Retatrutide: What It Is, Weight Loss Results, Side Effects and Availability

Medically reviewed by Dr Saira Bano, BMBS, BMedSci, MRCGP, PGCert MedEd, SCOPE-Certified Obesity Specialist

Retatrutide is one of the most-searched terms in weight loss right now, and the volume of interest has run well ahead of clear, accurate information. Headlines about trial results are easy to find; a straightforward answer to "can I actually get this" is harder to come by.

This article separates what has genuinely been shown in clinical trials from speculation about UK availability. It explains what retatrutide is, how it works, what the evidence says so far, and exactly where things stand with UK licensing today.

What is retatrutide and how does it work?

Retatrutide is an investigational medicine being studied for obesity and weight management. It is not currently an approved treatment anywhere in the world, including the UK, and remains in clinical development.

It is sometimes described as a "triple agonist" because it acts on three different hormone receptors: GLP-1, GIP, and glucagon. GLP-1 helps regulate appetite, food intake and blood glucose. GIP is also involved in appetite and glucose regulation and may complement GLP-1’s effects. Glucagon has roles in energy regulation and can influence how the body uses and stores energy.

The additional glucagon pathway is one of the reasons retatrutide has attracted so much interest: researchers are investigating whether combining all three mechanisms can produce greater effects on appetite and energy balance than medicines targeting fewer pathways. For background on how these hormone pathways work more generally, our explainer on GLP-1 weight-loss injections covers the mechanisms shared across this medicine class.

Clinical trials have reported substantial weight loss with retatrutide, but this is not proof that it will ultimately prove more effective than currently approved medicines for every patient. Trial results show how a medicine performed in a specific study population under controlled conditions; they do not mean retatrutide is currently available as a routine prescription. Researchers are continuing to study its effectiveness, safety, optimal dosing and long-term outcomes, all of which are required before regulatory approval can even be considered.

What weight loss results has retatrutide shown so far?

In the Phase 3 TRIUMPH-1 trial, adults with obesity or overweight and at least one weight-related condition, without diabetes, lost an average of around 25.9% of their body weight on the 9 mg dose and 28.3% on the 12 mg dose over 80 weeks. [1] In TRIUMPH-2, adults with obesity and type 2 diabetes lost an average of around 12.7% to 20.8% across the studied doses over the same period, alongside meaningful improvements in blood sugar control. [2]

These are headline trial results, not guaranteed outcomes. Trial populations are carefully selected and closely monitored, which differs from the general public, and individual results vary considerably. As with any investigational medicine, these figures should be read as an early indication of what retatrutide may achieve, not a promise of what any individual will experience.

How does retatrutide compare to Mounjaro and other GLP-1s?

Wegovy, Ozempic, Mounjaro and retatrutide all work through overlapping but distinct hormonal pathways:

The additional glucagon pathway is being investigated for its potential effects on energy balance, appetite and how the body uses energy, which is part of why retatrutide has generated so much interest. However, reported weight-loss results come from separate clinical trials with different populations and designs, so they should not be treated as a direct head-to-head comparison. Retatrutide has not been trialled against Mounjaro in the same study, and long-term, real-world comparisons between the two remain limited.

What side effects are associated with retatrutide?

The side-effect profile seen so far has been broadly consistent with what might be expected from medicines acting on GLP-1-related pathways, with gastrointestinal symptoms among the most commonly reported. In the trials to date, these have included:

• Nausea
• Diarrhoea
• Vomiting
• Constipation
• Reduced appetite
• Feeling full sooner than usual

These effects have generally been mild to moderate in the available trials, and have tended to occur more often during dose escalation, when the dose is being increased in steps. As with any newer medicine still in development, longer-term safety data continues to be gathered, and a fuller picture will emerge as more trials complete and results are published in peer-reviewed journals.

Is retatrutide available in the UK?

No. Retatrutide is not currently licensed or available for prescription in the UK, or anywhere else in the world. It remains an investigational medicine.

Before any new weight loss medication becomes available, it typically needs to complete its clinical trial programme, be submitted for regulatory review, and be granted a licence, a process that cannot be shortened. Eli Lilly has indicated it plans to submit its US regulatory application in early 2027, but a US filing is separate from UK licensing, and no confirmed timeline exists for when, or whether, retatrutide might become available in the UK. [4] Any product claiming to offer retatrutide for sale in the UK today is not a legitimate, regulated medicine.

If you are exploring weight loss treatments now, there are several clinically appropriate, currently licensed options worth discussing with a clinician rather than waiting on a medicine with no confirmed release date.

What are your options while retatrutide is not yet available?

Mounjaro, Wegovy and Foundayo are all currently available, clinically supported treatments in the UK, each with an established evidence base and a defined licensing position. Each works differently, and suitability depends on your individual health history.

SwiftDoctor clinicians can assess suitability for these existing treatments now, rather than leaving you to wait on a medicine that has no confirmed UK date. If you want to understand your current weight loss options, a consultation is a practical next step.

Frequently asked questions

Is retatrutide safe?

Safety data reported so far shows a side-effect profile broadly consistent with other GLP-1-related medicines, mostly mild-to-moderate gastrointestinal symptoms. However, retatrutide is still in clinical trials, and longer-term safety data is still being gathered. A full safety picture will only be available once trials are complete and independently reviewed by regulators.

How much weight can you lose on retatrutide?

In Phase 3 trials, participants have lost substantial amounts of weight on average, for example around 28.3% of body weight on the highest studied dose in the TRIUMPH-1 trial over 80 weeks. [1] These are trial averages from a specific study population, not a guarantee of individual results, and retatrutide is not yet an approved, prescribable treatment.

When might retatrutide be approved for use in the UK?

There is no confirmed date. Eli Lilly has said it plans to submit for regulatory approval in the US in early 2027, but that process is separate from UK licensing, which would follow its own timeline afterwards. [4] Any specific date you see quoted elsewhere is speculation rather than a confirmed regulatory decision.

Is retatrutide an injection or a tablet?

In the clinical trials conducted so far, retatrutide has been studied as a once-weekly injection, similar to how Mounjaro and Wegovy are administered. This could change as development continues, but no final, approved formulation exists yet.

Who could be eligible for retatrutide if it is approved?

It is too early to say. Eligibility criteria are set during the regulatory approval process based on the trial population and safety data, and retatrutide has not yet reached that stage. Once approved medicines typically become available, eligibility is usually based on BMI alongside other health factors, similar to existing GLP-1 treatments, but nothing has been confirmed for retatrutide specifically.

What did the latest retatrutide clinical trials find?

The most recent Phase 3 results, from the TRIUMPH-2 and TRIUMPH-3 trials, found that retatrutide led to substantial weight loss and improved blood sugar control in adults with obesity and type 2 diabetes or established cardiovascular disease. [2] Both trials met their primary endpoints, adding to the evidence base, though results still need full regulatory review before any conclusions can be drawn about approval.

Sources

1. Eli Lilly and Company, "Lilly’s triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial" (TRIUMPH-1), investor.lilly.com
2. Eli Lilly and Company / PR Newswire, "Lilly’s triple agonist, retatrutide, successful in two additional Phase 3 obesity trials" (TRIUMPH-2 and TRIUMPH-3), prnewswire.com
3. Eli Lilly and Company, "Lilly’s triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs...in first successful Phase 3 trial" (TRIUMPH-4), investor.lilly.com
4. BioPharma Dive, "Lilly, with new data, to seek FDA approval of obesity drug retatrutide", biopharmadive.com
5. Electronic Medicines Compendium (eMC), "Mounjaro 2.5 mg solution for injection: SmPC" (Eli Lilly), medicines.org.uk
6. Electronic Medicines Compendium (eMC), "Wegovy 0.25 mg–2.4 mg solution for injection: SmPC" (Novo Nordisk), medicines.org.uk