Tirzepatide vs Retatrutide: What the Trials Show (2026)

Tirzepatide is an FDA-approved medicine that acts on two hormone receptors. Retatrutide is an investigational drug that acts on three, and it is not approved anywhere we could verify. No completed trial compares the two directly [2][11].

Key points

• Tirzepatide activates the GIP and GLP-1 receptors. Retatrutide adds the glucagon receptor [2][6].
• Tirzepatide is FDA-approved (Zepbound for weight and sleep apnea; Mounjaro for type 2 diabetes). Lilly says it plans to submit retatrutide to the FDA in the first quarter of 2027 [2][3][9].
• In separate trials, the top retatrutide arm lost 25.0% of body weight at 80 weeks and the top tirzepatide arm 20.9% at 72 weeks. These are different trials, and the numbers are not a ranking [1][6].
• A head-to-head Phase 3 trial (TRIUMPH-5, about 800 adults) is registered, with primary completion listed as November 2026 and no results posted [11].

What is the difference between tirzepatide and retatrutide?

The FDA label describes tirzepatide as a glucose-dependent insulinotropic polypeptide (GIP) receptor and glucagon-like peptide-1 (GLP-1) receptor agonist [2]. Retatrutide (code LY3437943) is described in its trial papers as an agonist of the GIP, GLP-1 and glucagon receptors [4][6]. Lilly calls it a single molecule that activates all three [9].

In plain words: both drugs mimic gut hormones that act on appetite and blood sugar, and retatrutide additionally mimics glucagon, a hormone that works on the glucagon receptor. That is the design difference. The abstracts we read do not separate how much of retatrutide’s effect comes from the third receptor, so we do not claim a mechanism for any gap in results. Both are once-weekly injections given under the skin in the trials [1][6]. Both trial programmes were funded by Eli Lilly [1][6].

Approval status (as of 9 November 2026)

Tirzepatide, United States. The Zepbound label (revised August 2026) lists two uses: reducing excess body weight and maintaining the reduction in adults with obesity, or with overweight plus a weight-related condition; and moderate to severe obstructive sleep apnea in adults with obesity [2]. The Mounjaro label (revised August 2026) lists glycemic control in type 2 diabetes in adults and in children aged 10 and older, and reducing major cardiovascular events in adults with type 2 diabetes at high risk [3]. Both are prescription products and both carry a boxed warning for thyroid C-cell tumors seen in rats [2][3]. We did not check approval dates, or the EMA, UK or German registers.

Retatrutide. Lilly’s 23 July 2026 release says it is completing the manufacturing data package and plans to submit a Biologics License Application to the FDA in the first quarter of 2027 [9]. We found no announced submission. The company’s wording throughout is “investigational” [8][9]. Our separate article on Phase 3 results and regulator statements covers the programme in full.

Is retatrutide stronger than tirzepatide? The evidence side by side

Weight change is the mean percentage change from baseline, as published, using the treatment-regimen analysis (everyone counted as randomized) where the source says so. All trials listed were funded by Lilly [1][4][5][6][7].

Trial	Population	Size	Duration	Weight-change result as published
SURMOUNT-1 (tirzepatide, Phase 3) [1]	Adults with obesity or overweight plus a complication, no diabetes	2,539	72 weeks	5, 10, 15 mg arms: -15.0%, -19.5%, -20.9%; placebo -3.1%
SURMOUNT-2 (tirzepatide, Phase 3; “Study 2” in the label) [2]	Obesity or overweight with type 2 diabetes	938	72 weeks	10 and 15 mg arms: -12.8% and -14.7%; placebo -3.2%
Retatrutide Phase 2, obesity [4]	Adults with obesity, or overweight plus a condition	338	48 weeks	4, 8, 12 mg arms (combined where split): -17.1%, -22.8%, -24.2%; placebo -2.1%
Retatrutide Phase 2, type 2 diabetes [5]	Type 2 diabetes, BMI 25 to 50	281	36 weeks	Up to about -17% at the highest arms; placebo -3.0%; dulaglutide -2.0%
TRIUMPH-1 (retatrutide, Phase 3) [6]	Obesity, no diabetes	2,339	80 weeks	4, 9, 12 mg arms: -17.6%, -23.7%, -25.0%; placebo -3.9%
TRIUMPH-2 (retatrutide, Phase 3) [7]	Obesity with type 2 diabetes	1,152	80 weeks	4, 9, 12 mg arms: -11.9%, -16.8%, -18.8%; placebo -5.1%

Lilly has also announced TRIUMPH-3 (obesity with cardiovascular disease), reporting up to 22.6% average weight loss at 80 weeks [9]. We did not open the paper, so we do not tabulate it.

Read by population, the pattern is similar for both drugs: smaller average weight change in people with type 2 diabetes than without [1][2][6][7]. Read across drugs, the retatrutide numbers are larger. They also come from longer trials (80 versus 72 weeks), different dose ranges, different dose-escalation designs, different enrolment periods and different samples. The TRIUMPH-1 abstract reports its own estimand rules, and TRIUMPH-1 also analysed subgroups with knee osteoarthritis and sleep apnea [6]. Cross-trial comparison is not like-for-like. It shows both drugs lowered weight substantially against placebo in company-funded trials; it cannot show that one is stronger.

Is there a head-to-head trial?

A completed one, no. A registered one, yes. ClinicalTrials.gov lists TRIUMPH-5 (NCT06662383): a Lilly-sponsored, randomized, double-blind Phase 3 trial of retatrutide against tirzepatide in adults with obesity, planned at 800 participants and about 89 weeks, with percent change in body weight as the primary outcome. It started on 1 November 2024, is “active, not recruiting”, lists primary completion as November 2026 (estimated), and has posted no results (record last updated July 2026, read 9 November 2026) [11].

Two other Lilly records are not head-to-head efficacy trials of these two drugs. Lilly’s 29 October 2026 earnings release reports a Phase 3 comparison of retatrutide with Ozempic (semaglutide) 2 mg in type 2 diabetes, not with tirzepatide [10]. For tirzepatide against semaglutide see our head-to-head article. SYNERGY-Outcomes (NCT07165028, 4,500 planned adults with a fatty liver disease, primary completion listed as August 2030) lists both drugs, each with its own placebo arm; we did not establish that it compares them directly [12].

Which has more side effects? Safety signals as reported

The two sets of figures below use different trials and cannot be netted off against each other.

Tirzepatide. In the label’s pooled weight-reduction trials (SURMOUNT-1 and the type 2 diabetes trial), 4.8%, 6.3% and 6.7% on the 5, 10 and 15 mg arms permanently stopped for adverse reactions, against 3.4% on placebo [2]. Most stops occurred in the first months and were gastrointestinal [2]. Common reactions in the 15 mg arm included nausea (28% versus 8%), diarrhea (23% versus 8%), vomiting (13% versus 2%) and constipation (11% versus 5%) [2]. SURMOUNT-1 reported stops of 4.3%, 7.1% and 6.2% against 2.6% [1]. The label also warns of pancreatitis, gallbladder disease, severe gastrointestinal reactions and kidney injury from volume depletion [2].

Retatrutide. Gastrointestinal events were the most common in every trial paper we read [4][5][6][7]. Phase 2 reported dose-related heart-rate increases that peaked at 24 weeks and declined [4]. In TRIUMPH-1, Lilly reports dysesthesia (altered skin sensation) in 5.1%, 12.3% and 12.5% across the three arms against 0.9% on placebo, called mostly mild to moderate, and stops because of adverse events of 4.1%, 6.9% and 11.3% against 4.9% [8]. In TRIUMPH-2 (adults with type 2 diabetes), dysesthesia affected 4% to 7% against 1%, and stops for adverse events or death were 12% on the 9 mg arm and 8% on the 12 mg arm against 5% on placebo [7]. Six deaths occurred on retatrutide and one on placebo; investigators judged all unrelated to treatment [7]. Details are in our retatrutide side-effects article.

A dysesthesia signal is reported for retatrutide and we did not find a matching figure in the tirzepatide material we read; that is a gap in what we opened, not proof of absence. Stopping rates, event definitions and follow-up differ between programmes.

Risks and unknowns

• No completed head-to-head data on efficacy or tolerability; TRIUMPH-5 has not reported [11].
• Retatrutide has no regulatory-reviewed safety label. Long-term safety, cardiovascular outcomes and pregnancy data are not established in the sources we read. We opened abstracts, not full texts or trial protocols.
• Tirzepatide has a label with a boxed warning and warnings listed above; human relevance of the rodent thyroid finding is undetermined [2].
• Trial participants were selected, followed closely and (in the Lilly programmes) managed through dose escalation. Results outside trials are not covered here.
• The phase 2 retatrutide diabetes and obesity trials were small (281 and 338 adults) [4][5].

Research-use-only products are not the trial drug

Vials sold online under “research use only” are not the product used in these trials and come with none of the trial safety data. The trials used Lilly’s manufactured drug under medical supervision [1][6]. Retatrutide has no authorization, so any retail product is unapproved; for how US regulators treat compounded and online GLP-1 products see our FDA rules explainer. A certificate of analysis speaks only to what was in one batch; see what a CoA proves and what it does not.

Research-use-only listings are on the tirzepatide and retatrutide compound pages.

Frequently asked questions

What is the difference between tirzepatide and retatrutide? Tirzepatide activates two receptors (GIP and GLP-1); retatrutide activates three (GIP, GLP-1 and glucagon). Tirzepatide is FDA-approved, while retatrutide is still in Phase 3 testing [2][6][9].

Is retatrutide stronger than tirzepatide? In separate placebo-controlled trials, retatrutide’s top arm reached -25.0% at 80 weeks and tirzepatide’s -20.9% at 72 weeks. The trials differ in design, dose arms and length, so this is not a head-to-head result [1][6].

Is retatrutide approved? Not by any regulator we could verify. Lilly says it plans a US submission in the first quarter of 2027 [9]. We did not check the EMA, MHRA or BfArM.

Is there a head-to-head trial? TRIUMPH-5 (NCT06662383) is registered with about 800 adults and no posted results; primary completion is listed as November 2026 [11].

Which has more side effects? Both cause mostly gastrointestinal effects. Retatrutide trials also report dysesthesia and, at the highest arms, higher discontinuation for adverse events. Without a direct trial, we cannot say which is more tolerable [1][2][6][7][8].

Sources

1. Jastreboff AM et al., “Tirzepatide Once Weekly for the Treatment of Obesity” (SURMOUNT-1), N Engl J Med (2022), PMID 35658024, abstract via Europe PMC. https://pubmed.ncbi.nlm.nih.gov/35658024/ (supports SURMOUNT-1 population, size, results, discontinuations, funder)
2. DailyMed (NLM), Zepbound (tirzepatide) prescribing information, revised August 2026, read 2026-11-09. https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=487cd7e7-434c-4925-99fa-aa80b1cc776b (supports indications, mechanism wording, boxed warning, pooled adverse reactions, SURMOUNT-2 results)
3. DailyMed (NLM), Mounjaro (tirzepatide) prescribing information, revised August 2026, read 2026-11-09. https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=d2d7da5d-ad07-4228-955f-cf7e355c8cc0 (supports Mounjaro indications and boxed warning)
4. Jastreboff AM et al., “Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial,” N Engl J Med (2023), PMID 37366315, abstract via Europe PMC. https://pubmed.ncbi.nlm.nih.gov/37366315/ (supports Phase 2 obesity results, heart rate, funder)
5. Rosenstock J et al., retatrutide in type 2 diabetes, Phase 2, Lancet (2023), PMID 37385280, abstract via Europe PMC. https://pubmed.ncbi.nlm.nih.gov/37385280/ (supports diabetes Phase 2 size and weight results)
6. “Retatrutide, a Triple Hormone Receptor Agonist, for Treatment of Obesity” (TRIUMPH-1), N Engl J Med (online 2026-09-29), PMID 42814954, abstract via Europe PMC. https://pubmed.ncbi.nlm.nih.gov/42814954/ (supports TRIUMPH-1 design, results, subgroups, funder)
7. “Retatrutide in adults with obesity and type 2 diabetes (TRIUMPH-2),” The Lancet (online 2026-09-29), PMID 42810372, abstract via Europe PMC. https://pubmed.ncbi.nlm.nih.gov/42810372/ (supports TRIUMPH-2 results, dysesthesia, discontinuations, deaths)
8. Eli Lilly and Company, TRIUMPH-1 topline release (2026-05-21), via PR Newswire. https://www.prnewswire.com/news-releases/lillys-triple-agonist-retatrutide-delivered-powerful-weight-loss-in-pivotal-phase-3-obesity-trial-302778859.html (supports TRIUMPH-1 dysesthesia and discontinuation figures; “investigational”)
9. Eli Lilly and Company, TRIUMPH-2 and TRIUMPH-3 topline release (2026-07-23), via PR Newswire. https://www.prnewswire.com/news-releases/lillys-triple-agonist-retatrutide-successful-in-two-additional-phase-3-obesity-trials-delivering-significant-improvements-in-weight-and-a1c-302832674.html (supports Q1 2027 BLA plan, TRIUMPH-3 figure, three-receptor description)
10. Eli Lilly and Company, third-quarter 2026 results, Exhibit 99.1 to Form 8-K (2026-10-29). https://www.sec.gov/Archives/edgar/data/59478/000005947826000104/q326lillysalesandearningsp.htm (supports retatrutide versus Ozempic 2 mg Phase 3 trial; no figures read)
11. ClinicalTrials.gov, NCT06662383, TRIUMPH-5, retatrutide versus tirzepatide (record updated 2026-07-20), read via API 2026-11-09. https://clinicaltrials.gov/study/NCT06662383
12. ClinicalTrials.gov, NCT07165028, SYNERGY-Outcomes master protocol (record updated 2026-10-08), read via API 2026-11-09. https://clinicaltrials.gov/study/NCT07165028

Not verified: tirzepatide approval dates and non-US status; EMA, MHRA and BfArM status of either drug; Lilly’s own medical-information page and any press coverage of TRIUMPH-5 (not cited); TRIUMPH-3 and TRIUMPH-4 full results; TRIUMPH-1 nausea rates; dysesthesia rates for tirzepatide.

This article is information, not medical advice.