Retatrutide: what's known so far.

What retatrutide is

Retatrutide is an investigational medicine being developed by Eli Lilly for obesity and related conditions. It is a single molecule that acts on the receptors for three hormones: GIP, GLP-1 and glucagon. That is why it is described as a triple agonist. In trials it is given as an injection under the skin once a week.

For comparison, semaglutide (Wegovy) acts on GLP-1, and tirzepatide (Mounjaro) acts on GLP-1 and GIP. Retatrutide adds glucagon, a hormone involved in how the body uses energy.

You may see retatrutide called a “GLP-3” or “reta”. These are nicknames, not scientific terms.

Is it licensed in the UK?

No. Retatrutide has not been authorised by the MHRA, the UK medicines regulator, or by any other regulator. Lilly describes it as an investigational molecule that has not been approved anywhere in the world.

Until a regulator has reviewed the evidence and granted a licence, retatrutide is available only to people taking part in Lilly's clinical trials. No pharmacy, including PPRX, can legally supply it.

In July 2026 Lilly said it plans to apply for approval in the United States in the first quarter of 2027. It has not announced a UK application or date. A UK licence, if one is granted, would follow an MHRA review, so there is no date for when retatrutide might be prescribed in the UK.

Retatrutide sold online

Retatrutide is being sold illegally online and through social media, often labelled “for research use only”. The MHRA says products sold as retatrutide outside legitimate clinical trials are being supplied illegally and could pose serious risks to public health. They have not been assessed for safety, quality or effectiveness, and there is no guarantee of what they contain.

In October 2026 the MHRA and West Midlands Police seized more than 12,000 fake retatrutide and tirzepatide pens in Birmingham. Lilly has also taken legal action against companies selling products labelled as retatrutide.

• Don't buy or use anything sold as retatrutide. A “research use only” label doesn't make it legal or safe.
• If you have used it and feel unwell, speak to a doctor or pharmacist, or call 111. Take the product or packaging with you if you can.
• You can report a suspected fake medicine or a side effect to the MHRA through the Yellow Card scheme.

What the Phase 2 trial found

The first large trial of retatrutide for obesity was published in the New England Journal of Medicine in June 2023. It included 338 adults with a BMI of 30 or more, or 27 or more with a weight-related condition, and lasted 48 weeks.

At 48 weeks, average weight loss was 24.2% at the highest dose studied (12 mg), compared with 2.1% on placebo. Lower doses produced less weight loss: 8.7% at 1 mg, 17.1% at 4 mg and 22.8% at 8 mg.

What the Phase 3 trials have reported

Lilly is now running the larger Phase 3 TRIUMPH trials. The results below are topline figures announced by Lilly. They have not yet been published in full or reviewed by a regulator, so they are early evidence rather than a description of a licensed medicine.

• TRIUMPH-1 (reported May 2026): 2,339 adults with obesity, or overweight with a weight-related condition, who did not have type 2 diabetes. Over 80 weeks, average weight loss was 19.0% at 4 mg, 25.9% at 9 mg and 28.3% at 12 mg, compared with 2.2% on placebo.
• TRIUMPH-2 (reported July 2026): adults with obesity or overweight and type 2 diabetes. Average weight loss of up to 20.8% at 80 weeks.
• TRIUMPH-3 (reported July 2026): adults with severe obesity and established cardiovascular disease. Average weight loss of up to 22.6% at 80 weeks.

Trials of different medicines are run in different people, so these figures can't be compared directly with the results for licensed treatments. Our results guide covers what the trials of licensed medicines found.

Side effects seen in trials

Because retatrutide is not licensed, there is no approved patient information leaflet and no approved dose. What is known comes from the trials.

• The most common side effects were gastrointestinal: nausea, diarrhoea, constipation and vomiting. In the Phase 2 trial they were related to the dose, mostly mild to moderate, and less common with a lower starting dose.
• Heart rate rose with higher doses in the Phase 2 trial, peaking at 24 weeks and then falling.
• In TRIUMPH-1, some people reported altered or unpleasant skin sensations (dysaesthesia), mostly mild to moderate.
• In TRIUMPH-1, 11.3% of people on the 12 mg dose stopped treatment because of side effects, compared with 4.9% on placebo.