Key Takeaways
- Mechanism of action: Triple-hormone agonist targeting GLP-1, GIP, and glucagon receptors
- Efficacy: 24.2% average weight loss in 48 weeks
- Expected approval: 2026 (UK), potentially 2027 (FDA)
- Common side effects: Nausea, diarrhoea, vomiting, constipation
- Safety profile: Similar to existing GLP-1 medications but with concerns about rapid weight loss
Retatrutide is a new once-weekly injection for weight loss, currently in clinical trials. Early data shows it may lead to an average weight loss of 24.2% over 48 weeks—the highest seen so far among obesity treatments.
Developed by Eli Lilly (who make Mounjaro), it’s expected to be approved in the UK by 2026, pending final trial results.
Read on to find out how retatrutide works for weight loss.
What is retatrutide?
Retatrutide is a weekly weight loss injection that builds on the success of Wegovy and Mounjaro. While Wegovy (semaglutide) mimics one hormone (GLP-1) and Mounjaro (tirzepatide) mimics two (GLP-1 and GIP), retatrutide goes further by activating three key hormones—GLP-1, GIP, and glucagon—involved in appetite, metabolism, and energy use.
This “triple G” effect is what makes retatrutide one of the most advanced treatments in development. In clinical trials, it led to greater weight loss than any currently available medication
How does retatrutide work?
Retatrutide works to drive weight loss by mimicking the three hormones we just mentioned:
- GLP-1 stimulates the release of insulin, slows your digestion, and helps suppress your appetite.
- GIP enhances insulin release and may work to prevent fat buildup.
- GCG, a.k.a glucagon, may increase the amount of energy your body uses while helping to burn fat.
This triple mechanism helps reduce hunger, improve insulin sensitivity, and may increase the number of calories your body burns, all of which support sustained and meaningful weight loss.
Retatrutide clinical trials: How effective is it for weight loss?
Retatrutide is currently being investigated in a late-stage (phase III) clinical trial for obesity. The trial is due to end in May 2026.
But we can look at completed phase II research to get an idea of retatrutide weight loss results. Here, people on the highest dose of the medicine lost 24.2% of their starting weight after 48 weeks. What’s more, 26% of participants lost at least 30% of their initial weight.
For comparison:
- Mounjaro: 20.5% mean weight loss after 48 weeks (estimated from 72 week data)
- Wegovy (2.4 mg): 15% mean weight loss after 48 weeks (estimated from 72 week data)
- Wegovy (7.2 mg): 18% mean weight loss after 48 weeks (estimated from 72 week data
When will retatrutide be available in the UK?
For retatrutide to be approved in the UK, the Medicines and Healthcare products Regulatory Agency (MHRA) has to review clinical trial results and confirm that it’s safe and effective.
Since that research is due to be completed in mid-2026, we can’t expect retatrutide to be available before then.
Retatrutide vs other weight loss medications
Here’s a table summarising the key differences between retatrutide and the main weight loss medications on the market, a.k.a Wegovy and Mounjaro. If you’re curious to see how they stack up side by side in more depth, check out our separate article comparing retatrutide, Wegovy and Mounjaro.
Comparing retatrutide, Mounjaro, and Wegovy
What is retatrutide used for?
Retatrutide is likely to be approved for weight loss initially, but it’s also being studied as a treatment for type 2 diabetes.
What are the potential side effects of retatrutide?
In clinical research, the most commonly reported symptoms were nausea, diarrhoea, vomiting, and constipation. It’s a similar list to both Wegovy and Mounjaro's side effects.
However, because retatrutide is so potent, some people have reported losing weight too quickly.
Losing weight too quickly can sometimes cause problems such as nutritional deficiencies, so we’d advise you to monitor your weight loss and ensure you’re maintaining adequate nutrition whilst on treatment.
It can also lead to muscle loss, which can in turn increase the risk of bone fractures. Remember, when it comes to weight loss, slow and sustainable is best!
Who can use retatrutide?
Once approved, retatrutide will likely follow similar eligibility criteria to other weight loss medications. Typically, this means adults with:
- A BMI of 30 or higher (obesity)
- A BMI of 27 or higher with weight-related health conditions (like type 2 diabetes, high blood pressure, or sleep apnea)
But not everyone will be suitable. Based on similar medications, retatrutide probably won't be recommended for:
- People with a personal or family history of medullary thyroid cancer
- Those with multiple endocrine neoplasia syndrome type 2
- Anyone with a history of pancreatitis
- Pregnant or breastfeeding women
- Children and adolescents (under 18)
- People with severe kidney or liver disease
Now, what about people without diabetes who want to lose weight? Good news— retatrutide trials include plenty of participants without diabetes. The medication appears safe and effective for weight loss regardless of diabetes status.
For those interested in metabolic health improvements or reducing liver fat without meeting obesity criteria, this gets trickier.
While early data suggests retatrutide might help with fatty liver disease and metabolic markers, it's primarily being developed for people with obesity. Using it for other purposes would likely be off label, requiring careful discussion with a specialist. However, often weight loss can improve health conditions such as fatty liver disease or high cholesterol.
Your weight loss journey
Retatrutide isn’t available just yet, but there are plenty of other weight loss medications out there that work brilliantly for those they’re suitable for.
As part of our science-backed weight loss programme, you can get Mounjaro or Wegovy weight loss injections, or orlistat weight loss capsules, posted to you each month. Plus personalised support from our team of coaches.
Our Voy weight loss results truly speak for themselves. Most people lose 10-15% of their starting weight and 97% say they feel more confident.
Take our quiz to see if you’re eligible. It won’t take more than a few minutes.
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