Retatrutide Weight Loss Results: What the Trials Show

Published on: April 29, 2026

Medically reviewed by

Ashis Tandukar

Superintendent Pharmacist · Reg: GPhC No. 2084170

You might have seen the headlines — "the strongest weight loss drug yet", "24% of body weight lost in under a year". Striking numbers like these travel fast. What's harder to find is a calm, clinical look at what the trial data actually shows: how much weight people lose, when those changes start to appear on the scale, when the curve flattens, and what any of it might mean if retatrutide eventually reaches UK pharmacies.

That's what we'll do here.

What is retatrutide, in plain terms

Retatrutide (development code LY3437943, sometimes nicknamed "triple G") is an injectable weight loss medicine being developed by Eli Lilly. It's a triple agonist — meaning it activates three different hormone receptors at once: GLP-1, GIP and glucagon.

For context, Mounjaro (tirzepatide) targets two of those receptors. Wegovy (semaglutide) targets just GLP-1. Adding the glucagon arm appears to do something the others don't: it pushes up energy expenditure modestly while the GLP-1 and GIP effects suppress appetite. Whether that mechanism truly explains the bigger weight loss seen in trials is still being studied, but the clinical signal is hard to ignore.

If you'd like the bigger picture on availability and regulatory status, our guide on retatrutide in the UK covers that in detail. This article is specifically about the numbers — how much weight people are actually losing, and how quickly.

Which trials are we drawing on?

Most of the public data on retatrutide weight loss results comes from two sources.

The Phase 2 obesity trial (Jastreboff et al., NEJM, 2023). This was a 48-week, double-blind, placebo-controlled study of 338 adults with obesity (BMI ≥30, or BMI ≥27 with at least one weight-related comorbidity). Participants were randomised to placebo or one of several retatrutide doses (1 mg, 4 mg, 8 mg or 12 mg weekly), with two different titration schedules tested. It's the trial that produced the headline-making numbers.

The TRIUMPH Phase 3 programme. Lilly's larger, longer Phase 3 trials — TRIUMPH-1 through TRIUMPH-5 — are testing retatrutide in different populations: primary obesity, obesity with type 2 diabetes, cardiovascular outcomes, knee osteoarthritis, and metabolic dysfunction-associated steatohepatitis (MASH). Topline efficacy data from TRIUMPH-1 and the diabetes-focused TRANSCEND-T2D-1 trial has been released, and the results broadly track what Phase 2 showed at the highest doses.

A few smaller mechanistic and biomarker studies fill in detail around appetite, body composition and metabolic markers — but for weight loss specifically, the Phase 2 numbers and TRIUMPH topline data are what readers, clinicians and journalists are talking about.

Average weight loss in the Phase 2 trial

Here are the headline figures from Jastreboff et al., measured at week 48:

For someone weighing 100 kg at the start of the trial, that's around 24 kg lost on the highest dose — without surgery, after just under a year.

A few details worth noting before we go further. These are average figures from a controlled trial, with a placebo group losing 2.1% over the same period. Trial participants typically have more support and more frequent contact with clinicians than someone in real-world practice, which can flatter the numbers slightly. And not everyone tolerated the highest dose without meaningful side effects — we cover that in detail in our retatrutide side effects guide.

Still: even allowing for the trial-versus-real-world gap, the magnitude is striking. For comparison, Mounjaro produces roughly 22.5% weight loss at the highest dose over 72 weeks, and Wegovy around 15% over 68 weeks.

Month by month: when do results actually show up?

This is the question we're asked most often. People want to know when they'll see something on the scale, not just where they'll end up after a year.

The Phase 2 data tracks weight at multiple points across the 48 weeks, and a few patterns emerge clearly.

Weeks 1–4 (starting dose). Most participants on retatrutide started losing weight within the first month, even on the lowest starting dose of 2 mg. Average losses at week 4 were modest — typically around 2–4% of body weight — but they were real, and noticeably more than placebo. Many people in clinical trials report appetite changes within the first 7 to 14 days, well before the scale shifts much.

Months 2–3 (early titration). As the dose stepped up (4 mg, then higher), weight loss accelerated. By week 12 — three months in — participants on the higher dose arms were averaging around 8–12% body weight loss. This is a useful checkpoint. People who saw little change in month 1 but kept going often hit a meaningful drop here.

Months 4–6. Weight loss continued at a steady pace through this stretch. By week 24, average losses on the 8 mg and 12 mg arms were in the 16–18% range. For most people, this is the period when others start to notice — clothes fit differently, photographs look different, comorbidities like blood pressure and HbA1c improve.

Months 7–12. This is where retatrutide begins to differ from earlier GLP-1 medicines. With Wegovy and Mounjaro, the weight loss curve typically flattens noticeably after about 9–12 months. With retatrutide at 12 mg, the curve was still declining at week 48 — meaning people hadn't yet reached their plateau. The TRIUMPH-1 Phase 3 data, which followed people for 72 weeks, showed continued (smaller) losses through week 60 and beyond.

Does the weight loss plateau?

Yes — eventually. Every weight loss medication has a plateau, and retatrutide is unlikely to be the exception. The question is when.

Phase 2 data suggested the 12 mg arm hadn't yet plateaued by 48 weeks, which is unusual. By contrast, Wegovy's STEP trials show the curve flattening around month 12, and Mounjaro's SURMOUNT trials around month 15–18. The TRIUMPH-1 Phase 3 data, which extends to 72 weeks, looks to confirm continued (slowing) loss through that timeframe.

Whether retatrutide produces meaningful additional weight loss past 18–24 months will only be answered by longer-term studies. What we do know is that, like all weight loss medicines we have evidence for, weight tends to come back if treatment stops. The body's energy regulation system is biologically defended, and these medicines work by altering that defence — not by permanently resetting it. We've written more about what happens when you stop taking GLP-1 medication, and the principles broadly apply here too.

How retatrutide compares to Mounjaro and Wegovy

The most useful comparison isn't really retatrutide versus retatrutide at different doses — it's how the highest-dose retatrutide arm stacks up against the medicines you can actually be prescribed today.

A few honest caveats here. The Wegovy and Mounjaro figures come from longer trials, so they're not a perfectly like-for-like comparison. The Phase 2 retatrutide trial was also small (338 people) compared to the much larger SURMOUNT and STEP programmes. And the magnitude of difference between Mounjaro and retatrutide at their highest doses isn't huge — roughly 2 percentage points in head-to-head extrapolations. Whether that gap proves real and durable in Phase 3 is one of the biggest open questions in obesity medicine right now.

For a deeper look at how retatrutide is dosed across these trials, our retatrutide dosage guide walks through each titration step.

What about retatrutide before and after?

People search for retatrutide before and after photos for a reason — they want a visual sense of what 20–24% weight loss looks like.

We'd be cautious about the photos circulating online. Retatrutide isn't approved for general use anywhere yet. The before-and-after images you'll find are usually from clinical trial participants (rarely shared), unregulated peptide users (which we strongly discourage — see our how to get retatrutide in the UK article on why peptide pens aren't safe), or people who confuse retatrutide with Mounjaro.

If you want a realistic sense of what 20% weight loss looks like, the closest analogue is Mounjaro before and after photos at the higher doses — different mechanism, similar magnitude of change.

Why your real-world results might look different

Trial numbers are useful, but they're an idealised version of what's possible. A few reasons your own results might differ — in either direction:

Adherence. In Phase 2, participants were carefully monitored. Real-world adherence to weekly injections, especially through side effect-heavy titration weeks, is lower. The people who finish the year with the best results are usually the ones who stay on a therapeutic dose.

Dose tolerated. Not everyone can comfortably reach 12 mg. Some people stay at 8 mg, or even 4 mg, because higher doses bring more nausea, fatigue or other GI side effects. Lower-dose results — still excellent — were 17–22% over 48 weeks.

Starting BMI. People starting at higher body weights tend to lose more in absolute kilograms, but percentage-of-body-weight outcomes are generally similar across BMI ranges. Sex, age and metabolic health all play smaller roles too.

Lifestyle factors. Retatrutide (like every weight loss medicine) works alongside food, movement and sleep — not in spite of them. Adequate protein intake, resistance training and sleep quality all influence how much of the weight you lose is fat versus lean tissue. We've written about maintaining muscle on GLP-1 medication for that reason.

Comorbidities. People with type 2 diabetes typically lose somewhat less weight on GLP-1 class medicines than people with obesity alone. The TRANSCEND-T2D-1 trial showed retatrutide was still highly effective in diabetes — but average weight loss was a few percentage points lower than in non-diabetic populations.

A note on what these numbers mean clinically

Twenty-four percent of body weight is enough to do real metabolic work. In Phase 2, retatrutide produced clinically significant improvements in HbA1c, blood pressure, fasting insulin, triglycerides, hepatic steatosis markers and waist circumference — most of which improved progressively through the trial.

For someone with obesity-related conditions, that scale of weight loss is comparable to bariatric surgery in some metrics. It's also where a lot of the long-term value of these medicines is going to be measured: not the cosmetic outcome, but reductions in cardiovascular events, joint pain, sleep apnoea severity, fatty liver and so on. The TRIUMPH-3 cardiovascular outcomes trial will give us hard answers on whether retatrutide reduces heart attacks and strokes — and that data, when it lands, may matter more than any weight loss percentage.

What this means if you're waiting for retatrutide in the UK

Retatrutide isn't a UK option yet, and even with Phase 3 results emerging, an MHRA approval would still take time after that — typically a year or more from regulatory submission. A best-case timeline puts retatrutide at UK pharmacies in 2027 or beyond, and that's assuming everything goes smoothly with manufacturing and approval. Realistically, plan around late 2027 or 2028.

If you're searching for treatment now, the medicines that produce the closest results are already accessible. Mounjaro, in particular, gets within 2 percentage points of retatrutide's headline numbers and has years of real-world UK data behind it. Our broader weight loss medication UK guide walks through current options and how they compare on cost, side effects and eligibility.

Buying retatrutide as a research peptide online — which we see asked about often — isn't a clinically safe alternative, regardless of what the marketing says. The compounds sold this way aren't pharmaceutical-grade, aren't dose-controlled, and have no clinical oversight. We've written about that in detail in our piece on getting retatrutide in the UK, and the answer hasn't changed.

Medical disclaimer

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