GLP-1 & incretins

Retatrutide (Reta)

Reviewed 22 October 2026 · How we research

Also known as: Reta, RT-3, LY3437943, GLP-3 RT

What is Retatrutide?

Retatrutide — usually shortened to “Reta” — is an investigational once-weekly peptide from Eli Lilly that activates three hormone receptors at once: GIP, GLP-1 and glucagon. Tirzepatide (Mounjaro) hits two of these; semaglutide (Wegovy) hits one. That third target is why it is often called a “GLP-3” or “triple agonist”.

It is not licensed in the UK or anywhere else. Everything known about it comes from Lilly's clinical-trial programme, which is why it draws so much search interest: the phase 2 obesity trial reported the largest average weight reduction seen for an injectable at the time.

How Retatrutide works

Each receptor contributes something different. GLP-1 activation slows gastric emptying and reduces appetite through the brainstem and hypothalamus. GIP activation appears to enhance the GLP-1 effect on appetite and improves insulin response. Glucagon receptor activation is the new ingredient: it raises energy expenditure and promotes fat oxidation in the liver — which is why liver-fat reduction was one of the striking findings in the phase 2 sub-study.

The molecule carries a fatty-acid side chain that binds albumin in the blood, slowing clearance and giving the roughly six-day half-life that allows weekly dosing in trials.

What the research shows

Phase 2 obesity trial (338 adults, 48 weeks): mean body-weight change of −24.2% on the 12 mg weekly arm versus −2.1% on placebo. Weight was still falling when the trial ended.
Phase 2 type 2 diabetes trial: HbA1c reductions of up to 2.0 percentage points alongside dose-dependent weight loss.
Liver-fat sub-study: most participants on the higher doses reached normal liver-fat levels by 48 weeks.
The phase 3 TRIUMPH programme is testing retatrutide in obesity, knee osteoarthritis, sleep apnoea and cardiovascular outcomes. Lilly reported in December 2025 that TRIUMPH-4 (obesity with knee osteoarthritis) met its endpoints, with average weight loss of around 28% at 68 weeks on the top dose.

Compare: Retatrutide vs Tirzepatide

Retatrutide side effects

The side-effect pattern in trials looked like other incretin drugs, mostly gastrointestinal and mostly during dose increases:

Nausea, diarrhoea, vomiting and constipation — the most common, and dose-related.
Higher resting heart rate, peaking around week 24 and easing later.
Reduced appetite (the intended effect) and occasional fatigue.
Skin sensations (dysaesthesia) were noted on higher doses in phase 3 reporting.

Starting at a lower dose and stepping up reduced GI events in the phase 2 trial — the 4 mg and 8 mg arms that began at 2 mg had fewer events than those that started higher. Long-term safety data will only come from the phase 3 programme.

UK legal status

Retatrutide has no UK marketing authorisation. The MHRA has not approved it, it cannot be prescribed on the NHS, and no UK pharmacy can lawfully dispense it as a medicine. Products sold online are sold as unlicensed research chemicals; supplying them for human consumption is an offence under the Human Medicines Regulations 2012.

UK sites appearing as recruitment centres for TRIUMPH trials are the only legitimate route to receiving retatrutide as a person.

More detail, including current UK research-vial prices: Retatrutide UK guide. For the doses used in published studies and the reconstitution maths: Retatrutide dosing in the literature.

Retatrutide: frequently asked questions

What is Reta?

“Reta” is the common shorthand for retatrutide, an investigational weekly injection from Eli Lilly that activates GIP, GLP-1 and glucagon receptors. It is in phase 3 trials and is not licensed.

What does retatrutide do?

In trials it reduced appetite, slowed stomach emptying and increased energy expenditure, producing average weight loss of up to 24% at 48 weeks in phase 2 and around 28% at 68 weeks in the TRIUMPH-4 phase 3 trial.

How does retatrutide differ from Mounjaro?

Mounjaro (tirzepatide) activates GIP and GLP-1 receptors. Retatrutide adds glucagon-receptor activity, which is thought to raise energy expenditure and reduce liver fat further.

How long does retatrutide stay in your system?

Its half-life is about six days, so roughly five half-lives — around a month — pass before levels are negligible after the last dose.

Is Reta safe?

Phase 2 safety looked similar to other incretin drugs, with mainly gastrointestinal side effects. Long-term safety is still being studied, and products sold online are unregulated, so their contents are not guaranteed.

Is there a retatrutide pen?

No licensed retatrutide pen exists. Anything sold as a “retatrutide pen” is an unlicensed product. See our retatrutide pen guide.

References

Jastreboff AM, et al. Triple–hormone-receptor agonist retatrutide for obesity — a phase 2 trial. N Engl J Med. 2023;389:514–526.
Rosenstock J, et al. Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled, parallel-group, phase 2 trial. Lancet. 2023;402:529–544.
Sanyal AJ, et al. Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial. Nat Med. 2024.
Eli Lilly and Company. TRIUMPH-4 topline results press release, December 2025.

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