Resources - MECHANISMS
Retatrutide results: how much weight people lost in the trials, dose by dose, and what the side-effect data showed
Every published retatrutide result in one place: the 48-week phase 2 obesity trial (up to 24.2% at 12 mg), the type 2 diabetes trial, and the phase 3 TRIUMPH-4 (December 2025) and TRIUMPH-1 (May 2026) announcements (up to 28% at 80 weeks, 30% at 104 weeks in a high-BMI subgroup). Side effects and discontinuations by dose, why "before and after" photos tell you less than the tables, and why none of this is available on prescription in the UK yet.
Retatrutide is the most-searched weight-loss compound that nobody in the UK can legally be prescribed. It is Eli Lilly's once-weekly triple agonist of the GIP, GLP-1 and glucagon receptors, it is in phase 3 trials, and it has produced the largest average weight losses ever recorded in a drug trial for obesity. Because it is unlicensed, the only honest source of results is the trial data, and that is what this article collects: the two phase 2 trials published in 2023, the phase 3 announcements of 2026, what each one measured, at what dose, over how long, with what side effects, and how many people stopped. It also explains why the "before and after" material that fills social media tells you less than the tables, and where the compound actually stands in the UK. It is an account of the evidence, not advice, and it describes a medicine that does not yet exist as a medicine.
The trials that have reported so far
Five sets of results are described here. The phase 2 obesity trial, published in the New England Journal of Medicine in June 2023, randomised 338 adults with obesity or overweight plus a weight-related condition to placebo or one of six retatrutide regimens for 48 weeks. The phase 2 type 2 diabetes trial, published in the Lancet the same month, randomised 281 adults to placebo, dulaglutide or one of six retatrutide regimens for 36 weeks. The phase 3 TRIUMPH-1 obesity trial, with 2,339 participants over 80 weeks and a 104-week extension in a subgroup, was announced by topline press release on 21 May 2026 and presented in full at the American Diabetes Association Scientific Sessions on 6 June 2026. TRIUMPH-4, in people with obesity and knee osteoarthritis, was announced on 11 December 2025. Most phase 3 results are, at the time of writing, company summaries and conference presentations rather than peer-reviewed papers; the exception is TRANSCEND-T2D-1, published in the Lancet in June 2026. The article flags where that matters.
Phase 2 obesity trial: weight loss by dose at 24 and 48 weeks
The primary endpoint was percentage weight change at 24 weeks; 48 weeks was a secondary endpoint. The dose groups were 1 mg, 4 mg (two different starting doses), 8 mg (two starting doses) and 12 mg, all weekly. The table gives the least-squares mean changes reported in the paper.
| Weekly dose | 24 weeks | 48 weeks |
|---|
| Placebo | -1.6% | -2.1% |
| 1 mg | -7.2% | -8.7% |
| 4 mg (combined groups) | -12.9% | -17.1% |
| 8 mg (combined groups) | -17.3% | -22.8% |
| 12 mg | -17.5% | -24.2% |
Two features of this table matter more than the headline. First, the curves had not flattened at 48 weeks at any dose above 1 mg, so the trial ended before the full effect was seen, which is why the phase 3 programme runs to 80 weeks. Second, the difference between 8 mg and 12 mg was small at both timepoints, while the difference in side effects, discussed below, was not. At 48 weeks, a loss of at least 15% was reached by 60% of people on 4 mg, 75% on 8 mg and 83% on 12 mg, against 2% on placebo; at least 5% was reached by 92%, 100% and 100% respectively, against 27% on placebo.
Phase 2 type 2 diabetes trial: smaller losses, large HbA1c falls
The Lancet trial measured blood sugar first and weight second, in people with a mean eight years of diabetes. At 24 weeks, HbA1c fell by about 1.3 to 1.4 percentage points at 4 mg, 1.9 to 2.0 at 8 mg and 2.0 at 12 mg, against essentially no change on placebo and 1.4 on dulaglutide 1.5 mg, the active comparator. Weight at 36 weeks fell by 3.2% at 0.5 mg, 7.9% to 10.4% at 4 mg, 16.3% to 16.8% at 8 mg and 16.9% at 12 mg, against 3.0% on placebo and 2.0% on dulaglutide. As with tirzepatide and semaglutide, people with type 2 diabetes lost less weight than people without it at the same dose, a pattern that runs through the whole class and is not fully explained. Gastrointestinal adverse events affected 35% of retatrutide participants overall, 13% of the placebo group and 35% of the dulaglutide group; there were no severe hypoglycaemia events and no deaths.
Phase 3 TRIUMPH-1: the May 2026 announcement
TRIUMPH-1 randomised 2,339 adults with obesity or overweight, without diabetes, to placebo or retatrutide 4 mg, 9 mg or 12 mg weekly for 80 weeks. Lilly reported two versions of the result, and the difference between them causes most of the confusion in coverage. The "treatment-regimen" estimate, which counts everyone randomised whether or not they stayed on the drug, was about 17.6% at 4 mg, 23.7% at 9 mg and 25.0% at 12 mg, against 3.9% on placebo. The "efficacy" estimate, which describes people who stayed on treatment, was about 19.0%, 25.9% and 28.3%. The first number is the fairer description of what a prescribing decision would produce across a population; the second is the drug's effect in people who tolerate it. A subgroup of 532 participants with a baseline BMI of 35 or more who tolerated their dose were escalated to the highest tolerated dose for a further 24 weeks and the 12 mg arm of that subgroup reached a mean loss of 30.3% at 104 weeks on the efficacy estimand (29.9% treatment-regimen), from a mean baseline of about 122 kg; the placebo arm, switched to retatrutide for the extension, lost about 19%.
At 12 mg, Lilly reported that around 45% of participants lost 30% or more of their weight, and around 65% of those starting with a BMI of 30 or more finished below it. Waist circumference fell by about 24 cm at the top dose, and blood pressure, triglycerides and non-HDL cholesterol improved. These are the figures from the company's release and the trade press that reported it; until the peer-reviewed paper appears, the exact estimands, confidence intervals and subgroup definitions are not independently checkable, and secondary outcomes announced by press release have a history of looking less tidy in print.
TRIUMPH-4 and the diabetes phase 3
TRIUMPH-4 enrolled people with obesity and knee osteoarthritis, with pain as a co-primary outcome. Lilly reported a mean weight loss of 28.7% at 68 weeks on 12 mg (efficacy estimand) together with substantial improvement in knee pain scores. TRANSCEND-T2D-1, in type 2 diabetes, reported 16.8% at 40 weeks on 12 mg on the efficacy estimand (15.3% treatment-regimen, against about 2.5% on placebo), again lower than in people without diabetes, and consistent with phase 2. Lilly said in August 2026 that three further phase 3 obesity trials had completed with positive results; TRIUMPH-2 in type 2 diabetes and TRIUMPH-3 in established cardiovascular disease had not been announced in detail at the time of writing. Of the phase 3 results, TRANSCEND-T2D-1 has been published in the Lancet (June 2026); the obesity trials have so far been presented at conferences and in company releases rather than in peer-reviewed papers.
Side effects, dose by dose
In phase 2, the most common adverse events were gastrointestinal, dose-related, mostly mild to moderate, and partially reduced by starting at 2 mg rather than 4 mg, which is why the phase 3 protocols start low. Heart rate rose in a dose-dependent way, peaking at 24 weeks and declining thereafter; this is a feature of glucagon-receptor activity and is one of the things the cardiovascular outcomes trial is designed to examine. One death was reported in the phase 2 obesity trial, a drowning that the investigator assessed as unrelated to the drug and an independent committee recorded as undetermined. TRIUMPH-1 gives the first large-sample side-effect table, and it looks like the rest of the class at higher intensity.
| Adverse event, TRIUMPH-1 | 4 mg | 9 mg | 12 mg | Placebo |
|---|
| Nausea | 28.6% | 38.4% | 42.4% | 14.8% |
| Diarrhoea | 25.2% | 34.1% | 32.0% | 13.5% |
| Constipation | 23.8% | 25.9% | 26.1% | 10.9% |
| Vomiting | 10.6% | 22.8% | 25.3% | 4.8% |
| Stopped for adverse events | 4.1% | 6.9% | 11.3% | 4.9% |
Two other events were reported at around one in ten people at the top dose: dysaesthesia, meaning altered or unpleasant skin sensations such as tingling or crawling, which appeared in the phase 2 data and also features in the tirzepatide licence at a lower rate, and urinary tract infections, whose relationship to the drug is not yet explained. The discontinuation row is the one to read most carefully: at 4 mg and 9 mg, retatrutide participants stopped for side effects at roughly the placebo rate, but at 12 mg more than one in ten did, more than double placebo. Since 9 mg delivered most of the weight loss of 12 mg with a tolerability profile close to the lower dose, the dose that eventually gets licensed, if one does, may not be the one the headlines quote.
How retatrutide compares with tirzepatide and semaglutide
There is no head-to-head trial, so any comparison lines up separate trials with different durations, populations and definitions. On that basis, retatrutide 12 mg at 80 weeks in TRIUMPH-1 (about 25% treatment-regimen, 28% efficacy) sits above tirzepatide 15 mg at 72 weeks in SURMOUNT-1 (about 21% and 23% on the same two estimands) and well above semaglutide 2.4 mg at 68 weeks in STEP 1 (about 15% and 17%). Nausea, diarrhoea and vomiting rates at the top retatrutide dose are higher than at the top tirzepatide dose, and so is discontinuation. In short, the third receptor appears to buy more weight loss at the cost of more side effects, which is the trade the whole field expected. Whether it also buys better long-term outcomes, on heart disease, joints, liver or sleep apnoea, is what the outcome trials are for. Our articles on Mounjaro side effects and Ozempic and Wegovy side effects give the comparators in the same format.
Why "before and after" tells you less than the tables
Search for retatrutide results and most of what comes back is photographs and testimonials. None of it can be checked. Retatrutide is not licensed anywhere in the world, so nobody outside a trial is using pharmaceutical retatrutide under medical supervision; what people are photographing themselves on is a vial bought online whose contents, purity and concentration are unknown, taken at a dose they chose, alongside diet changes they may not mention. A photograph also cannot show the placebo group, and in TRIUMPH-1 the placebo group lost 3.9% on lifestyle advice alone. The trial tables are unglamorous, but they are the only results in existence that were measured on a known compound, at a known dose, against a control, by people who did not know which group they were in.
Where retatrutide stands in the UK
Retatrutide has no marketing authorisation from the MHRA or any other regulator. It is an investigational medicine, available in the UK only inside a clinical trial. Lilly said in its second-quarter 2026 results (5 August 2026) that it plans to file with the US FDA in the first quarter of 2027; it has not announced an MHRA submission date. On that timetable a UK licensing decision before 2028 looks unlikely, though that is an estimate that assumes the UK decision follows the US one, and NHS availability would follow a NICE technology appraisal after licensing, a process that took tirzepatide more than a year after its licence. Anything sold in the UK today as retatrutide is unlicensed, is not covered by any of the safety monitoring described above, and is the subject of MHRA warnings about unlicensed weight-loss injections. The licensed alternatives are semaglutide and tirzepatide, through NHS weight-management or diabetes services or a regulated prescriber.
How to read the next retatrutide headline
- Ask which estimand: "treatment-regimen" includes everyone randomised and is the fairer figure; "efficacy" describes people who stayed on the drug and is always higher.
- Ask which dose and how long: 30% was reached in a high-BMI subgroup on the highest tolerated dose at 104 weeks, not by everyone at 80.
- Look for the placebo group: in TRIUMPH-1 it lost 3.9%, so the drug's effect is the difference, not the raw number.
- Check whether it is a press release or a paper: as of September 2026, the phase 3 obesity data are press releases and conference presentations; only the diabetes trial has been published.
- Look at the discontinuation row before the weight-loss row; the top dose lost the most weight and lost the most participants.
- Remember that nothing here is available to buy legally as a medicine, and that anything sold as retatrutide in the UK is unverified.
This article summarises published and announced clinical trial results for an investigational medicine. Retatrutide is not licensed in the UK or anywhere else, and nothing here is medical advice or a suggestion that anyone should obtain or use it. The trial figures are population averages under monitored conditions and are not predictions for any individual. Anyone seeking help with weight or type 2 diabetes should speak to a GP, an NHS weight-management service or a regulated prescriber about the licensed options. Vivera Education sells nothing.
Frequently asked
- How much weight did people lose on retatrutide?
- In the 48-week phase 2 trial, average loss was 8.7% at 1 mg, 17.1% at 4 mg, 22.8% at 8 mg and 24.2% at 12 mg, against 2.1% on placebo. In the phase 3 TRIUMPH-1 trial announced in May 2026, the treatment-regimen averages at 80 weeks were about 17.6% at 4 mg, 23.7% at 9 mg and 25% at 12 mg, against 3.9% on placebo, with a high-BMI subgroup reaching about 30% at 104 weeks.
- What are retatrutide results at 12 weeks or 3 months?
- The trials report at 24 weeks, not 12. At 24 weeks in phase 2, average loss was 7.2% at 1 mg, 12.9% at 4 mg, 17.3% at 8 mg and 17.5% at 12 mg. Loss was still continuing at 48 weeks at every dose above 1 mg, so early figures understate the eventual result.
- Is retatrutide better than Mounjaro or Wegovy?
- No head-to-head trial has been published. Comparing across separate trials, retatrutide phase 3 averages at 80 weeks (about 25% at 12 mg) are higher than tirzepatide at 72 weeks in SURMOUNT-1 (about 21% at 15 mg) and semaglutide at 68 weeks in STEP 1 (about 15%). Different trials, populations and durations make this an indication, not a proof.
- What were the retatrutide side effects in the trials?
- Mostly gastrointestinal and dose-related: in TRIUMPH-1 at 12 mg, nausea affected about 42% of people, diarrhoea 32%, constipation 26% and vomiting 25%, against 15%, 14%, 11% and 5% on placebo. Skin sensation changes and urinary tract infections affected around one in ten at the top dose. Heart rate rose in a dose-dependent way in phase 2, peaking at 24 weeks.
- How many people stopped retatrutide because of side effects?
- In TRIUMPH-1, discontinuation for adverse events was 4.1% at 4 mg, 6.9% at 9 mg and 11.3% at 12 mg, against 4.9% on placebo. The top dose roughly doubled the placebo rate.
- Did anyone die in the retatrutide trials?
- One death was reported in the phase 2 obesity trial, a drowning that the investigator assessed as unrelated to the drug and an independent committee recorded as undetermined. The Lancet type 2 diabetes phase 2 trial reported no deaths. Most phase 3 safety data have so far been released as company summaries; the type 2 diabetes trial, TRANSCEND-T2D-1, was published in the Lancet in June 2026.
- Are retatrutide before and after photos reliable?
- Photos posted online cannot be verified: the product, the dose, the timescale and the diet are unknown, and the compound is not licensed anywhere, so nobody posting is using pharmaceutical-grade retatrutide under supervision. The trial tables above are the only checked results that exist.
- When will retatrutide be available in the UK?
- It is unlicensed and investigational in the UK as of September 2026. Lilly plans to file with the US FDA in the first quarter of 2027 and has not announced a UK submission; a UK decision before 2028 looks unlikely on the US timetable, though that is an estimate, and NHS access would follow a NICE appraisal after that.
- Was retatrutide taken daily or weekly in the trials?
- Once weekly in every published trial that gave repeated doses; the first-in-human study gave a single injection. No published trial has tested daily dosing. This describes trial protocols for an unlicensed, experimental medicine and is not a guide to taking anything.
- What happens when people stop retatrutide?
- No published trial has yet reported what happens after stopping retatrutide. A randomised withdrawal trial, TRIUMPH-6, is under way and is not due to complete before 2028. For semaglutide and tirzepatide, withdrawal trials showed roughly half to two-thirds of the lost weight returning within a year of stopping, and there is no evidence yet on whether retatrutide differs.
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Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.