Retatrutide keeps appearing in headlines with weight-loss numbers that make tirzepatide look modest. It is worth understanding what is actually being compared, because one of these drugs is in pharmacies and the other is in clinical trials.
The mechanism, plainly
Your gut releases hormones after you eat. These drugs mimic them.
- Tirzepatide activates two receptors: GLP-1 and GIP. It is a dual agonist, sold as Zepbound® for weight management and Mounjaro® for type 2 diabetes.
- Retatrutide activates three: GLP-1, GIP, and glucagon. Adding glucagon is the difference, and it is why the internet started calling it a “GLP-3.”
There is no hormone called GLP-3. It is a nickname, not a drug class. If a site sells you something labelled GLP-3, that tells you more about the site than the molecule.
The glucagon receptor is the interesting part. Glucagon agonism increases energy expenditure — the body burns more rather than simply wanting less. That is a genuinely different lever, and it is also why the safety profile is less predictable than the two established classes.
What the trials show
| Tirzepatide | Retatrutide | |
|---|---|---|
| Receptors | GLP-1, GIP | GLP-1, GIP, glucagon |
| Mean weight loss | ~20.9–22.5% | 24.2% (Ph2) · 28.7% (Ph3) |
| Trial duration | 72 weeks | 48–68 weeks |
| Status | Approved, on the market | Phase 3, not approved |
| Available today | Yes | No |
The phase 2 trial published in the New England Journal of Medicine reported roughly 24.2% mean body-weight loss at 48 weeks on the 12 mg dose. Eli Lilly’s TRIUMPH-4 topline, released in December 2025, reported about 28.7% — roughly 71 lbs — over 68 weeks in participants with obesity and knee osteoarthritis.
A separate phase 2 substudy reported liver fat reduction of around 82% at 24 weeks, which is the finding that has metabolic-disease researchers paying closest attention.
Mean weight loss reported in trials
Phase 2 results
Best reported phase 3
Different trials, populations and durations. No head-to-head study exists.
Read the numbers carefully. These are different trials, different populations, different endpoints. A 28.7% figure from one study and a 22.5% figure from another are not a head-to-head result. No trial has yet compared the two drugs directly.
The part that decides it
Retatrutide is not approved. You cannot get a prescription for it. It is not sold in pharmacies, and it is not eligible for compounding under current enforcement.
Eli Lilly is completing the TRIUMPH phase 3 programme through 2026, with a possible approval submission late in the year. Even on an optimistic timeline, availability is a 2027 question at the earliest.
What this means practically: if you are choosing a treatment for obesity today, there is no choice to make. Tirzepatide is available; retatrutide is not.
About buying it anyway
Retatrutide is sold online by peptide vendors labelled “for research use only.” Do not treat those as a supply route.
They are not pharmacies. The product has not been through pharmaceutical manufacturing controls. There is no clinician overseeing dose, titration, or adverse events — and glucagon agonism is precisely the mechanism where you would want that oversight. “Research use only” is a legal disclaimer, not a quality statement.
The gap between trial results and what arrives in an unmarked vial is the whole point.
Side effects
Both drugs produce the gastrointestinal effects characteristic of the class: nausea, vomiting, diarrhoea, constipation, most pronounced during dose escalation.
Reported discontinuation rates for retatrutide in trials ran 6–10%, broadly consistent with semaglutide and tirzepatide. That is reassuring as far as it goes, but trial populations are screened and monitored in ways that real-world use is not, and glucagon activity introduces variables the dual agonists do not have. Long-term data does not exist yet.
The verdict
Choose tirzepatide if you are treating obesity now. It works, it is available through a licensed prescriber, and it has years of post-marketing data.
Watch retatrutide if you are interested in where this is going. The numbers are genuinely striking and the liver-fat finding may end up mattering more than the weight-loss headline. But watching is all that is available.
If cost is what is holding you back on tirzepatide, our tirzepatide cost breakdown covers every payment route, including the refill rule that quietly doubles some prices.
Educational information, not medical advice. Treatment decisions belong with a licensed clinician.