phase 2 publications the safety profile resembled the incretin class, with some features attributed to glucagon-receptor activity:

• Gastrointestinal events (nausea, diarrhoea, vomiting, constipation) were the most common, mostly mild to moderate, more frequent at higher doses, and concentrated during dose escalation. In the obesity trial, starting at a lower dose reduced their frequency.
• Heart rate increased in a dose-dependent manner, peaking around week 24 and declining afterwards. Heart-rate increases are known for GLP-1 agonists, and glucagon receptor activity may contribute. Phase 3 cardiovascular data will clarify the significance.
• Discontinuation due to adverse events was more common at higher doses than on placebo.

Phase 2 trials are not large enough to detect rare events. Questions such as pancreatitis, gallbladder events, arrhythmia and long-term lean-mass effects are what phase 3 programs and post-approval surveillance are built to answer.

The TRIUMPH phase 3 program

Lilly's phase 3 obesity program for retatrutide is named TRIUMPH. According to trial registrations, it includes studies in adults with obesity or overweight, in those with type 2 diabetes, in those with established cardiovascular disease, and in those with knee osteoarthritis, plus studies with obstructive sleep apnea and other complications as outcomes. A parallel program, TRANSCEND, studies retatrutide in type 2 diabetes. Lilly has begun announcing topline results through company releases. Full peer-reviewed phase 3 publications, and then regulatory submissions, are the milestones that determine whether retatrutide becomes an approved medicine.

Is retatrutide FDA approved, or approved in Canada?

Not as far as public information shows at the time of writing. Retatrutide is investigational in both countries. Approval typically follows completion of phase 3, submission of a full data package, and regulatory review, which together take time even after positive results. For current status, Health Canada's Drug Product Database and the FDA's drug approval listings are authoritative; news articles and supplier pages are not.

This matters for research teams in a practical way. Retatrutide used in laboratory work is a research reference standard, not clinical-trial material and not a pharmaceutical product. Its value in the lab depends entirely on identity and purity, which is why the 10 mg lot has a published HPLC-UV certificate at Lab Results and a COA is available on request for the 5 mg and 20 mg vials.

What is still unknown

• Durability. What happens to weight and metabolic markers after treatment stops, and over multi-year exposure.
• Body composition. How much of the lost mass is lean tissue, and whether the glucagon arm changes that ratio compared with dual agonists.
• Cardiovascular outcomes. Whether the heart-rate signal has clinical relevance, and whether outcome benefits seen with semaglutide (SELECT trial) extend to retatrutide.
• Glucagon-specific effects on liver, amino acid metabolism and bone, which dual agonists do not engage.

For bench work with the reference standard, reconstitution math is in how to reconstitute retatrutide and in the per-vial pages at /reconstitution/retatrutide-10mg/.

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