In short
Retatrutide is an investigational drug from Eli Lilly that acts on three hormones (GLP-1, GIP and glucagon). In the phase 3 TRIUMPH-1 trial, published in 2026, the highest dose produced an average weight loss of 28.3% over 80 weeks, versus 2.2% on placebo. But it is not approved in Canada: Lilly states that it is legally available only to participants in its clinical trials. Retatrutide sold online is not authorized, and Health Canada found product contaminated with bacteria in November 2026.
After semaglutide and tirzepatide, retatrutide is the molecule everyone is talking about. Its results are spectacular, and that is exactly what fuels a dangerous grey market. Here is what the scientific publications and the authorities say, with no dosages or protocols.
In this article
How does retatrutide work?
Semaglutide (Ozempic, Wegovy) mimics a single hormone, GLP-1. Tirzepatide (Mounjaro, Zepbound) mimics two: GLP-1 and GIP. Retatrutide targets three: GLP-1, GIP and glucagon, which is why it is called a “triple agonist.” Adding glucagon may increase energy expenditure on top of reducing appetite. These mechanisms explain researchers’ interest, but they also come with effects to monitor.
Trial results
| Trial | Participants | Duration | Weight loss (highest dose) | Placebo |
|---|---|---|---|---|
| Phase 2 (NEJM, 2023) | Adults with obesity | 48 weeks | Up to −24.2% | −2.1% |
| TRIUMPH-1, phase 3 (NEJM, 2026) | 2,339 adults | 80 weeks | −28.3% (efficacy estimand); −25.0% (all randomized participants) | −2.2% |
Two figures are reported for TRIUMPH-1. The first (−28.3%) estimates the effect in people who took the treatment as intended. The second (−25.0%) includes everyone, including those who stopped. The second is closer to what happens in real life.
How to read these numbers
Trial results are averages. Some people lose much more, others much less. Participants also received diet and physical activity counselling and regular medical follow-up.
It is also important not to compare different trials directly. For a sense of scale, the STEP 1 semaglutide trial reported an average loss of 14.9% at 68 weeks, versus 2.4% on placebo. But populations, durations and analysis methods differ from trial to trial: only a head-to-head study could say which drug produces more weight loss, and at what cost in side effects.
Finally, more weight loss is not automatically better. What matters for health is also what you lose (fat or muscle), how well you tolerate the treatment, and whether the benefits can be maintained over the long term.
Side effects observed
Like other drugs in this family, retatrutide mainly causes digestive side effects, which increased with dose in the phase 2 trial.
| Side effect (TRIUMPH-1) | Highest dose | Placebo |
|---|---|---|
| Nausea | 42.4% | 14.8% |
| Dysesthesia (abnormal skin sensations) | Up to 12.5% | Not specified in the press release |
| Stopped treatment due to side effects | 11.3% | 4.9% |
Dysesthesia, abnormal skin sensations such as tingling or unusual sensitivity, is a lesser-known effect reported in TRIUMPH-1. Its long-term significance remains to be clarified.
What we do not know yet
- Long-term effects beyond the length of the trials
- The share of lean mass lost compared with fat in large studies
- What happens when treatment stops
- Effects in people excluded from trials (pregnancy, certain conditions)
The muscle question matters especially: the more weight you lose, the more lean mass you lose in absolute terms. With GLP-1s already authorized, about a quarter of the weight lost is lean mass (see our article on muscle loss). The same protective principles should apply, but data specific to retatrutide remain limited.
Status in Canada
In its May 2026 press release, Lilly writes that retatrutide is an investigational molecule that is legally available only to participants in Lilly’s clinical trials. At the time of writing, it is not authorized in Canada.
Yet it circulates. Health Canada included it in the list of unauthorized seized products in its April 2026 advisory, and in November 2026 the department announced that retatrutide made by Shanghai ERP Peptide was among the products contaminated with bacteria (Acinetobacter and Cutibacterium acnes). Two Canadian retailers were shut down and more than 11,400 vials seized. The risk of serious infection, including sepsis, is real. Our guide are peptides legal in Canada? details these actions.
Why it is especially risky
A product sold online as “retatrutide” may contain another substance, an unknown dose, impurities or bacteria. Even if genuine, an investigational molecule used without medical supervision exposes you to side effects that are still poorly understood.
What if I want to join a trial?
The only legal way to access retatrutide in Canada is to take part in a clinical trial. Talk to your doctor, who can point you to ongoing studies. You can also search public clinical trial registries. Joining a trial involves close medical follow-up, informed consent and the possibility of receiving a placebo.
A legitimate trial never asks you to buy the study drug. It is approved by a research ethics board, gives you a written consent form explaining the risks and your right to withdraw at any time, and is run by an identified research team, often in a hospital or specialized clinic. If someone offers you “trial access” in exchange for payment, or ships vials to your home without a medical visit, it is not a clinical trial.
Authorized options today
If you are looking for obesity treatment, several drugs are authorized in Canada, including semaglutide (with generic versions since 2026) and tirzepatide. Read our article on generic semaglutide in Canada and discuss options with your doctor.
Questions to ask your doctor
- Am I eligible for an authorized obesity medication?
- What results and side effects can I reasonably expect?
- How can I protect my muscle during treatment?
- Is there a relevant clinical trial for me, and what would taking part involve?
- How would we decide whether, and when, to stop?
Talk to a health professional
This article summarizes clinical trial data. It is not medical advice and does not replace a consultation.
Frequently asked questions
Is retatrutide approved in Canada?
No. According to Lilly, it is an investigational molecule legally available only to participants in its clinical trials.
Is retatrutide more effective than Ozempic?
In separate trials, it produced larger average weight loss. But no head-to-head comparison is summarized here, and its long-term safety profile has yet to be established.
When will retatrutide be available?
We do not know. Lilly would have to file a submission and Health Canada would have to review it. Be wary of any offer claiming to sell it today.
Is retatrutide sold online dangerous?
It is not authorized, and Health Canada found retatrutide contaminated with bacteria in November 2026. Injecting a contaminated product can cause a serious infection.
What are the side effects of retatrutide?
In TRIUMPH-1, nausea affected 42.4% of participants on the highest dose versus 14.8% on placebo, and 11.3% stopped because of side effects. Dysesthesia was also reported.
Sources
- Jastreboff AM et al. Triple-hormone-receptor agonist retatrutide for obesity: a phase 2 trial. NEJM, 2023 (PubMed)
- TRIUMPH-1 phase 3 trial. NEJM, 2026
- Eli Lilly. Press release on TRIUMPH-1 results, May 2026
- Health Canada. Think twice before injecting peptides bought online, April 2026
- Health Canada. Contaminated, unauthorized injectable peptides, November 2026
- Wilding JPH et al. STEP 1 semaglutide trial. NEJM, 2021
- Look M et al. Body composition with tirzepatide (SURMOUNT-1). Diabetes Obes Metab, 2025 (PubMed)
This article is for information only and does not replace advice from a health professional. Talk to your doctor, pharmacist or a registered dietitian before changing your treatment, diet or exercise.
