30.3% of body weight in two years. That's not a typo — that's Retatrutide's headline result from Phase 3 trials.

If you've been following the GLP-1 space, you'll know the numbers keep improving. But Retatrutide, developed by Eli Lilly, has just posted the highest weight loss figure ever recorded for any anti-obesity medication in a Phase 3 clinical trial: 30.3% of body weight at 104 weeks. For a 17-stone man, that's roughly five and a half stone.

Here's everything you need to know about it — including why you can't get it yet, and what that means for your options right now.

What Is Retatrutide?

Retatrutide is a once-weekly injectable drug developed by Eli Lilly — the same company behind Mounjaro. It belongs to a new class called triple hormone receptor agonists, sometimes shortened to 'triple agonists' or 'triple incretin agonists'.

In plain terms: it works on three separate hormonal pathways at the same time.

How Does the Triple Agonist Mechanism Work?

Think of it this way:

• Wegovy works on one receptor: GLP-1 (reduces appetite, slows gastric emptying)
• Mounjaro works on two: GLP-1 + GIP (adds a stronger insulin response and greater fat burning)
• Retatrutide works on three: GLP-1 + GIP + Glucagon

That third mechanism — glucagon — is the key differentiator. Glucagon directly increases energy expenditure in the liver and promotes fat breakdown. It's what's driving Retatrutide's results beyond anything currently on the market.

What Do the Clinical Trials Actually Show?

The clinical data for Retatrutide is genuinely exceptional. Here's the full picture across Phase 2 and Phase 3.

Phase 2 Trial (published in NEJM, June 2023):

• 48-week randomised controlled trial
• 338 adults with obesity or overweight, without type 2 diabetes
• Tested against placebo

At the 12mg dose, participants lost an average of 24.2% of their body weight over 48 weeks. Every single person on 12mg lost at least 5% of body weight. 83% lost at least 15%.

Phase 3: TRIUMPH-4 (December 2025): Topline results showed 28.7% weight loss at 68 weeks in adults with obesity and knee osteoarthritis. A 75.8% reduction in knee pain scores was also observed — a notable benefit for men carrying significant weight around the joints.

Phase 3: TRIUMPH-1 (May 2026): The pivotal obesity trial. With 2,339 participants across 80–104 weeks, results showed:

• 28.3% average weight loss at 80 weeks
• 30.3% at 104 weeks at the 12mg dose

That last number is the one making headlines. No obesity drug has ever produced results like it in a Phase 3 trial.

Retatrutide Dosage: How It Works in Trials

Retatrutide is not approved for prescription in the UK. The information below comes from published clinical trial protocols only.

In trials, Retatrutide was administered as a once-weekly subcutaneous injection — the same delivery method as Wegovy and Mounjaro. Dosing started low and escalated gradually every four weeks, following the 'start low, go slow' protocol that reduces gastrointestinal side effects during the adjustment period.

The drug has a half-life of approximately six days, making a once-weekly injection appropriate. It would take roughly 30 days to fully clear the body.

Escalation schedule used in trials:1mg → 2mg → 4mg → 8mg → 12mg (maintenance), escalating every 4 weeks over approximately 20 weeks.

Side Effects and Safety: What the Data Shows

The side effect profile is broadly similar to Wegovy and Mounjaro — but with some unique additions due to the glucagon mechanism.

Most common side effects at 12mg (TRIUMPH-4):

• Nausea: 43.2% (vs 10.7% placebo)
• Diarrhoea: 33.1% (vs 13.4% placebo)
• Vomiting: 20.9% (vs 0% placebo)
• Constipation: 25.0% (vs 8.7% placebo)

The majority were moderate in severity, mostly during dose escalation, and reduced once participants stabilised on their maintenance dose.

Retatrutide-Specific Safety Signals

Dysaesthesia is the most notable new finding — abnormal skin sensations including tingling, burning, numbness, or unusual sensitivity to touch. This was reported in 20.9% of participants on 12mg in TRIUMPH-4, compared to just 0.7% on placebo. Most cases were mild and didn't lead to participants stopping treatment.

Other signals worth noting:

• Heart rate increase: An average rise of 5–10 bpm at 12mg, peaking around week 24 then declining. This is slightly higher than tirzepatide (+2–3 bpm) or semaglutide (+3–4 bpm), though no increase in serious cardiovascular events was observed in Phase 2.
• Liver enzyme elevations: Temporary ALT/AST increases in a minority of participants, typically tied to dose escalation steps and not associated with liver damage signals.
• Discontinuation rates: 18.2% at 12mg in TRIUMPH-4 and 5.1% in the diabetes-specific TRANSCEND-T2D-1 trial. Direct comparison with semaglutide or tirzepatide trials is difficult given different trial designs.

Retatrutide vs Mounjaro vs Wegovy: How Does It Stack Up?

RetatrutideMounjaro (Tirzepatide)Wegovy (Semaglutide)MechanismTriple (GLP-1 + GIP + Glucagon)Dual (GLP-1 + GIP)Single (GLP-1)Best weight loss in trials30.3%22.5%20.7%Trial duration104 weeks72 weeks72 weeksUK approvalNot yetYes (Mounjaro)Yes (Wegovy)Injection frequencyOnce weeklyOnce weeklyOnce weekly

One important caveat: the 30.3% figure comes from a 104-week trial, while the Mounjaro and Wegovy results are from 72-week trials. Longer trials tend to accumulate more weight loss over time. There is no direct head-to-head comparison yet.

Can You Get Retatrutide in the UK Now?

No. Retatrutide has not been approved by the MHRA and cannot legally be prescribed in the UK outside of a clinical trial.

As of May 2026, the TRIUMPH Phase 3 programme is still ongoing. Eli Lilly is expected to file for FDA approval in 2026 following the TRIUMPH-1 results. MHRA authorisation typically follows FDA approval by twelve months or more.

If regulatory steps proceed smoothly, Retatrutide could realistically be available in the UK in late 2027.

Seven further TRIUMPH trials are expected to report results throughout 2026, covering maintenance dosing, cardiovascular outcomes, and additional patient groups.

A Word on Buying Retatrutide Online

Products labelled 'Retatrutide peptides' or 'Reta for research' are being advertised on TikTok, Telegram, and grey-market supplement sites. The results are compelling — but these are powerful compounds. Without clinical supervision, using unregulated peptides carries serious risk: unknown ingredients, incorrect dosing, bacterial contamination, and zero medical support if something goes wrong.

Under the Human Medicines Regulations 2012, any product presented as treating disease becomes a 'medicinal product' requiring MHRA authorisation — regardless of any 'not for human consumption' disclaimer on the label. Buying from unauthorised sellers offers no legal or medical protection whatsoever.

Phase 3 Timeline: Where Are We Now?

Here's the current state of the TRIUMPH programme as of May 2026:

• TRIUMPH-4 (December 2025): 28.7% average weight loss at 68 weeks in adults with obesity and knee osteoarthritis. 75.8% reduction in knee pain scores.
• TRIUMPH-1 (May 2026): 30.3% weight loss at 104 weeks in 2,339 participants. All three doses (4mg, 8mg, 12mg) tested.
• Seven further TRIUMPH trials: Ongoing — expected to cover maintenance strategies, cardiovascular outcomes, and other populations throughout 2026.

Retatrutide has received Fast Track Designation from the FDA, which may accelerate the US review process. MHRA approval would follow.

Summary

Retatrutide is the most effective weight loss drug ever tested in clinical trials. For men carrying significant excess weight, the 30.3% body weight reduction figure at two years represents a genuine step change from anything currently available.

But it isn't approved yet. It may be 18 months or more before it reaches UK prescribing. And like any medication, individual results vary — what works in a trial population won't look identical for every person.

The smart move is to keep an eye on the TRIUMPH data as it releases, and in the meantime, make progress with the clinically proven options that are already available and working right now.

Your Options While You Wait

Mounjaro and Wegovy are both available now through HeMed's doctor-led programme. Both are MHRA-approved, clinically proven, and delivered with an at-home blood test, clinical review, and ongoing medical support — built specifically for men.

Start your HeMed programme from £59 →

Frequently Asked Questions

Is Retatrutide approved in the UK?
‍No. As of June 2026, Retatrutide has not received approval from the FDA, MHRA, or the European Medicines Agency. It remains an investigational drug in Phase 3 clinical trials. Eli Lilly is expected to file for FDA approval in 2026 following TRIUMPH-1 results.

Will Retatrutide come as a pen injector?
‍Eli Lilly hasn't confirmed the final delivery format. Given that Mounjaro uses a KwikPen injector, a similar device is plausible — but this is speculation at this stage. UK pricing has not been disclosed.

When will Retatrutide be available in the UK?

‍If Eli Lilly files for FDA approval in 2026 and the process runs smoothly, MHRA authorisation could follow by late 2027 or early 2028. Retatrutide's Fast Track FDA designation may accelerate the US timeline, which could bring the UK timeline forward slightly.

How does Retatrutide compare to Mounjaro for men?
‍Retatrutide's trial results exceed Mounjaro's (30.3% vs 22.5% average weight loss), but the trials were different lengths — 104 weeks for Retatrutide vs 72 weeks for Mounjaro. No head-to-head comparison exists yet. Mounjaro is available now; Retatrutide is not.

Sources & Further Reading

1. Jastreboff AM et al. — Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial. NEJM, 2023
2. AJMC — Retatrutide Achieves Up to 30.3% Average Weight Loss in Phase 3 TRIUMPH-1 Trial, May 2026
3. PMC12190491 — Efficacy and Safety of Retatrutide: Systematic Review and Meta-analysis
4. ClinicalTrials.gov — NCT05929066: TRIUMPH Phase 3 Programme
5. Eli Lilly — What to Know About Retatrutide
6. MHRA — Medicines and Healthcare Products Regulatory Agency

‍