Everything you need to know about retatrutide

There’s a new weight-loss drug making its way through clinical trials, and the results are hard to ignore.

In the TRIUMPH-1 phase 3 trial, people taking the highest dose of retatrutide lost an average of 28.3% of their starting weight over 80 weeks (if they stayed on treatment) [1]. That’s one of the largest average weight losses ever recorded for a medicine. For comparison, Mounjaro produced losses of 22.5% over 72 weeks [2].

There’s a catch, though. The full phase 3 results haven’t yet been published in a peer-reviewed journal. It’s not been approved in the UK or anywhere else yet. But it could be soon.

So, how does retatrutide work, what makes it different to Wegovy and Mounjaro, and when might it actually become available?

In this article, we cover:

• What is retatrutide?
• How does retatrutide work?
• How much weight can you lose?
• How long does it take to work?
• How does it compare with Mounjaro and Wegovy?
• How is retatrutide taken?
• Is it safe, and what are its side effects?
• Can you buy it in the UK?
• When will it become available?
• Does retatrutide have benefits beyond weight loss?
   

What is retatrutide?

Retatrutide is a weight-loss medicine being developed for obesity, type 2 diabetes, and several related conditions. It’s a type of synthetic peptide, not a steroid.

You may see it called 'Reta' online, but there’s no approved brand or pharmacy product. In clinical trials, retatrutide is given as a once-weekly injection. It’s often grouped with GLP-1 weight-loss medicines, although it acts on additional hormone pathways [3].
 

How does retatrutide work?

Many weight-loss medications work by acting on hormone receptors that reduce hunger. For example, Wegovy acts on GLP-1 receptors. Whereas, Mounjaro acts on GLP-1 and GIP.

Retatrutide is a 'triple agonist'. This means it activates three different hormone receptors: GLP-1, GIP, and glucagon.

The third pathway, glucagon, is what sets retatrutide apart. Glucagon can increase energy use and help the body mobilise stored fat, although researchers are still working out how much this contributes to weight loss in people.

The aim of combining all three actions is to reduce calorie intake while also influencing how the body uses energy [4]. This combination may help explain why it produces greater weight loss than a single or dual agonist.
 

How much weight can you lose with retatrutide?

The most rigorous evidence so far comes from TRIUMPH-1, a trial involving over 2,000 adults with obesity or who are overweight with a weight-related condition.

Assuming participants stayed on treatment, the average weight loss at 80 weeks was:

• 19.0% with 4mg
• 25.9% with 9mg
• 28.3% with 12mg [1]

A smaller extension followed 532 people who had started with a BMI of at least 35, completed the first 80 weeks, and tolerated treatment. Those originally assigned to 12mg reached an average loss of 30.3% at 104 weeks while taking their maximum tolerated dose [1]. This is encouraging, but it should not be presented as the result everyone can expect.

People with type 2 diabetes were studied separately and generally lost less weight. In the separate TRIUMPH-2 trial, average weight loss was 20.8% at 80 weeks with 12mg [5].

How long does retatrutide take to work?

In a small interview study of people who had taken part in the phase 2 trial, most reported changes in their appetite and eating behaviour within the first eight weeks [6].

Weight loss, on the other hand, is modest in the first few weeks. The dose is increased gradually to let the body adjust. By week 24, average weight loss in the phase 2 trial reached 17.5% at the highest dose, rising to 24.2% by week 48 [4]. In the later phase 3 trial, average weight loss reached 28.3% by week 80, rising to 30.3% in the extension group [1].

These are group averages over many months. They don’t necessarily mean that weight will fall smoothly every week, and results vary by person.

Is retatrutide better than Mounjaro or Wegovy?

On paper, yes.

Retatrutide’s 28.3% average weight-loss result at 80 weeks, mentioned above, is higher than both Mounjaro and Wegovy. In the direct trial comparing Mounjaro with standard Wegovy, the average weight loss for participants who remained on treatment was 21.6% and 15.4% at 72 weeks, respectively [7]. The newer 7.2mg Wegovy injection produced 20.7% average weight loss in a separate trial [8].

Those figures come from different studies, with different people, treatment periods, and study designs, so they can’t prove that retatrutide is better. A phase 3 head-to-head trial comparing retatrutide with tirzepatide, the medicine in Mounjaro, is under way and is expected to complete in late 2026 [9].

Effectiveness is only part of the comparison. Tolerability, long-term safety, availability, and cost also matter if retatrutide is approved.
 

How do you take retatrutide?

Retatrutide currently has no licensed dose or approved pen. Its final strengths and dose schedule could change if it’s approved.

In trials, participants started with 2mg once weekly and increased the dose in steps every four weeks until they reached a target dose of 4mg, 9mg, or 12mg [1].
 

Is retatrutide safe?

Phase 3 results suggest that retatrutide has similar side effects seen with other weight-loss medicines, but it’s not yet completed regulatory safety review. It’s too early to describe it as safe for routine public use.

The most common side effects of retatrutide were:

• Nausea and vomiting
• Diarrhoea and constipation [1]

Side effects were more common at higher doses. On the highest dose (12mg), just over 1 in 10 people stopped treatment because of side effects [1].

Some users have also reported urinary tract infections as well as dysaesthesia, which is altered skin sensations like tingling, burning, or tenderness. In one trial, dysaesthesia affected 1 in 8 people taking 12mg. Most cases were described as mild to moderate [1].

The final contraindications, rare risks, interactions, and advice for pregnancy or breastfeeding will only be clear if a regulator approves the medicine and publishes official product information.

Can you buy retatrutide in the UK?

No. As of the last update to this article, retatrutide is not authorised in the UK. The only legal access is through an approved clinical trial.

The MHRA has specifically warned against products advertised as retatrutide online or through social media. They have not been checked for quality, safety, or effectiveness, and there’s no guarantee that they contain retatrutide at all [10].

A label saying 'for research use only' does not make a product safe or legal for human use. Anyone offering retatrutide without a clinical trial is not selling an approved medicine.

 

Why buying retatrutide now is riskier than it looks

Even though it isn't approved anywhere, retatrutide is already being sold online through 'research chemical' sites, private Telegram and WhatsApp groups, and some beauticians offering it as a low-cost injection. If you're tempted, it's worth understanding exactly what you'd be risking, beyond the fact that selling it is illegal

You don't know what's actually in the vial. 

Products sold as retatrutide aren't made under any regulatory oversight, so there's no guarantee of purity, correct concentration, or even that they contain retatrutide at all [10]. This isn't a theoretical worry. The MHRA has seized 369 potentially fake Ozempic pens in the UK since January 2023, and testing by Austrian authorities found some contained insulin instead of semaglutide, leading to hypoglycaemic shock, seizures, and hospital admissions in people who had no idea they'd been given the wrong drug entirely [11]. There's no reason to assume a product sold as retatrutide, through the same kind of unregulated channels, is any safer.

There's no clinical supervision. 

In a trial, a doctor checks your medical history, screens for contraindications, and monitors you as the dose is increased. None of that happens when you buy from an online seller or an unregulated salon, so problems that would normally be caught early – an unsuitable medical history, an early bad reaction, the wrong dose for your situation – often aren't.

The safety profile isn't finished yet, even for the genuine drug.

Retatrutide's phase 3 results are still being gathered and haven't completed regulatory review. Taking it outside a monitored trial means absorbing risks that aren't yet fully understood for anyone, on top of the separate risk that the product itself isn't what it claims to be.

It's illegal, and you'd have no recourse if something went wrong. 

The MHRA has explicitly warned against products sold as retatrutide online, and anyone selling it outside a clinical trial is breaking the law [10]. If an unlicensed product harms you, there's no manufacturer, regulator, or formal reporting route standing behind it the way there would be for a licensed medicine.

A 'for research use only' label doesn't change any of this. It's wording sellers use to sidestep the rules, not a genuine safety claim, and it won't protect you if you use the product on yourself.

For now, licensed weight-loss medications, like Mounjaro and Wegovy are the safest alternatives. 

 

When will retatrutide be available?

There’s no confirmed launch date for retatrutide.

Eli Lilly says it plans to submit retatrutide to the US Food and Drug Administration (FDA) in the first quarter of 2027, with global submissions also planned [5]. Regulators will then decide whether the evidence is strong enough to approve it and what warnings or restrictions are needed.
 

Does retatrutide have benefits beyond weight loss?

Possibly. In TRIUMPH-3, retatrutide also improved waist circumference, triglycerides, non-HDL cholesterol, blood pressure, and a marker of inflammation called high-sensitivity CRP [5].

It’s also being studied for type 2 diabetes, obstructive sleep apnoea, knee osteoarthritis pain, cardiovascular and kidney outcomes, fatty liver disease, and chronic low back pain. Early phase 3 results suggest improvements in blood sugar, sleep apnoea severity, and knee pain, but retatrutide is not approved to treat any of these conditions [12].

Some of these improvements likely result partly from weight loss itself. Longer trials are still needed to show whether retatrutide reduces major events such as heart attacks, strokes, or kidney failure.

 

The bottom line

Retatrutide may become the most effective weight-loss medicine so far. In the main phase 3 obesity trial, the highest dose produced 28.3% average weight loss at 80 weeks, and the results continued beyond this in a smaller, selected group.

However, nothing has been approved yet and its long-term safety is still being studied. Until then, avoid anything sold online as ‘Reta’ or retatrutide.
 

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References

1. Lilly’s triple agonist, retatrutide, delivered powerful weight loss in pivotal Phase 3 obesity trial | Eli Lilly and Company. 21 May 2026 [cited 31 Aug 2026]. Available: https://investor.lilly.com/news-releases/news-release-details/lillys-triple-agonist-retatrutide-delivered-powerful-weight-loss
2. Jastreboff AM, Aronne LJ, Ahmad NN, Wharton S, Connery L, Alves B, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387: 205–216. doi:10.1056/NEJMoa2206038
3. Urva S, Coskun T, Loh MT, Du Y, Thomas MK, Gurbuz S, et al. LY3437943, a novel triple GIP, GLP-1, and glucagon receptor agonist in people with type 2 diabetes: a phase 1b, multicentre, double-blind, placebo-controlled, randomised, multiple-ascending dose trial. Lancet. 2022;400: 1869–1881. doi:10.1016/S0140-6736(22)02033-5
4. Jastreboff AM, Kaplan LM, Frías JP, Wu Q, Du Y, Gurbuz S, et al. Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. 2023;389: 514–526. doi:10.1056/NEJMoa2301972
5. Lilly’s triple agonist, retatrutide, successful in two additional Phase 3 obesity trials, delivering significant improvements in weight and A1C | Eli Lilly and Company. 23 July 2026 [cited 31 Aug 2026]. Available: https://investor.lilly.com/news-releases/news-release-details/lillys-triple-agonist-retatrutide-successful-two-additional
6. Goetz IA, Kanu C, Hoover A, Jimenez-Moreno C, Karn H, Kimel M, et al. Perceived benefits of treatment for obesity with retatrutide: A qualitative study of patients in a phase 2 clinical trial. Obes Pillars. 2025;16: 100220. doi:10.1016/j.obpill.2025.100220
7. Aronne LJ, Horn DB, le Roux CW, Ho W, Falcon BL, Gomez Valderas E, et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity. N Engl J Med. 2025;393: 26–36. doi:10.1056/NEJMoa2416394
8. Wharton S, Freitas P, Hjelmesæth J, Kabisch M, Kandler K, Lingvay I, et al. Once-weekly semaglutide 7·2 mg in adults with obesity (STEP UP): a randomised, controlled, phase 3b trial. Lancet Diabetes Endocrinol. 2025;13: 949–963. doi:10.1016/S2213-8587(25)00226-8
9. Eli Lilly and Company. A Phase 3, Randomized, Double-Blind Study to Evaluate the Efficacy and Safety of Retatrutide Compared to Tirzepatide in Adults Who Have Obesity. clinicaltrials.gov; 2026 July. Report No.: NCT06662383. Available: https://clinicaltrials.gov/study/NCT06662383
10. No summer shortcut for safe weight loss. In: GOV.UK [Internet]. [cited 31 Aug 2026]. Available: https://www.gov.uk/government/news/no-summer-shortcut-for-safe-weight-loss
11. Patients hospitalised after using counterfeit weight-loss drugs. In: euronews [Internet]. [cited 3 Sept 2026]. Available: https://www.euronews.com/health/2023/10/30/fake-ozempic-pens-patients-hospitalised-after-using-counterfeit-weight-loss-drugs 
12. Lilly's triple agonist, retatrutide, drove substantial improvements in weight, A1C, knee osteoarthritis pain, and obstructive sleep apnea, demonstrating its remarkable potential to treat obesity and its complications | Eli Lilly and Company. 6 Jun 2026 [cited 31 Aug 2026]. Available: https://investor.lilly.com/news-releases/news-release-details/lillys-triple-agonist-retatrutide-drove-substantial-improvements