What is retatrutide?

Retatrutide is a weekly weight loss injection being developed by Eli Lilly, the same pharmaceutical company that makes Mounjaro.
Like Mounjaro it works by mimicking hormones your gut produces naturally after eating. Those hormones tell your brain you are full and slow down digestion. The result is significantly reduced appetite and sustained weight loss.
The difference is how many hormone pathways it targets.
Ozempic targets one receptor. Mounjaro targets two. Retatrutide targets three, GLP-1, GIP and glucagon simultaneously. That third receptor, glucagon, affects how your body burns stored fat for energy. Targeting all three at once appears to produce substantially greater weight loss than anything currently approved.
Researchers have called it a triple agonist. You do not need to remember the science. What matters is that the clinical trial results are the most impressive ever recorded for a weight loss medicine.

What did the TRIUMPH-1 trial actually show?

The TRIUMPH-1 Phase 3 trial results were published by Eli Lilly and ClinicalTrials.gov in May 2026. Phase 3 is the final and most rigorous stage of drug testing before regulatory submission. It involves thousands of participants, runs for long enough to capture sustained results, and uses placebo controls so results can be compared fairly.
The headline result was this. Participants on the highest dose lost an average of 28.3 percent of their body weight over 80 weeks, roughly a year and a half.
To put that in plain terms. Someone starting at 14 stone would lose on average nearly four stone. Someone starting at 16 stone would lose closer to four and a half stone.
But the number that genuinely surprised researchers was this one. 45.3 percent of participants on the highest dose lost 30 percent or more of their body weight. Nearly half of all patients crossed a threshold that doctors use to measure results equivalent to bariatric surgery, the surgical procedures that physically reduce the size of the stomach.
No injectable treatment had ever consistently reached that territory before.
For comparison, Ozempic and Wegovy produce average weight loss of around 15 to 17 percent. Mounjaro produces around 20 to 22 percent. Retatrutide at 28.3 percent represents the single biggest step forward in this class of medicine yet recorded.

Retatrutide UK vs Mounjaro

This is the question most UK patients are actually asking. Here is the straight answer.
Based on trial data retatrutide produces significantly greater average weight loss than Mounjaro. The gap is meaningful, 28.3 percent versus 20 to 22 percent. For many patients that difference represents an additional stone or more of weight loss.
The 45.3 percent of patients reaching the bariatric surgery threshold is the most striking comparison. Mounjaro trials showed around 10 to 15 percent of patients reaching that level. Retatrutide nearly tripled that proportion.
However there are important caveats worth understanding.
Trial results represent best-case scenarios under controlled conditions. Real-world results for Mounjaro are typically somewhat lower than trial data suggested. The same pattern is likely for retatrutide once it reaches everyday clinical use.
Side effect data from the full Phase 3 trial has not yet been published in peer-reviewed journals. Phase 2 data published in the New England Journal of Medicine in 2023 showed similar side effects to Mounjaro, nausea, vomiting, diarrhoea and constipation being the most common. The full picture will emerge when complete Phase 3 data is published.
The honest comparison is this. If the trial results translate to real-world use at anything close to the same level, retatrutide will be more effective than Mounjaro for most patients. The question for UK patients is whether that additional effectiveness is worth waiting for compared with starting treatment now. A separate 2026 BMJ analysis pooling all available trial data across 19 drugs took a more cautious view of retatrutide specifically, reflecting how few trials exist so far, alongside a wider look at quality of life, dropout rates and long-term outcomes across every major option, worth reading in our weight loss jabs and pills compared guide.

Should I wait for retatrutide or start Mounjaro now?

This is the most important practical question on this page and the answer is genuinely personal.
For most people who are eligible for Mounjaro right now, start now.
Mounjaro is producing genuinely life-changing results for many UK women and men right now. Average weight loss of 20 to 22 percent is clinically significant. The health risks of carrying excess weight, type 2 diabetes, cardiovascular disease, joint problems, sleep apnoea, continue accumulating every month you delay treatment.
Waiting 18 months or more for retatrutide while carrying excess weight and its associated health risks is not obviously the right choice for most people. The gap between what Mounjaro achieves and what retatrutide might achieve is real, but it needs to be weighed against the cost of waiting.
If you start Mounjaro now and retatrutide reaches the UK in late 2027 or 2028, switching at that point is a conversation you can have with your prescriber. You do not have to choose permanently between them.
There are specific situations where waiting might make sense. If you have tried GLP-1 or dual GLP-1/GIP treatments before and responded poorly, retatrutide’s different mechanism might work better for you. If you are managing conditions that make current options less suitable, the additional efficacy might tip the balance. These are conversations for a doctor not a website.
What you should not do under any circumstances is purchase anything claiming to be retatrutide from an online clinic, compounding pharmacy or supplement site. Retatrutide is not approved in the UK. It is not available legally through any channel. Any product claiming to be retatrutide is unregulated, unverified and potentially dangerous.

UK approval timeline – realistic expectations

Retatrutide is not approved in the UK. The MHRA, the Medicines and Healthcare products Regulatory Agency, which is the UK body responsible for approving medicines, has not yet received a submission for retatrutide.
The expected timeline based on current information is as follows.
Eli Lilly is expected to begin regulatory submissions in major markets later this year. The United States FDA submission is likely to come first. A UK MHRA submission would follow, after which formal review begins. The MHRA review process typically takes 12 to 18 months.
Realistic UK availability, even if everything proceeds smoothly, is not expected before late 2027. Mid-2028 is considered more likely by analysts tracking the regulatory timeline.
After MHRA approval there would still be a separate health technology assessment by NICE, the National Institute for Health and Care Excellence, which decides what the NHS will fund and at what price. The NHS rollout of Mounjaro was gradual and rationed following its approval. A similar pattern for retatrutide is the realistic expectation.
Private clinic availability would likely precede NHS availability, as happened with Mounjaro and Wegovy.

Where things stand right now

The TRIUMPH-1 Phase 3 trial results have been published showing 28.3 percent average weight loss. Full peer-reviewed data is expected to be submitted to medical journals in the coming months.
Eli Lilly has not yet made a formal MHRA submission. No UK approval date has been announced.
Retatrutide cannot legally be prescribed or purchased in the UK through any channel.
ThePeptideBrief will update this page every time a significant development is announced, MHRA submission, FDA approval, NHS appraisal decision, pricing announcement or availability date.
Bookmark this page if you want to track the retatrutide story as it develops.

Retatrutide side effects – what we know so far

Phase 3 safety data is now available, published following the TRIUMPH-4 trial results announced by Eli Lilly in December 2025. This gives a far more complete picture than the earlier Phase 2 data alone.
The most common side effects remain gastrointestinal, consistent with the wider GLP-1 and dual GLP-1/GIP class. In TRIUMPH-4, nausea affected around 43 percent of participants on the 12mg dose, compared with roughly 11 percent on placebo. Diarrhoea affected around 33 percent, constipation around 25 percent, and vomiting around 21 percent [Eli Lilly, TRIUMPH-4 topline results, December 2025]. These effects were most common during dose escalation and tended to settle at a stable dose.
Phase 3 data also confirmed a side effect that is genuinely distinctive to retatrutide, dysesthesia, an altered skin sensation described as tingling, prickling or mild burning. This affected around 21 percent of participants on the 12mg dose in TRIUMPH-4, compared with under 1 percent on placebo. It was generally mild, did not typically lead patients to stop treatment, and is thought to be linked to retatrutide’s additional glucagon receptor activity, since it has not been reported at similar rates with Mounjaro or standard-dose Wegovy [Eli Lilly, TRIUMPH-4 topline results, December 2025].
A modest, dose-dependent increase in resting heart rate was also recorded, typically 5 to 10 beats per minute, peaking around week 24 before tapering. No unexpected pattern of serious adverse events emerged beyond the pancreatitis and gallbladder risks already tracked across this entire drug class.
Discontinuation because of side effects was dose-dependent. In TRIUMPH-4, around 12 percent of participants on 9mg and 18 percent on 12mg stopped treatment due to adverse events, compared with 4 percent on placebo [Eli Lilly, TRIUMPH-4 topline results, December 2025]. A separate Phase 3 trial in people with type 2 diabetes, TRANSCEND-T2D-1, published in The Lancet in 2026, found a broadly similar safety pattern, though with somewhat lower rates of gastrointestinal side effects in that specific patient group.
The January 2026 MHRA safety update remains relevant context. The regulator strengthened warnings across the entire GLP-1 and dual GLP-1/GIP class, including Mounjaro and Wegovy, following reports of rare but serious pancreatitis cases, including some fatal cases [MHRA, January 2026]. Retatrutide will face the same level of post-approval safety scrutiny once available.
This is now genuine Phase 3 data from two separate, large trials, a much stronger evidence base than was available even a few months ago. Longer-term safety, particularly beyond the 68-week trial period studied so far, will only be confirmed through continued monitoring after approval.

Retatrutide vs Wegovy – the honest comparison

Wegovy is the branded version of semaglutide specifically licensed for weight management in the UK. It targets one hormone receptor, GLP-1. It was approved by the MHRA and is available both privately and, in limited circumstances, through the NHS.
In clinical trials, Wegovy produced average weight loss of around 15 percent of body weight over 68 weeks [NICE]. That is a clinically meaningful result. For a person starting at 14 stone, that represents just over two stone lost on average.
Retatrutide produced average weight loss of 28.3 percent over 80 weeks, nearly double that of Wegovy [Eli Lilly / ClinicalTrials.gov, May 2026]. The trial duration is different, which makes a direct comparison imperfect. But the gap is large enough that researchers consider it genuinely significant rather than a product of timing differences.
The mechanism explains much of this difference. Wegovy’s single receptor action is effective at suppressing appetite and slowing digestion. Retatrutide’s triple receptor action does all of that while also increasing the rate at which the body burns stored fat for energy. That additional metabolic effect appears to be the key driver of the greater weight loss.
For people who have tried Wegovy and found it effective but want better results, retatrutide, when available, may represent a meaningful upgrade. For people who have not responded well to Wegovy, the different mechanism offers a genuinely different option rather than simply a higher dose of the same approach.
The side effect profiles are similar in nature, nausea, vomiting and gastrointestinal effects dominate in both cases, though the severity and frequency comparisons between the two will only be fully understood when Phase 3 data is published alongside real-world post-approval monitoring.
On cost, see our Mounjaro price comparison for a live sense of current private GLP-1 pricing. Retatrutide is expected to be priced at a premium to Wegovy given its greater efficacy, though no official pricing has been announced.

What will retatrutide cost in the UK?

No official pricing has been set for retatrutide in the UK. Eli Lilly has not announced a list price for any market. What we can do is use the pricing history of Mounjaro and Wegovy as a framework for realistic expectations.
When Mounjaro launched privately in the UK it was priced significantly lower than it is today. Following a price increase from Eli Lilly, private costs for Mounjaro now typically start at around £164 to £200 per month depending on the provider and dose, see our Mounjaro price comparison for current figures.
Wegovy launched at a similar price point and has remained broadly comparable.
Retatrutide is a more efficacious drug than either. Pharmaceutical companies typically price more effective treatments at a premium. Based on the Mounjaro and Wegovy pricing precedents, private clinic costs for retatrutide in the UK are likely to start at somewhere between £200 and £300 per month for starter doses, potentially higher for maintenance doses.
This is speculation based on pricing patterns rather than confirmed information. Actual pricing will depend on Eli Lilly’s commercial decisions, competition from other manufacturers, and the negotiating position of private providers.
For NHS patients, the picture is more uncertain. The NHS does not pay list price for any medicine, it negotiates a confidential commercial agreement with the manufacturer. The NICE health technology assessment process, which follows MHRA approval, will determine whether retatrutide represents sufficient value for money to be funded by the NHS and at what level of patient eligibility.
Based on the pattern of Mounjaro and Wegovy, NHS access to retatrutide is likely to be initially rationed to patients with a high BMI and multiple qualifying health conditions. Widespread NHS availability is unlikely before 2028 or 2029 at the earliest, and may be later depending on the outcome of NICE negotiations.
If cost is a significant factor in your decision-making, the most sensible approach is to continue with whatever treatment you are currently on or are eligible for, and revisit the retatrutide question when actual UK pricing is confirmed.

Beyond TRIUMPH-1: what the other retatrutide trials show

TRIUMPH-1 is the headline general obesity trial, but Eli Lilly is running a much larger programme, and two further trials have reported results since.
TRIUMPH-4, announced in December 2025, studied retatrutide specifically in adults with obesity and knee osteoarthritis. Over 68 weeks, the 12mg dose produced average weight loss of 28.7 percent, alongside a meaningful reduction in knee pain scores, with more than one in eight participants becoming completely free of knee pain by the end of the trial [Eli Lilly, TRIUMPH-4 topline results, December 2025]. For anyone carrying excess weight alongside joint pain, this is a genuinely relevant, separate data point from the general TRIUMPH-1 figures.
TRANSCEND-T2D-1, announced in March 2026 and now published in The Lancet, tested retatrutide specifically in adults with type 2 diabetes whose blood sugar was not well controlled by diet and exercise alone. Over 40 weeks, retatrutide produced significant reductions in both HbA1c, a measure of long-term blood sugar control, and body weight, meeting all its primary and key secondary goals [The Lancet, 2026]. This is the first Phase 3 evidence specifically covering how retatrutide performs in people with diabetes, a group that responds differently to weight loss drugs than people without diabetes.
A further, larger trial is also underway. Registered as NCT06383390 on ClinicalTrials.gov, this study is testing whether retatrutide reduces major adverse cardiovascular events, heart attacks, strokes and cardiovascular death, in around 10,000 adults with obesity and existing cardiovascular disease. It is designed along the same lines as the SELECT trial that gave semaglutide its cardiovascular risk-reduction evidence. Results are not expected before 2027 at the earliest, since trials measuring cardiovascular outcomes need years of follow-up to accumulate enough events to analyse reliably.
Taken together, these trials suggest Eli Lilly is building a genuinely broad evidence base for retatrutide, covering general obesity, joint pain, diabetes and cardiovascular risk, not just a single headline weight loss figure. We will update this section as further results are published.

Frequently Asked Questions

Sources and editorial methodology

Page last reviewed: 12 July 2026. This guide was compiled by The Peptide Brief editorial team and is checked against primary regulatory and clinical sources, not secondhand reporting. We accept no payment, sponsorship or affiliate commission from pharmaceutical manufacturers or private clinics.
Regulatory status: MHRA medicines information (mhra.gov.uk). No marketing authorisation application for retatrutide has been submitted as of this review.
Trial data: TRIUMPH-1 topline results, Eli Lilly, May 2026. TRIUMPH-4 topline results, Eli Lilly, December 2025 (ClinicalTrials.gov NCT05931367). TRANSCEND-T2D-1, published in The Lancet, 2026 (PubMed ID 42250575, ClinicalTrials.gov NCT06354660). Phase 2 data: Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity, a Phase 2 Trial. New England Journal of Medicine 2023;389:514-526.
Cardiovascular outcomes trial: registered as NCT06383390 on ClinicalTrials.gov.
Safety alerts: MHRA class-wide GLP-1 and dual GLP-1/GIP safety update, January 2026.
Spotted something out of date or incorrect? Email corrections@thepeptidebrief.co.uk and we will investigate.

This guide is for information purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting any treatment.