Retatrutide: what the evidence shows

What the human research shows

The strongest weight-loss numbers ever reported for one molecule — and, so far, reported in press releases. Two phase 3 trials have read out topline; neither has been published in a journal. That is why the grade is B and not A: the size of the effect is not in doubt, the independent scrutiny is still pending.

Outcome	Grade	Effect size	Human studies	Where the evidence comes from
Body weight	BProbable	Large	1 phase 2 RCT published; 2 phase 3 RCTs topline	Phase 2: −24.2% at 48 weeks on 12 mg. Phase 3 TRIUMPH-4: −28.7% at 68 weeks vs −2.1% placebo. TRIUMPH-1 (2,339 adults): −28.3% at 80 weeks on 12 mg, −30.3% at 104 weeks in the BMI ≥35 extension. Press releases; peer review pending.1 2 3
Knee osteoarthritis pain	BProbable	Moderate	1 phase 3 RCT topline	TRIUMPH-4 enrolled adults with obesity and knee osteoarthritis: WOMAC pain fell by up to 4.5 points (75.8%) vs 2.4 (40.3%) on placebo. Weight loss likely explains much of it; not yet peer-reviewed.2
GI side effectsadverse	BProbable	Large	Phase 2 + phase 3 topline	TRIUMPH-4: nausea 38–43% vs 11%, vomiting 20–21% vs 0%; discontinuation for adverse events 12–18% vs 4%. TRIUMPH-1: 11.3% stopped on 12 mg vs 4.9% placebo.1 2 3
Dysesthesia (skin sensation changes)adverse	BProbable	Small	Phase 3 topline	Reported in 9–21% of treated participants vs 0.7% on placebo in TRIUMPH-4 — an effect not prominent with older GLP-1 drugs.2

AMultiple good human trials, consistentBSome human trials, mostly consistentCFew or weak human studiesDAnimal or anecdotal onlyeffect size · red = adverse

Literature searched to 2 September 2026. Human studies only; animal and cell work informs the mechanism section and nothing else.

What it is

Retatrutide is a single peptide engineered to act on three hormone receptors that older drugs hit one or two at a time: GLP-1 (the semaglutide target), GIP (added in tirzepatide) and glucagon. It is given as a weekly injection and is being developed by Eli Lilly under the TRIUMPH programme.

It is the first of the so-called triple agonists to reach late-stage trials, and the reason “GLP-3” has become shorthand online. That shorthand is wrong — there is no GLP-3 hormone — but it captures the idea: a third receptor added to the pair that already works.

How it is thought to work

GLP-1 and GIP activation reduce appetite and slow stomach emptying, as with semaglutide and tirzepatide. Adding glucagon-receptor activation is thought to raise energy expenditure and increase fat breakdown in the liver, which may explain why the weight loss runs higher than the dual agonist and why liver-fat reductions have been striking in earlier trials.

The same glucagon action is the reason to watch heart rate and blood sugar closely; glucagon raises both in isolation. The trials so far report the net effect on glucose as favourable.

What is still unknown

How much of the reported effect survives peer review and full publication of the TRIUMPH programme.
Cardiovascular outcomes: no outcomes trial has reported.
What happens to weight after stopping; no withdrawal data yet.
The dysesthesia signal — what causes it and whether it resolves.

Regulatory status

European Union

Not authorised — investigational

No marketing application decided. Any product sold as retatrutide in the EU is an unlicensed medicine of unknown provenance.

United States

Not approved — in phase 3

Available only inside clinical trials. Lilly has said it expects to file for approval after the TRIUMPH programme completes.

United Kingdom

Not authorised

MHRA has warned about unlicensed weight-loss injectables sold online.

Sweden

Not authorised

Not on the Läkemedelsverket register; cannot be prescribed.

Questions people actually ask

How much weight do people lose on retatrutide?

In the phase 3 TRIUMPH-1 topline, adults on the 12 mg dose lost 28.3% of body weight on average over 80 weeks, and 30.3% at 104 weeks in a heavier subgroup. In phase 2 the figure was 24.2% at 48 weeks. These are averages from press releases; the peer-reviewed papers will give the spread.

Is retatrutide better than tirzepatide?

Probably more weight loss, on the numbers so far — but no trial has compared them head-to-head, and cross-trial comparisons are unreliable. The side-effect burden also appears higher. Treat “better” as unsettled until a direct comparison exists.

Is retatrutide available?

Not legally, anywhere. It is an investigational medicine available only in clinical trials. Products sold online under the name are unlicensed and, in enforcement seizures, frequently not what the label says.

Why is the grade B and not A?

Because the phase 3 results exist only as company press releases. LYO's A grade requires peer-reviewed publication of multiple trials. The grade will be reviewed when the TRIUMPH papers appear.

References

1
Jastreboff AM, et al. Triple-hormone-receptor agonist retatrutide for obesity — a phase 2 trial.N Engl J Med, 2023;389:514–526338 adults; −24.2% at 48 weeks on 12 mg.
2
Eli Lilly. 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 (TRIUMPH-4).Press release, 11 Dec 2025445 adults, 68 weeks. Topline; not peer-reviewed.
3
Eli Lilly. Retatrutide TRIUMPH-1 topline results (NCT05929066).Press release, May 20262,339 adults, 80 weeks plus 104-week extension. Detailed data promised for the ADA Scientific Sessions; not peer-reviewed at time of review.

Change log

2026-09-02Full page: mechanism, regulatory status and FAQ added. Phase 3 topline results (TRIUMPH-4, TRIUMPH-1) with osteoarthritis-pain and dysesthesia rows. Grade held at B pending peer-reviewed publication.
2026-09-01Initial publication.