Retatrutide vs Tirzepatide: Phase 3 Data, Safety and Legal Status

Tirzepatide is an approved medicine that activates two hormone receptors, GIP and GLP-1. Retatrutide is an investigational drug that adds a third, the glucagon receptor. In phase 3 trials reported in 2025 and 2026, retatrutide produced average weight loss of up to 28.7%, but it has not yet been compared directly with tirzepatide and is not approved anywhere.^2,3

Both come from the same company, Eli Lilly. Tirzepatide is sold as Mounjaro for type 2 diabetes and Zepbound for weight management and obstructive sleep apnoea.⁸ Retatrutide has no brand name and no licence. Lilly has said it plans to file for US approval in the first quarter of 2027.⁴

That gap matters for readers. Retatrutide is widely sold online as a “research peptide”, and the FDA says it cannot legally be compounded.⁹ This page sets out what the trials actually show, where the comparison with tirzepatide holds and where it breaks down, and what is known about safety. It gives no dosing, sourcing or switching advice.

At a glance

Property	Retatrutide	Tirzepatide
Status	Investigational, not approved anywhere	FDA-approved (Mounjaro, Zepbound)
Receptors activated	GIP, GLP-1 and glucagon	GIP and GLP-1
Developer	Eli Lilly	Eli Lilly
Largest obesity trial	TRIUMPH-1: up to -28.3% at 80 weeks	SURMOUNT-1: up to -20.9% at 72 weeks
Peer-reviewed phase 3 data	TRANSCEND-T2D-1 only; TRIUMPH results topline	Multiple NEJM publications
Head-to-head trial	TRIUMPH-5 under way, no results	TRIUMPH-5 under way, no results
Dysaesthesia (skin sensation)	5.1% to 12.5% in TRIUMPH-1	0.2% to 0.4% in label trial pool
Stopped for side effects	4.1% to 11.3% in TRIUMPH-1	4.3% to 7.1% in SURMOUNT-1
Compounding	Not permitted under federal law	Shortage ended December 2024
Legal access	Clinical trials only	Prescription only

Two receptors or three: how retatrutide and tirzepatide work

Tirzepatide is a dual agonist. It is built on the GIP sequence, activates GIP and GLP-1 receptors, and has a half-life of about five days thanks to an albumin-binding fatty acid chain.⁸ Both receptors are found in brain areas that regulate appetite, and GLP-1 activity also slows stomach emptying.

Retatrutide (development code LY3437943) is a triple agonist of the GIP, GLP-1 and glucagon receptors.¹ Glucagon’s role is the novel part. It is thought to raise energy expenditure and act on fat handling in the liver, which would add to the appetite effects of the other two. How much each receptor contributes in people has not been pinned down.

Glucagon activity may also help explain some differences in side effects. In the phase 2 trial, retatrutide produced dose-dependent increases in heart rate that peaked at 24 weeks and declined afterwards.¹ For comparison, the Zepbound label reports a mean heart rate increase of 1 to 3 beats per minute.⁸

Online sellers often call retatrutide a “GLP-3”. That is marketing, not pharmacology: there is no GLP-3 hormone. Our explainer on GLP-1 vs GLP-2 vs GLP-3 unpicks the labels.

Retatrutide’s phase 3 results so far: TRIUMPH and TRANSCEND

Retatrutide moved quickly from a 338-person phase 2 trial, where the 12 mg dose produced -24.2% mean weight change at 48 weeks versus -2.1% on placebo, to a large phase 3 programme.¹ The first results are now in.

Trial	Who took part	Length	Mean weight change
TRIUMPH-42	445 adults with obesity or overweight and knee osteoarthritis	68 weeks	-26.4% (9 mg), -28.7% (12 mg) vs -2.1% placebo
TRIUMPH-13	2,339 adults with obesity or overweight plus a related condition, no diabetes	80 weeks	-19.0% (4 mg), -25.9% (9 mg), -28.3% (12 mg) vs -2.2% placebo
TRIUMPH-24	1,152 adults with type 2 diabetes and obesity or overweight	80 weeks	-12.7%, -19.1%, -20.8% vs -4.0% placebo
TRIUMPH-34	1,949 adults with severe obesity and cardiovascular disease	80 weeks	-21.6% (9 mg), -22.6% (12 mg) vs -3.2% placebo
TRANSCEND-T2D-15	537 adults with type 2 diabetes on diet and exercise	40 weeks	-11.5%, -13.9%, -15.3% vs -2.6% placebo

The TRIUMPH figures above use Lilly’s “efficacy estimand”, which estimates the effect if people stayed on treatment. On the stricter treatment-regimen estimand, TRIUMPH-1 results were -17.6%, -23.7% and -25.0% versus -3.9%.³ In a 104-week extension of participants with a BMI of 35 or more, the 12 mg group reached -30.3%.³

Weight was not the only outcome. In TRIUMPH-4, knee pain scores fell by about three-quarters on retatrutide versus 40.3% on placebo.² In TRANSCEND-T2D-1, HbA1c fell by 1.69 to 1.94 percentage points against 0.81 on placebo.⁵

Evidence quality: why the numbers are hard to compare

The honest answer to “which is stronger?” is that no trial has tested it yet. TRIUMPH-5, a double-blind phase 3 trial comparing retatrutide with tirzepatide in about 800 adults with obesity, has finished recruiting. Its primary endpoint is weight change at 80 weeks, and primary completion is estimated for November 2026.¹¹

Until then, the temptation is to line up TRIUMPH-1 against SURMOUNT-1. In SURMOUNT-1, 2,539 adults without diabetes lost -15.0%, -19.5% and -20.9% on tirzepatide 5, 10 and 15 mg over 72 weeks, on the treatment-regimen estimand.⁶ Set beside TRIUMPH-1’s -25.0% on the same estimand at 80 weeks, retatrutide looks ahead. But the trials differ in length, era, population and background care, so the gap is an estimate, not a finding.

How much weight to put on each source

• Tirzepatide: multiple large phase 3 trials published in peer-reviewed journals, including a head-to-head win over semaglutide in SURMOUNT-5, and years of post-marketing use.^6,7
• Retatrutide: one peer-reviewed phase 2 obesity trial and one peer-reviewed phase 3 diabetes trial.^1,5 TRIUMPH-1 to TRIUMPH-4 are, so far, topline results in company press releases.^2,3,4
• Funding: every trial on this page was funded by Lilly, which makes both drugs.

Press-release data are not fabricated, but they are selected. Full publications add subgroup results, adverse-event detail and independent peer review. Expect the retatrutide picture to sharpen through 2026 and 2027.

Side effects compared: GI effects, dysaesthesia and dropouts

Both drugs cause the familiar GLP-1 class effects: nausea, diarrhoea, constipation and vomiting, mostly during dose escalation. In TRIUMPH-1, nausea affected 28.6%, 38.4% and 42.4% of people on the three retatrutide doses versus 14.8% on placebo.³ In SURMOUNT-1, tirzepatide’s gastrointestinal events were also mostly mild to moderate.⁶

Dysaesthesia: the signal that stands out

Dysaesthesia means abnormal skin sensations such as tingling, burning or sensitivity to touch. In TRIUMPH-1 it was reported by 5.1%, 12.3% and 12.5% on retatrutide 4, 9 and 12 mg versus 0.9% on placebo.³ In TRIUMPH-4, the figure reached 20.9% on 12 mg.² The Zepbound label, pooling its obesity trials, reports dysaesthesia in 0.2% to 0.4% of tirzepatide users versus 0.1% on placebo.⁸ Its cause and long-term course with retatrutide are not yet described in peer-reviewed detail.

Stopping because of side effects

Discontinuation due to adverse events was 4.1%, 6.9% and 11.3% across retatrutide doses in TRIUMPH-1 versus 4.9% on placebo, and reached 18.2% on 12 mg in TRIUMPH-4.^2,3 In SURMOUNT-1, the equivalent figures for tirzepatide were 4.3%, 7.1% and 6.2% versus 2.6%.⁶ In TRIUMPH-4, rates were lower among participants with a BMI of 35 or more (8.8% and 12.1% versus 4.8% on placebo), a pattern full publications will need to explain.²

Known label warnings

Tirzepatide’s label carries a boxed warning for thyroid C-cell tumours in rodents, plus warnings on pancreatitis, gallbladder disease, kidney injury from dehydration, hypoglycaemia with insulin or sulfonylureas, and aspiration during anaesthesia. It also advises a non-oral or added barrier contraceptive for four weeks after starting and after each dose increase.⁸ Retatrutide has no approved label, so there is no equivalent regulatory summary of its risks yet.

Grey-market retatrutide and “research” tirzepatide: FDA warnings

Retatrutide’s trial headlines created a grey market before the drug was approved. Vials sold online as “research use only”, “r3ta” or “GLP-3” are not the drug studied in TRIUMPH. They are unapproved products with no verified identity, purity, sterility or dose.

The FDA’s position is plain. Retatrutide “cannot be used in compounding under federal law” and has not been shown safe or effective for any condition.⁹ Tirzepatide can no longer be mass-compounded either, because its shortage was declared resolved in December 2024. The agency has logged more than 730 adverse event reports involving compounded tirzepatide as of 31 May 2026.⁹

Warning letters show the pattern. In March 2026 the FDA told one online seller that its retatrutide and tirzepatide products were drugs intended for human use, despite “not intended for human consumption” disclaimers, because the website described appetite, insulin and fat-oxidation effects.¹⁰ A June 2026 Public Citizen investigation counted 14 FDA warning letters to retatrutide sellers since 2024. It found that eight of those companies were still selling retatrutide in May 2026, and that 14 hospitalisations linked to retatrutide had been reported to the FDA.¹²

A “research use only” label is a legal disclaimer, not a quality mark; see what “research use only” really means. We do not give sourcing, dosing or preparation guidance for any of these products. Anyone who has used one and feels unwell should seek medical care and mention it.

Legal status and approval timeline as of September 2026

Tirzepatide is FDA-approved and prescription-only in the US, and is not a controlled substance. It is licensed and prescription-only in the UK, where Mounjaro is available privately and through NHS routes set by NICE guidance, and it is a prescription drug in Canada.

Retatrutide is not approved in any country. In the US it is available legally only inside clinical trials. Lilly plans to submit a Biologics License Application to the FDA in the first quarter of 2027, so any US decision depends on that filing and the review that follows.⁴ The UK’s MHRA has said retatrutide cannot be legally supplied outside clinical trials. Health Canada has not authorised it.

State rules add a further layer in the US, because state pharmacy boards license pharmacies and oversee telehealth prescribing. None of that changes retatrutide’s status: no US state can make an unapproved drug legal to sell. Our guide to weight-loss peptides in 2026 follows the regulatory story as it moves.

Frequently asked questions

Is retatrutide stronger than tirzepatide?

Retatrutide’s phase 3 weight results are larger than tirzepatide’s in separate trials, for example -28.3% at 80 weeks in TRIUMPH-1 versus -20.9% at 72 weeks in SURMOUNT-1.^3,6 But these trials used different designs and analysis methods. The direct comparison, TRIUMPH-5, is not expected to complete until late 2026.¹¹

Is retatrutide FDA approved?

No. Retatrutide is investigational and not approved in any country. Eli Lilly has said it plans to submit a US Biologics License Application in the first quarter of 2027.⁴ Until approval, it is legally available only to people enrolled in clinical trials, and the FDA says it cannot be used in compounding.⁹

Has retatrutide been tested against tirzepatide?

Not yet with published results. TRIUMPH-5 is a phase 3, double-blind trial randomising about 800 adults with obesity to retatrutide or tirzepatide, with weight change at 80 weeks as the primary endpoint. ClinicalTrials.gov lists it as active and not recruiting, with primary completion estimated for November 2026.¹¹

What side effects does retatrutide have that tirzepatide does not?

The most notable difference so far is dysaesthesia, altered or unpleasant skin sensation. It affected up to 12.5% of retatrutide users in TRIUMPH-1 and 20.9% on the top dose in TRIUMPH-4, compared with under 0.5% in tirzepatide’s pooled label data.^2,3,8 Gastrointestinal effects are common with both.

Is “research grade” retatrutide the same as the trial drug?

No. Products sold as research peptides are not the manufacturer’s product, have no approved quality standard and have not been evaluated by the FDA. The agency has issued warning letters to sellers whose research-use disclaimers did not match how they marketed retatrutide.¹⁰ Hospitalisations linked to unapproved retatrutide have been reported.¹²

Sources

1. Jastreboff AM, Kaplan LM, Frías JP, et al; Retatrutide Phase 2 Obesity Trial Investigators. Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial. N Engl J Med. 2023;389(6):514-526. DOI: 10.1056/NEJMoa2301972. PMID: 37366315. pubmed.ncbi.nlm.nih.gov
2. Eli Lilly and Company. Lilly’s triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs along with substantial relief from osteoarthritis pain in first successful Phase 3 trial [press release, TRIUMPH-4 topline results]. 11 December 2025. investor.lilly.com
3. Eli Lilly and Company. Lilly’s triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial [press release, TRIUMPH-1 topline results]. 21 May 2026. investor.lilly.com
4. Eli Lilly and Company. Lilly’s triple agonist, retatrutide, successful in two additional Phase 3 obesity trials, delivering significant improvements in weight and A1C [press release, TRIUMPH-2 and TRIUMPH-3 topline results]. 23 July 2026. investor.lilly.com
5. Bajaj HS, Welch M, Shah P, et al. Efficacy and safety of retatrutide, a GIP, GLP-1, and glucagon receptor agonist, in people with type 2 diabetes and inadequate glycaemic control with diet and exercise (TRANSCEND-T2D-1): a double-blind, randomised, phase 3 trial. Lancet. 2026;407(10546):2402-2413. DOI: 10.1016/S0140-6736(26)00967-0. PMID: 42250575. pubmed.ncbi.nlm.nih.gov
6. Jastreboff AM, Aronne LJ, Ahmad NN, et al; SURMOUNT-1 Investigators. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387(3):205-216. DOI: 10.1056/NEJMoa2206038. PMID: 35658024. pubmed.ncbi.nlm.nih.gov
7. Aronne LJ, Horn DB, le Roux CW, et al; SURMOUNT-5 Trial Investigators. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity. N Engl J Med. 2025;393(1):26-36. DOI: 10.1056/NEJMoa2416394. PMID: 40353578. pubmed.ncbi.nlm.nih.gov
8. Eli Lilly and Company. ZEPBOUND (tirzepatide) injection: US prescribing information, including boxed warning. DailyMed, US National Library of Medicine; label version published 2 September 2026. dailymed.nlm.nih.gov
9. US Food and Drug Administration. FDA’s Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. Content current as of 1 September 2026. fda.gov
10. US Food and Drug Administration. Warning letter to Gram Peptides (MARCS-CMS 721806). 31 March 2026. fda.gov
11. ClinicalTrials.gov. A Phase 3, Randomized, Double-Blind Study to Evaluate the Efficacy and Safety of Retatrutide Compared to Tirzepatide in Adults Who Have Obesity (TRIUMPH-5). NCT06662383. Record last updated July 2026. clinicaltrials.gov
12. Public Citizen. Hospitalizations, Enforcement Gaps, Influencers Tied to Unapproved Retatrutide Products. 16 June 2026. citizen.org

Editorial note: This article is for general information only and is not medical or legal advice. Tirzepatide is a prescription medicine; use it only as prescribed and supervised by a licensed clinician. Retatrutide is investigational and not approved anywhere, and products sold as research retatrutide are not the drug studied in clinical trials. Laws vary by location and change often, so verify the rules that apply where you are. Always talk to a licensed healthcare provider before use, especially if you are under 21, pregnant or nursing, taking medication, or managing a health condition.