Tirzepatide vs retatrutide: which is stronger, and can you get either?

Choose by what exists. Tirzepatide is an FDA-approved drug you can get with a prescription today. Retatrutide is an investigational drug that cannot be bought or prescribed normally; outside clinical trials there is only a narrow compassionate use program. On trial numbers, retatrutide’s top dose looks likely to take off more weight, with more stomach side effects and more people stopping. That is an estimate from separate trials, because the direct comparison has not reported. Both are once-weekly injections from Eli Lilly.

Tirzepatide vs retatrutide, side by side

All weight figures above are treatment-regimen estimates, which count everyone randomized whether or not they kept taking the drug. Lilly’s retatrutide press releases lead with a different measure, the efficacy estimand, which estimates the effect had everyone stayed on the drug and gave 28.3% for 12 mg in TRIUMPH-1. Putting that number next to tirzepatide’s 20.9% compares two different measures and overstates the gap.

Who each one suits, as things stand

Tirzepatide suits adults who want a treatment now and meet the label: obesity, or overweight with a weight-related condition, or moderate-to-severe sleep apnea with obesity for Zepbound, and type 2 diabetes for Mounjaro. It has years of real-world use, a full label and a known price. It is not for anyone with a personal or family history of medullary thyroid carcinoma or MEN 2. Those choices belong with a prescriber, and semaglutide sits on the same list of approved options.

Retatrutide suits two groups today: people who may qualify for a trial, and people following the science to judge whether waiting makes sense. A narrow compassionate use program also exists for adults with refractory obesity and serious complications who cannot join a trial, but a physician has to apply, reach is limited, and some obesity specialists want more safety data before using it.

What the comparison cannot tell you

The two obesity trials look similar, but they are not the same experiment.

• Different lengths. SURMOUNT-1 ran 72 weeks and TRIUMPH-1 ran 80. Eight extra weeks of treatment is a real difference when comparing totals.
• Different people. TRIUMPH-1’s participants started at an average weight of 112.7 kg and a BMI of 40.0, according to Lilly. Enrollment rules, countries and background care also differed between the programs.
• Different years and settings. SURMOUNT-1 was published in 2022; the TRIUMPH program began in 2023. Placebo groups behaved differently, too: placebo nausea was 14.8% in TRIUMPH-1 against 8% in the tirzepatide label trials, which suggests side effects were recorded or experienced differently.
• Different estimands in headlines. Press releases and papers often use different statistical approaches, as explained above. Mixing them inflates or shrinks the gap.
• Different time on the market. Tirzepatide’s safety record comes from a label and years of prescribing. Retatrutide’s comes from trials alone, and rare problems often appear only after a drug reaches many more people.

The fix is a direct trial. TRIUMPH-5 (NCT06662383) randomized about 800 adults with obesity to retatrutide or tirzepatide, double-blind, with percent weight change at 80 weeks as the primary outcome. ClinicalTrials.gov lists its primary completion as November 2026, and we found no results as of November 2. Retatrutide’s only reported head-to-head so far is against Ozempic 2 mg in type 2 diabetes, where 12 mg lowered A1c by 2.2 points versus 1.8 and weight by 20.9% versus 9.4% at 80 weeks (Lilly topline, efficacy estimand, open-label). That says nothing directly about tirzepatide.

Does type 2 diabetes change the picture?

Yes: both drugs take off less weight in people with type 2 diabetes. Tirzepatide 15 mg produced 14.7% in SURMOUNT-2, against 20.9% in people without diabetes. Retatrutide 12 mg produced 18.8% in TRIUMPH-2, against 25.0%. For someone with diabetes the bigger difference today is regulatory: Mounjaro is approved for it, and retatrutide is not, although Lilly’s TRANSCEND trials are testing that use.

What happens if you stop?

Tirzepatide has an answer and retatrutide does not yet. In SURMOUNT-4, adults who took tirzepatide for 36 weeks and then switched to placebo regained 14.0% over the next year, while those who stayed on it lost another 5.5%. We found no published retatrutide withdrawal result. Regain after stopping has been seen across weight-loss drug classes, so it would be surprising if retatrutide were different, but that is an expectation, not data.

How the mechanisms differ

Both drugs copy gut hormones that the body releases after eating. GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide) boost insulin after meals, slow the stomach and signal fullness to the brain. Tirzepatide activates both.

Retatrutide also activates the receptor for glucagon, a pancreatic hormone that normally raises blood sugar between meals. The phase 2 authors wrote that adding glucagon activity “may further reduce energy intake, increase energy expenditure, or both.” Glucagon and GLP-1 can also raise heart rate, which is one reason heart rate is tracked closely in the retatrutide program. The retatrutide side effects page has the phase 2 heart rate and rhythm data.

Are 12 mg of retatrutide and 15 mg of tirzepatide comparable?

No, the milligram numbers are not interchangeable. Tirzepatide’s top dose is 15 mg; retatrutide’s is 12 mg. They are different molecules with different potency at each receptor, so 12 mg of one is not “less” than 15 mg of the other. Both are stepped up gradually, and both the trials and the tirzepatide label use four-week steps, because stomach side effects cluster in the weeks after each increase. The tirzepatide dosage chart sets out the approved ladder, and retatrutide dosing sets out the trial ladders.

Availability and cost

Tirzepatide is available now as Zepbound and Mounjaro, with Zepbound also sold through Lilly’s self-pay channel. The tirzepatide cost guide covers list prices, self-pay and insurance, and the cheapest GLP-1 without insurance page compares options across drugs.

Retatrutide has no price, no brand name and no retail source. Lilly says it “cannot be legally sold or marketed for human use,” and the FDA says it cannot be used in compounding. Vials sold online as “research use only” retatrutide are not legal to use on people, whatever the label says; the FDA has warned sellers, and calls these products “of unknown quality.” The FDA status tracker will record Lilly’s filing and the FDA’s decision.

Waiting has its own trade-offs. Retatrutide has no approval date, and an approved drug can be started now and changed later if something better arrives. Another investigational option, Novo Nordisk’s amylin-based cagrilintide, combined with semaglutide as CagriSema, is further along: Novo filed for FDA approval in December 2025.