Retatrutide vs. tirzepatide: how the two Lilly drugs compare

Both molecules are once-weekly subcutaneous peptides. Tirzepatide activates two receptors: GLP-1 and GIP. Retatrutide activates those same two plus a third, the glucagon receptor.

GLP-1 and GIP signaling drive appetite suppression, slowed gastric emptying, and improved insulin response. Glucagon-receptor activation, somewhat counterintuitively, increases resting energy expenditure and mobilizes fat from the liver. The combination appears to add weight-loss effect on top of the GLP-1/GIP base — though it is also the most likely source of retatrutide's modest heart-rate signal.

In the Phase 2 obesity trial published in NEJM in 2023, participants on the highest 12 mg dose of retatrutide lost an average of 24.2% of body weight at 48 weeks. Placebo participants lost about 2.1%. The 8 mg dose delivered roughly 22.8%. [1]

In the Phase 3 SURMOUNT-1 trial, participants on the 15 mg dose of tirzepatide lost an average of 20.9% of body weight at 72 weeks, vs. 3.1% on placebo. [2]

Two important caveats. First, retatrutide's data is from a smaller, shorter Phase 2 trial; Phase 3 readouts may regress somewhat toward typical Phase-2-to-3 attenuation. Second, the trials enrolled different populations, used different titration schedules, and reported at different time points. The 24% vs. 21% gap is real in the published literature, but it is not a head-to-head result.

The most common adverse events in both programs were gastrointestinal: nausea, diarrhea, vomiting, and constipation. These were mostly mild-to-moderate, concentrated during dose escalation, and largely abated once participants reached their maintenance dose.

Retatrutide trials also reported a modest, dose-dependent increase in resting heart rate — typically a few beats per minute. The clinical significance of this signal is still being evaluated in Phase 3. [1]

Discontinuation rates due to adverse events were broadly similar across published trials, though direct comparison is limited by the same trial-design issues noted above.

Tirzepatide is currently approved at maintenance doses of 5, 10, and 15 mg once weekly, with a four-week titration step from a 2.5 mg starting dose.

In Phase 2, retatrutide was studied across maintenance doses up to 12 mg once weekly, with monthly titration. The maintenance doses Lilly will pursue commercially are still being finalized based on the TRIUMPH Phase 3 results.

Tirzepatide was approved by the FDA in 2022 for type 2 diabetes (Mounjaro) and in 2023 for chronic weight management (Zepbound). It is also available through several telehealth platforms and, in some jurisdictions, through compounding pharmacies.

Retatrutide is not approved by any regulator anywhere. Lilly has signaled intent to file with the FDA following positive Phase 3 readouts. Independent analyst estimates put a possible US approval in late 2026 or during 2027. [4]

Products marketed online as "retatrutide for sale" or "research retatrutide" are unregulated, of unverified identity and purity, and may be illegal to import or use in many jurisdictions. They are not the same molecule that's being evaluated in clinical trials.

The Zepbound list price is currently around $1,060 per month, with Lilly's self-pay vials program available for as low as roughly $550 per month for the lowest doses. Insurance coverage for chronic-weight-management indications remains uneven across US payers.

Industry analysts generally expect Lilly to price retatrutide at a premium to Zepbound, given its larger weight-loss effect. Working assumption from sell-side coverage: list price ~$1,200 – $1,400 per month, with a similar direct-to-consumer self-pay tier roughly 30–40% below list. Real numbers will only be clear at launch.

For people who can already access tirzepatide and are getting good results, there is no clinical reason to wait for retatrutide. Both produce historically large weight loss.

For people considering long-term treatment who haven't started yet, the choice is between a drug with several years of real-world post-approval data (tirzepatide) and a drug with somewhat better trial numbers but no approval, no commercial supply, and limited long-term safety data (retatrutide).

Treatment decisions belong with a clinician who knows your medical history. This page is informational and is not medical advice.