Retatrutide explained: what is the “triple agonist” weight-loss injection?

July 28, 2026

If you follow weight-loss news, you may have noticed a new name appearing alongside Wegovy and Mounjaro: Retatrutide.

It has been called “Triple G”, a “triple agonist” and, rather inaccurately, “GLP-3”. Whatever name has reached your news feed, the excitement comes from the same place: retatrutide acts on three hormone receptors rather than one or two.

Early and late-stage trial results have certainly attracted attention. But before anyone starts mentally clearing a space in the fridge, there is something important to know:

Retatrutide is still an investigational medicine. It is not approved for public use in the UK or anywhere else at the time of writing.

So, what exactly is it - and why is everyone talking about it?

What is retatrutide?

Retatrutide is an experimental medicine being developed by pharmaceutical company Eli Lilly. In clinical trials, it is given as an injection under the skin once a week.

It belongs to a growing family of medicines designed to copy the effects of hormones involved in appetite, digestion and blood-sugar control.

What makes retatrutide unusual is that one molecule activates three different receptors:

GLP-1
GIP
Glucagon

That is why it is known as a triple hormone receptor agonist, or simply a triple agonist.

You might also see it called “GLP-3” online. It is a catchy nickname, but it is not scientifically accurate: GLP-3 is not the name of the medicine or a hormone that retatrutide copies.

How does retatrutide work?

Think of the three receptors as members of the same team, each with a slightly different job.

GLP-1: the “I’m full” signal

GLP-1 can reduce appetite and slow the speed at which food leaves the stomach. This may help someone feel satisfied after eating less and stay full for longer.

This is the main pathway targeted by semaglutide, the active ingredient in Wegovy.

GIP: the blood-sugar helper

GIP is involved in the release of insulin after eating. It may support blood-sugar control and work alongside GLP-1 to influence appetite and body weight.

Tirzepatide, the active ingredient in Mounjaro, activates both GIP and GLP-1 receptors.

Glucagon: the extra player

Glucagon helps regulate blood sugar and energy use. Researchers believe that adding activity at the glucagon receptor may increase energy expenditure and contribute to fat loss.

That does not mean retatrutide simply “burns fat”, however. Its overall effect is more complicated, and scientists are still investigating how the three pathways interact.

How much weight did people lose in the trials?

This is where most of the headlines come from.

In a peer-reviewed Phase 2 study involving adults with obesity or overweight, participants receiving the highest studied dose lost an average of 24.2% of their starting weight after 48 weeks. Individual results varied, and trial participants also received lifestyle guidance.

More recently, Eli Lilly announced results from its Phase 3 TRIUMPH-1 trial. In the company’s analysis of participants who remained on their assigned treatment, average weight loss after 80 weeks was:

19.0% with 4 mg
25.9% with 9 mg
28.3% with 12 mg
2.2% with placebo

A study extension involving a selected group of participants with a starting BMI of at least 35 reported average weight loss of up to 30.3% after 104 weeks.

Those are striking figures - but they need context.

The Phase 3 numbers were initially announced by the manufacturer. Some findings have been presented at a medical conference, while detailed results from several retatrutide trials are still due to be published in peer-reviewed journals. Trial averages also cannot predict how much weight any one person might lose.

Promising? Yes. A guaranteed result? Definitely not.

Is retatrutide better than Wegovy or Mounjaro?

It is too early to give retatrutide the winner’s trophy.

Results from separate trials may make retatrutide look more effective, but comparing headline percentages from different studies is not the same as conducting a direct comparison. Trials can differ in their participants, length, doses, lifestyle support and methods of analysing results.

Retatrutide is being compared directly with tirzepatide in the ongoing TRIUMPH-5 trial. Until suitable head-to-head evidence is available - and regulators have reviewed the complete safety and effectiveness data—we cannot say that retatrutide is definitively “better”.

There is also more to a medicine than the largest possible weight-loss figure. Tolerability, safety, health conditions, personal preferences and the ability to continue treatment all matter.

What else is retatrutide being studied for?

Obesity is the best-known area of research, but it is not the only one.

Clinical studies are investigating retatrutide in people with conditions including:

Type 2 diabetes
Knee osteoarthritis pain
Obstructive sleep apnoea
Metabolic dysfunction-associated steatotic liver disease, or MASLD
Cardiovascular and kidney disease
Chronic lower-back pain

This does not mean retatrutide has been proven or approved to treat all these conditions. It means researchers are testing whether it could be helpful.

What are the possible side effects?

As with other medicines acting on GLP-1-related pathways, the most frequently reported problems in trials have involved the digestive system.

These have included:

Nausea
Diarrhoea
Vomiting
Constipation
Reduced appetite

Side effects have generally been more common at higher doses and during dose increases. In TRIUMPH-1, the manufacturer also reported altered skin sensations - known as dysaesthesia - and urinary tract infections in some participants.

Researchers are still collecting information about retatrutide’s safety. That is one of the main reasons clinical trials and regulatory reviews cannot be skipped, no matter how encouraging the weight-loss figures appear.

Can you buy retatrutide in the UK?

No legitimate UK pharmacy can currently prescribe or sell retatrutide for public use.

It has not received marketing authorisation from the Medicines and Healthcare products Regulatory Agency (MHRA). Its future availability will depend on the completion of clinical studies and a successful regulatory review.

You may see websites or social-media accounts advertising “research-grade” retatrutide. These are not a cheeky shortcut to a future treatment. There may be no reliable way to know what such a product contains, how strong it is or whether it is contaminated.

The MHRA has taken enforcement action involving unlicensed weight-loss medicines, including products claiming to contain retatrutide. Buying an injectable substance from an unofficial seller can place your health at serious risk.

Outside an authorised clinical trial, the safest amount of retatrutide to take is none.

When will retatrutide become available?

There is no confirmed UK launch date.

Eli Lilly has now disclosed results from several Phase 3 trials, but more research and regulatory work remain. Even if retatrutide receives authorisation, that would not automatically mean it becomes immediately available through the NHS.

The bottom line

Retatrutide is a once-weekly experimental injection that activates GLP-1, GIP and glucagon receptors. Trial results suggest that it could become a significant new option for treating obesity and related health problems.

But “could” is doing important work in that sentence.

Retatrutide remains under investigation and is not currently an approved UK treatment. The sensible approach is to watch the evidence develop, avoid unlicensed products and speak to a qualified healthcare professional about weight-management treatments that are properly regulated and available today.

This article is for general information only and is not a substitute for medical advice, diagnosis or treatment. Information and regulatory status were checked in July 2026. Medical content should be reviewed by a suitably qualified UK healthcare professional before publication.