In short
- Tirzepatide is FDA-approved as Mounjaro (2022) and Zepbound (2023). As of October 2026, retatrutide is approved nowhere; Lilly plans to submit it to the FDA in the first quarter of 2027.
- Both are once-weekly injections from Eli Lilly. Tirzepatide acts on 2 hormone receptors (GIP and GLP-1); retatrutide acts on 3, adding glucagon.
- In TRIUMPH-1, published in the New England Journal of Medicine in September 2026, 2,339 adults without diabetes lost 17.6% to 25.0% of body weight on retatrutide over 80 weeks, against 3.9% on placebo, counting everyone assigned.
- In SURMOUNT-1, adults without diabetes lost 15.0% to 20.9% on Zepbound over 72 weeks, against 3.1% on placebo. The trials differ in length and design, so this isn’t a head-to-head result.
- TRIUMPH-5, a Phase 3 trial of retatrutide against tirzepatide in about 800 adults, lists November 2026 as its estimated primary completion date. No results have been posted.
Tirzepatide is an FDA-approved medicine you can get with a prescription today, as Zepbound for weight reduction and Mounjaro for type 2 diabetes. Retatrutide is not. It’s an investigational drug from the same company, Eli Lilly, and as of October 2026 no regulator anywhere has approved it. Lilly plans to ask the FDA for approval in the first quarter of 2027.
So this comparison is between a prescription drug and trial results. Retatrutide’s trials have reported larger average weight loss than tirzepatide’s, but the two haven’t been tested against each other yet, and the trials that exist differ in ways that matter.
Tirzepatide and retatrutide at a glance
| Tirzepatide | Retatrutide | |
|---|---|---|
| Status (October 2026) | FDA-approved | Investigational; not approved anywhere |
| Maker | Eli Lilly | Eli Lilly |
| Receptors it acts on | GIP and GLP-1 | GIP, GLP-1 and glucagon |
| Brand names | Zepbound, Mounjaro | None |
| Approved for | Weight reduction and sleep apnea with obesity (Zepbound); type 2 diabetes (Mounjaro) | Nothing |
| Main obesity trial | SURMOUNT-1: 15.0% to 20.9% over 72 weeks | TRIUMPH-1: 17.6% to 25.0% over 80 weeks |
| Boxed warning | Thyroid C-cell tumors | No FDA label yet |
| Self-pay price | Zepbound $299 to $449 a month at LillyDirect with on-time refills (read Oct 26, 2026) | Not sold; can’t be legally bought |
| How to get it | Prescription from a licensed clinician | Clinical trial or Lilly’s expanded access program |
Sources: the Zepbound label, TRIUMPH-1, LillyDirect and Lilly.
How they differ
Both are once-weekly injections that mimic gut hormones. Tirzepatide acts on the GIP and GLP-1 receptors. Retatrutide adds a third, the glucagon receptor, which is why Lilly calls it a triple hormone receptor agonist. Semaglutide, the drug in Wegovy, acts on GLP-1 alone.
The trials reported here tested retatrutide at three weekly doses, 4 mg, 9 mg and 12 mg. Those are trial doses, not a prescribing plan: no approved label sets how retatrutide should be used.
Weight loss in the trials
These figures count everyone assigned to each drug, including people who stopped taking it, which is how FDA labels report results. They come from separate trials, so compare each drug with its own placebo.
Scroll sideways to see the full table
| Trial | Who took part | Drug and doses | Length | Average weight change | Placebo |
|---|---|---|---|---|---|
| SURMOUNT-1 | 2,539 adults with obesity or overweight, no diabetes | Tirzepatide 5 / 10 / 15 mg | 72 weeks | -15.0% / -19.5% / -20.9% | -3.1% |
| TRIUMPH-1 | 2,339 adults with obesity or overweight, no diabetes | Retatrutide 4 / 9 / 12 mg | 80 weeks | -17.6% / -23.7% / -25.0% | -3.9% |
| SURMOUNT-2 | 938 adults with type 2 diabetes | Tirzepatide 10 / 15 mg | 72 weeks | -12.8% / -14.7% | -3.2% |
| TRIUMPH-2 | 1,152 adults with type 2 diabetes | Retatrutide 4 / 9 / 12 mg | 80 weeks | -11.9% / -16.8% / -18.8% | -5.1% |
Sources: Zepbound label (Studies 1 and 2); TRIUMPH-1 in the New England Journal of Medicine (September 2026); TRIUMPH-2 in The Lancet (October 2026). Lilly, which makes both drugs, funded both TRIUMPH trials.
Retatrutide’s numbers are higher at the top doses, but three things keep this from being a fair race. TRIUMPH ran 8 weeks longer than SURMOUNT. The trials enrolled different volunteers. And each compared its drug with placebo, not with the other drug.
You may also see 28.3% quoted for retatrutide 12 mg. That’s from Lilly’s May 2026 announcement, which led with an efficacy estimand: an estimate of the result if everyone had stayed on treatment. The journal paper’s 25.0% is the figure to set against a drug label. Our how to get retatrutide guide uses Lilly’s figures, labeled as such.
An earlier Phase 2 study in the New England Journal of Medicine, published in 2023, enrolled 338 adults with obesity or overweight. At 48 weeks, the 12 mg group had lost 24.2% on average, against 2.1% on placebo.
The head-to-head trial: TRIUMPH-5
The answer to “which is stronger” will come from TRIUMPH-5 (NCT06662383), a Phase 3, double-blind trial randomly assigning adults with obesity to retatrutide or tirzepatide. It plans to enroll about 800 people, measures percent change in body weight at week 80, and was active but no longer recruiting when we checked on October 26, 2026. Its estimated primary completion date is November 2026, and no results have been posted. Lilly is the sponsor.
Side effects
Stomach side effects lead for both, and on retatrutide they rose with dose. Retatrutide’s rates are from Lilly’s TRIUMPH-1 announcement; tirzepatide’s are from the Zepbound label’s pool of its two main weight trials. Compare each with its own placebo.
Scroll sideways to see the full table
| Side effect | Retatrutide 4 / 9 / 12 mg | Its placebo | Zepbound 5 / 10 / 15 mg | Its placebo |
|---|---|---|---|---|
| Nausea | 28.6% / 38.4% / 42.4% | 14.8% | 25% / 29% / 28% | 8% |
| Diarrhea | 25.2% / 34.1% / 32.0% | 13.5% | 19% / 21% / 23% | 8% |
| Vomiting | 10.6% / 22.8% / 25.3% | 4.8% | 8% / 11% / 13% | 2% |
| Constipation | 23.8% / 25.9% / 26.1% | 10.9% | 17% / 14% / 11% | 5% |
| Stopped because of side effects | 4.1% / 6.9% / 11.3% | 4.9% | 4.8% / 6.3% / 6.7% | 3.4% |
Two other findings stand out. Dysesthesia, an abnormal skin sensation such as tingling or burning, affected 5.1%, 12.3% and 12.5% at the three retatrutide doses in TRIUMPH-1, against 0.9% on placebo; on the Zepbound label it affected 0.2% to 0.4%, against 0.1%. Low blood pressure affected 1% to 6% on retatrutide in TRIUMPH-2, against under 1% on placebo, and 1.6% on Zepbound in its label’s weight trials, against 0.1%. The 2023 Phase 2 retatrutide trial also saw heart rate rise with dose, peaking at 24 weeks and then declining.
Zepbound carries a boxed warning for thyroid C-cell tumors, and its label warns about pancreatitis, gallbladder disease, low blood sugar with insulin or sulfonylureas, kidney injury from dehydration and severe stomach problems. Retatrutide has no FDA label, so its full safety profile won’t be set out until the FDA reviews it. See our Zepbound side effects guide.
Can you get either one?
Tirzepatide, yes, with a prescription. Zepbound costs $299 to $449 a month self-pay at LillyDirect, as of October 2026, with $449 at 7.5 mg and up if you refill within 45 days. Our Zepbound cost guide has every dose, our Mounjaro vs Zepbound guide explains the two brands, and our Zepbound rankings compare programs that prescribe it. Compounded tirzepatide is not Zepbound or Mounjaro and isn’t FDA-approved; see compounded tirzepatide vs Zepbound.
Retatrutide, only through a clinical trial or, for some adults with severe obesity, Lilly’s expanded access program. In August 2026, Lilly said that no medicine containing retatrutide “has yet been approved for human use by any regulatory agency in the world” and that “it cannot be sold to consumers.” Lilly also cites the FDA’s position that retatrutide “cannot be lawfully compounded” and that products labeled for research use “are of unknown quality and may be harmful.” We don’t list or link to any seller of retatrutide. Our how to get retatrutide guide covers the legal routes and the expected timeline.
The bottom line
Tirzepatide is approved and available now: in SURMOUNT-1, adults lost up to 20.9% of body weight on Zepbound over 72 weeks. Retatrutide reached up to 25.0% over 80 weeks in TRIUMPH-1, counting everyone assigned, but it’s an investigational drug that can’t be prescribed or legally sold, and Lilly doesn’t plan to file for FDA approval until the first quarter of 2027. The direct comparison, TRIUMPH-5, hasn’t reported. If you want treatment now, talk to a licensed clinician about the approved options on our weight-loss hub.
Questions, answered
Is retatrutide better than tirzepatide?
Is retatrutide FDA-approved?
Can I switch from Zepbound to retatrutide?
Why do retatrutide results show two different numbers?
Sources
- Jastreboff et al., Retatrutide, a Triple Hormone Receptor Agonist, for Treatment of Obesity (TRIUMPH-1), N Engl J Med 2026
- Retatrutide in adults with obesity and type 2 diabetes (TRIUMPH-2), Lancet 2026
- Jastreboff et al., Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial, N Engl J Med 2023
- Eli Lilly: Retatrutide TRIUMPH-1 topline results (May 21, 2026)
- Eli Lilly: TRIUMPH-2 and TRIUMPH-3 results and Q1 2027 filing plan (Jul 23, 2026)
- Eli Lilly: Call to shut down the illegal retatrutide black market (Aug 12, 2026)
- ClinicalTrials.gov: Retatrutide compared with tirzepatide in adults with obesity (TRIUMPH-5), NCT06662383 (updated Jul 20, 2026)
- ClinicalTrials.gov: Pre-approval expanded access of retatrutide, NCT07629401 (posted Jun 5, 2026)
- Eli Lilly: Zepbound prescribing information (rev. 08/2026)
- Eli Lilly: Mounjaro prescribing information (rev. 08/2026)
- FDA: Zepbound approval for chronic weight management (Nov 8, 2023)
- Eli Lilly: LillyDirect Zepbound self-pay prices (read Oct 26, 2026)