Retatrutide vs Semaglutide: Trial Data, Safety and Legal Status

Semaglutide is an approved drug that activates one receptor, GLP-1. Retatrutide is an investigational drug that activates three: GIP, GLP-1 and glucagon. In its main phase 3 obesity trial, retatrutide produced average weight loss of up to 28.3% at 80 weeks, against 14.9% at 68 weeks for semaglutide 2.4 mg in STEP 1. No head-to-head results exist yet.^2,5

The other difference is status. Semaglutide, sold as Ozempic, Rybelsus and Wegovy, has years of use and a completed cardiovascular outcomes trial behind it.^7,8 Retatrutide is not approved in any country, and its developer, Eli Lilly, plans to file for US approval in early 2027.³

That has not stopped retatrutide being sold online as a “research peptide”. This page compares the two on the published evidence, explains why cross-trial numbers mislead, sets out the safety signals, and covers what regulators have said about grey-market supply. It gives no dosing, sourcing or switching advice.

At a glance

Property	Retatrutide	Semaglutide
Status	Investigational, not approved anywhere	Approved (Ozempic, Rybelsus, Wegovy)
Receptors activated	GIP, GLP-1 and glucagon	GLP-1
Developer	Eli Lilly	Novo Nordisk
Main obesity trial	TRIUMPH-1: up to -28.3% at 80 weeks	STEP 1: -14.9% at 68 weeks (2.4 mg)
Highest-dose data	12 mg in TRIUMPH trials	STEP UP: -18.7% at 72 weeks (7.2 mg)
Cardiovascular outcomes	No outcomes trial reported	SELECT: 20% fewer major events vs placebo
Dysaesthesia	5.1% to 12.5% in TRIUMPH-1	6% (2.4 mg) and 22% (7.2 mg) in label trial
Head-to-head trial	TRANSCEND-T2D-2 in diabetes, no results	TRANSCEND-T2D-2 in diabetes, no results
Compounding	Not permitted under federal law	US shortage ended February 2025
Legal access	Clinical trials only	Prescription only

One receptor versus three: how semaglutide and retatrutide work

Semaglutide is a GLP-1 analogue with 94% homology to the human hormone. A C18 fatty diacid chain binds it to albumin, giving an elimination half-life of about one week, so it is taken weekly by injection or daily as a tablet.⁸ It acts on appetite regions of the brain and slows stomach emptying.

Retatrutide (LY3437943) is a single peptide that activates the GIP, GLP-1 and glucagon receptors.¹ The GIP and GLP-1 components resemble tirzepatide’s dual action. Glucagon is the new element. It is thought to increase energy expenditure and shift how the liver handles fat.

The liver data are striking but early. In a phase 2a substudy of 98 people with fatty liver disease, liver fat fell by 82.4% at 24 weeks on the 12 mg dose, and 86% of that group reached normal liver fat levels.⁹ The authors linked the reductions closely to weight loss and metabolic changes. Semaglutide, by comparison, already holds an accelerated approval for MASH with moderate to advanced fibrosis, based on biopsy outcomes.⁸

Glucagon activity is not free. The phase 2 obesity trial found dose-dependent increases in heart rate with retatrutide that peaked at 24 weeks and then declined.¹ Sellers who call retatrutide a “GLP-3” are using a marketing term, not a hormone name; see GLP-1 vs GLP-2 vs GLP-3.

Weight-loss trials side by side, and why the numbers mislead

The table below lists the main obesity trials for each drug. Read it as context, not as a contest: the trials ran at different times, in different people, for different lengths and with different statistical approaches.

Trial	Who took part	Length	Mean weight change
STEP 1 (semaglutide 2.4 mg)5	1,961 adults, no diabetes	68 weeks	-14.9% vs -2.4% placebo
STEP UP (semaglutide 7.2 mg)6	1,407 adults, no diabetes	72 weeks	-18.7% vs -15.6% (2.4 mg) and -3.9% placebo
Retatrutide phase 21	338 adults with obesity or overweight	48 weeks	-24.2% (12 mg) vs -2.1% placebo
TRIUMPH-1 (retatrutide)2	2,339 adults with obesity or overweight, no diabetes	80 weeks	-19.0% (4 mg), -25.9% (9 mg), -28.3% (12 mg) vs -2.2% placebo
TRIUMPH-4 (retatrutide)4	445 adults with obesity or overweight and knee osteoarthritis	68 weeks	-26.4% (9 mg), -28.7% (12 mg) vs -2.1% placebo
TRIUMPH-3 (retatrutide)3	1,949 adults with severe obesity and cardiovascular disease	80 weeks	-21.6% (9 mg), -22.6% (12 mg) vs -3.2% placebo

The estimand matters. STEP 1 and STEP UP report effects regardless of whether people stopped treatment.^5,6 The TRIUMPH figures above use Lilly’s efficacy estimand, which assumes people stayed on the drug. On the stricter treatment-regimen estimand, TRIUMPH-1 reported -17.6%, -23.7% and -25.0% versus -3.9%.²

In people with type 2 diabetes, weight loss is smaller with every drug in this class. In TRIUMPH-2, 1,152 adults with diabetes lost -12.7% to -20.8% on retatrutide versus -4.0% on placebo over 80 weeks.³

Evidence quality: outcome data versus topline results

Semaglutide’s evidence base is among the deepest in obesity medicine. Beyond STEP 1 and STEP UP, SELECT randomised 17,604 adults with cardiovascular disease and overweight or obesity. Cardiovascular death, heart attack or stroke occurred in 6.5% on semaglutide versus 8.0% on placebo (hazard ratio 0.80) over a mean 39.8 months.⁷ That is an outcome, not a surrogate marker, and it underpins Wegovy’s cardiovascular indication.⁸

Retatrutide’s evidence is promising but younger:

• Peer-reviewed: the phase 2 obesity trial in the New England Journal of Medicine, the phase 2a liver substudy, and one phase 3 diabetes trial, TRANSCEND-T2D-1, published in The Lancet in 2026.^1,9
• Topline only: TRIUMPH-1 to TRIUMPH-4 results so far come from company press releases, without full adverse-event tables or independent peer review.^2,3,4
• No outcomes data: TRIUMPH-3 enrolled people with cardiovascular disease but reported weight, not heart attacks or strokes.³

Is there a head-to-head trial?

Not in obesity. TRANSCEND-T2D-2 is an open-label phase 3 trial comparing retatrutide with semaglutide in about 1,250 adults with type 2 diabetes on metformin, with HbA1c change at 80 weeks as the primary endpoint. ClinicalTrials.gov lists it as active, with primary completion estimated for August 2026 and no results posted.¹¹

Funding is worth stating plainly: the semaglutide trials were funded by Novo Nordisk and the retatrutide trials by Eli Lilly, commercial rivals.^1,5,7

Side effects compared, including dysaesthesia

Both drugs cause the gastrointestinal effects typical of the class: nausea, diarrhoea, constipation and vomiting, mostly during dose escalation. In STEP 1, 4.5% of semaglutide users stopped because of gastrointestinal events versus 0.8% on placebo.⁵ In TRIUMPH-1, nausea affected 28.6% to 42.4% on retatrutide versus 14.8% on placebo.²

Dysaesthesia appears with both

Dysaesthesia means altered skin sensation such as tingling, burning or sensitivity to touch. It is often presented online as a retatrutide problem, but the Wegovy label shows it with semaglutide too, rising with dose: 22% on 7.2 mg, 6% on 2.4 mg and 0% on placebo in the higher-dose trial.⁸ In TRIUMPH-1, rates were 5.1%, 12.3% and 12.5% on the three retatrutide doses versus 0.9%, and in TRIUMPH-4 they reached 20.9% on 12 mg.^2,4 What causes it, and how long it lasts, is not well described for either drug.

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 13.5% on 12 mg in TRIUMPH-3.^2,3 In SELECT, over a much longer period, 16.6% stopped semaglutide for adverse events versus 8.2% on placebo.⁷

Label warnings

Semaglutide carries a boxed warning for thyroid C-cell tumours in rodents and warnings on pancreatitis, gallbladder disease, kidney injury from dehydration, hypoglycaemia with insulin or sulfonylureas, raised heart rate and aspiration during anaesthesia.⁸ Suicidal behaviour and ideation was removed from its warnings in early 2026 after an FDA review.⁸ Retatrutide has no approved label, so there is no regulator-reviewed summary of its risks yet.

Compounded semaglutide and grey-market retatrutide: regulator warnings

Both molecules circulate outside the regulated system, but in different ways.

Compounded semaglutide was widely made during the shortage, which the FDA declared resolved in February 2025. As of 31 May 2026 the agency had received 990 adverse event reports involving compounded semaglutide, including hospitalisations tied to dosing errors. It also warns that semaglutide sodium and acetate salts are not the approved form.¹⁰

Retatrutide has no legitimate supply outside trials. The FDA states it “cannot be used in compounding under federal law” and has not been shown safe or effective for any condition. It also warns about sellers labelling products “for research purposes” or “not for human consumption” while marketing them to consumers.¹⁰ In the UK, the MHRA has said retatrutide cannot be legally supplied outside clinical trials, and its enforcement unit has seized thousands of doses of unlicensed weight-loss medicines, including retatrutide, in raids since October 2025.¹²

A vial sold as research retatrutide is not the drug studied in TRIUMPH. Its identity, purity, sterility and strength are unverified, and a certificate from the seller is not independent oversight. See what “research use only” really means and our coverage of the 503A bulks list and research peptides. We do not give sourcing, dosing or preparation guidance.

Legal status in September 2026: US, UK and Canada

United States: semaglutide is FDA-approved and prescription-only, and is not a controlled substance. Retatrutide is investigational and legally available only in clinical trials. Lilly has said it plans to submit a Biologics License Application in the first quarter of 2027.³ No state law can make an unapproved drug lawful to sell.

United Kingdom: semaglutide is a licensed prescription-only medicine, with NHS access set by NICE guidance. Retatrutide is unlicensed and cannot be legally supplied outside trials.¹²

Canada: semaglutide is a prescription drug, and generic versions were approved in 2026. Retatrutide is not authorised by Health Canada.

For the broader regulatory story, including oral GLP-1s and the end of mass compounding, see our guide to weight-loss peptides in 2026.

Frequently asked questions

Is retatrutide more effective than semaglutide?

In separate trials, retatrutide produced larger average weight loss: up to -28.3% at 80 weeks in TRIUMPH-1 versus -14.9% at 68 weeks with semaglutide 2.4 mg in STEP 1.^2,5 The trials differ in design and analysis, and semaglutide now has a 7.2 mg dose. No head-to-head obesity trial has reported, so a direct answer is not yet possible.

Is retatrutide approved like semaglutide?

No. Semaglutide is approved in the US, UK, Canada and elsewhere. Retatrutide is investigational and not approved anywhere. Eli Lilly has said it plans to submit a US Biologics License Application in the first quarter of 2027.³ The FDA says retatrutide cannot be used in compounding.¹⁰

Has retatrutide been compared directly with semaglutide?

A head-to-head trial exists in type 2 diabetes, not obesity. TRANSCEND-T2D-2 compares retatrutide with semaglutide in about 1,250 adults on metformin, measuring HbA1c at 80 weeks. It is open-label, and no results had been posted as of September 2026.¹¹

Does semaglutide cause dysaesthesia like retatrutide?

Yes, at higher doses. The Wegovy label reports dysaesthesia in 22% on the 7.2 mg dose and 6% on 2.4 mg in its higher-dose trial.⁸ Retatrutide trials reported 5.1% to 12.5% in TRIUMPH-1 and up to 20.9% in TRIUMPH-4.^2,4 Anyone noticing new skin sensations on either should tell their prescriber.

Is research retatrutide safe to use instead of semaglutide?

Products sold as research retatrutide are unapproved, unverified and outside any safety monitoring. The FDA and the UK’s MHRA have both warned against them.^10,12 They are not the drug tested in clinical trials. Any decision about weight-management medicine belongs with a licensed prescriber using approved products.

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 powerful weight loss in pivotal Phase 3 obesity trial [press release, TRIUMPH-1 topline results]. 21 May 2026. investor.lilly.com
3. 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
4. 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
5. Wilding JPH, Batterham RL, Calanna S, et al; STEP 1 Study Group. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021;384(11):989-1002. DOI: 10.1056/NEJMoa2032183. PMID: 33567185. pubmed.ncbi.nlm.nih.gov
6. Wharton S, Freitas P, Hjelmesæth J, et al. Once-weekly semaglutide 7·2 mg in adults with obesity (STEP UP): a randomised, controlled, phase 3b trial. Lancet Diabetes Endocrinol. 2025;13(11):949-963. DOI: 10.1016/S2213-8587(25)00226-8. PMID: 40961952. pubmed.ncbi.nlm.nih.gov
7. Lincoff AM, Brown-Frandsen K, Colhoun HM, et al; SELECT Trial Investigators. Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes. N Engl J Med. 2023;389(24):2221-2232. DOI: 10.1056/NEJMoa2307563. PMID: 37952131. pubmed.ncbi.nlm.nih.gov
8. Novo Nordisk. WEGOVY (semaglutide) injection and tablets: US prescribing information, including boxed warning. DailyMed, US National Library of Medicine; label version published 30 June 2026. dailymed.nlm.nih.gov
9. Sanyal AJ, Kaplan LM, Frias JP, et al. Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial. Nat Med. 2024;30(7):2037-2048. DOI: 10.1038/s41591-024-03018-2. PMID: 38858523. pubmed.ncbi.nlm.nih.gov
10. 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
11. ClinicalTrials.gov. A Phase 3, Randomized, Multicenter, Open-label Study to Investigate the Efficacy and Safety of Retatrutide Once Weekly Compared With Semaglutide Once Weekly in Adult Participants With Type 2 Diabetes (TRANSCEND-T2D-2). NCT06260722. clinicaltrials.gov
12. The Pharmacist. Warning over unlicensed weight-loss drugs being sold on social media [report including MHRA statement on retatrutide]. August 2026. thepharmacist.co.uk

Editorial note: This article is for general information only and is not medical or legal advice. Semaglutide 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.