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Retatrutide UK 2026: TRIUMPH-1 Phase 3 Results, 28.3% Weight Loss and the Complete Clinical Guide

What Is Retatrutide?

Retatrutide (development code LY3437943) is an investigational once-weekly injectable weight loss medication developed by Eli Lilly and Company — the same pharmaceutical company behind Mounjaro (tirzepatide) and the oral GLP-1 pill orforglipron (Foundayo). It is the world’s first triple hormone receptor agonist to reach Phase 3 clinical trials for obesity treatment.

Retatrutide is currently investigational — it has not received marketing authorisation from the MHRA, FDA, or EMA and cannot be prescribed or dispensed anywhere in the world outside of clinical trials. The TRIUMPH Phase 3 clinical programme is ongoing. Regulatory submission is expected in 2026–2027, with UK private availability most likely in 2027–2028 under an optimistic timeline.

It is generating extraordinary clinical interest because the Phase 3 trial data — published on 21 May 2026 — shows weight loss results that no licensed medication has achieved before. At the highest dose, participants on retatrutide lost an average of 28.3% of their body weight at 80 weeks, with 45.3% of patients losing 30% or more — a threshold previously only achievable with bariatric surgery.

The Triple Agonist Mechanism: Why Retatrutide Is Different

To understand why retatrutide produces greater weight loss than any previous medication, you need to understand what makes it mechanistically different. Every licensed weight loss injection currently available in the UK acts on one or two hormone receptor pathways. Retatrutide acts on three simultaneously.

Here is how each receptor contributes, and why three is fundamentally different from two:

GLP-1 (Glucagon-Like Peptide-1)

GLP-1 is a gut hormone released after eating. It signals fullness to the brain, slows gastric emptying, and stimulates insulin release. GLP-1 receptor activation is the mechanism shared by all current UK-licensed weight loss injections — Wegovy (semaglutide), Saxenda (liraglutide), and Mounjaro (tirzepatide). For a full explanation of how GLP-1 activation drives weight loss, see our guide to how Mounjaro works and our article on GLP-1 agonists and systemic inflammation.

GIP (Glucose-Dependent Insulinotropic Polypeptide)

GIP is the second gut hormone targeted by Mounjaro’s dual mechanism. GIP receptor activation enhances insulin secretion and has additional metabolic effects on adipose tissue, amplifying the appetite-suppressing effect of GLP-1 stimulation. This is why Mounjaro produces greater average weight loss (22.5%) than GLP-1-only medications like Wegovy (14.9–20.7%). Retatrutide activates GIP receptors as well — building on Mounjaro’s dual mechanism.

Glucagon (GCG Receptor) — The Key Differentiator

This is what makes retatrutide unique. The glucagon receptor is activated by glucagon — a hormone that raises blood sugar and, critically, increases energy expenditure by promoting fat burning from adipose tissue. No other currently licensed weight loss medication in the UK activates the glucagon receptor.

By adding glucagon receptor activation to GLP-1 and GIP agonism, retatrutide adds a third distinct mechanism to the weight loss equation: not just reduced calorie intake (GLP-1, GIP) but increased calorie burning (glucagon). This is theorised to be the key reason why retatrutide’s trial results exceed even Mounjaro’s impressive numbers — and it is a mechanism that is explored in much more depth in the full section below.

The Glucagon Receptor: Why It Matters So Much

Most guides to retatrutide mention the glucagon receptor but do not explain it. This section does — because understanding it is essential to understanding why retatrutide may be in a genuinely different category from all previous weight loss medications.

Glucagon is a hormone produced by the alpha cells of the pancreas. Its primary function in normal physiology is to raise blood sugar when it falls too low — it does this by stimulating the liver to release stored glucose. This is why glucagon is used in emergency treatment of severe hypoglycaemia.

But glucagon has a second, metabolically important function: it increases energy expenditure by stimulating the breakdown of stored fat (lipolysis) and promoting thermogenesis — heat production from metabolic activity. In simple terms: glucagon receptor activation causes the body to burn more calories, even at rest.

The reason glucagon was not targeted in previous weight loss medications is a counterintuitive paradox: glucagon also raises blood sugar, which seemed counterproductive in a medication also targeting GLP-1 receptors for insulin regulation. The insight behind retatrutide’s design is that when glucagon receptor activation is combined with GLP-1 and GIP agonism — which themselves stimulate insulin release and improve insulin sensitivity — the blood sugar-raising effect of glucagon is counterbalanced, leaving only the beneficial energy-expenditure and fat-burning effects.

The clinical result is a medication that works through three complementary pathways simultaneously:

• Reduced appetite — via GLP-1 and GIP receptor activation (eating less)
• Improved metabolic efficiency — via GIP receptor activation on adipose tissue (storing fat less efficiently)
• Increased energy expenditure — via glucagon receptor activation (burning more calories)

This triple mechanism explains why retatrutide’s Phase 3 trial results exceed Mounjaro’s, which already uses two pathways. The addition of the glucagon component appears to produce a synergistic effect that goes beyond simple additive benefit.

TRIUMPH-1 Phase 3 Results: The Data in Full

On 21 May 2026, Eli Lilly published topline results from TRIUMPH-1 — the largest and longest Phase 3 obesity trial ever conducted for retatrutide. These are the most important numbers in the guide.

Trial design

TRIUMPH-1 was a randomised, double-blind, placebo-controlled Phase 3 trial. It enrolled 2,339 adults with obesity or overweight (BMI ≥27) with at least one weight-related health condition, without type 2 diabetes. Mean baseline BMI was approximately 40 kg/m². Mean starting weight was 112.7 kg. All participants received lifestyle intervention alongside medication or placebo. Trial duration was 80 weeks, with a pre-specified blinded extension to 104 weeks for a subset.

Primary results at 80 weeks

All three doses met the primary and all key secondary endpoints of the trial.

Weight Loss Outcome Distribution: Who Achieves What

Average weight loss figures tell you the middle of the distribution. This table shows what happens across the full range of participants — and why the figures at the top end are genuinely unprecedented in obesity pharmacotherapy.

The 45.3% of participants on the highest dose losing 30% or more of their body weight is the figure that has generated the most clinical discussion globally. This is the threshold associated with bariatric surgery outcomes in published literature. Achieving it through pharmacotherapy — in nearly half of a trial population — is without precedent in the history of obesity medicine.

TRIUMPH-1 Extension: Results to 104 Weeks

A pre-specified blinded extension of TRIUMPH-1 followed 532 participants with a baseline BMI of 35 or higher who completed the main 80-week study. The results are particularly significant because they address a critical clinical question: does weight loss plateau or continue with extended treatment?

The extension data shows that weight loss at the 12mg dose continued to accumulate between week 80 and week 104 — without an obvious plateau. This is an important finding because it suggests that, unlike some medications where efficacy plateaus at maintenance dose, retatrutide may continue producing results with longer treatment duration. The full implications of this for treatment duration recommendations will be addressed in formal prescribing guidance if and when the medication is approved.

TRIUMPH-4: Knee Osteoarthritis — The Most Overlooked Trial

TRIUMPH-4 is the retatrutide trial that most guides overlook — but for patients with obesity-related joint pain, it may be the most clinically relevant.

TRIUMPH-4 evaluated retatrutide in adults with obesity and knee osteoarthritis. Published by Eli Lilly in December 2025, the results showed:

• Average weight loss of 28.7% at the 12mg dose over 68 weeks — comparable to TRIUMPH-1 at the same dose
• Significant reductions in WOMAC pain scores — a validated measure of knee pain and function
• More than 1 in 8 patients on retatrutide were completely free of knee pain by the end of the trial — compared with less than 1 in 25 on placebo
• Significant improvements in physical function, stair climbing, and walking distance

This matters clinically because knee osteoarthritis and obesity are deeply intertwined — each kilogram of body weight lost reduces knee joint load by approximately 4 kilograms during walking. The combination of direct weight loss and the anti-inflammatory GLP-1 mechanism (which we cover in our guide to GLP-1 agonists and joint pain) may produce benefits in joint disease that go beyond what weight loss alone would achieve.

This is also consistent with data from the STEP 9 trial for semaglutide, which showed significant improvements in knee pain scores in adults with obesity and osteoarthritis — suggesting a class effect beyond simply the weight loss itself.

The Full TRIUMPH Clinical Programme

TRIUMPH-1 is the headline trial, but Eli Lilly’s retatrutide programme spans multiple Phase 3 studies covering different patient populations and indications. Understanding the full programme helps explain both the clinical breadth of retatrutide’s potential and the regulatory timeline for UK approval.

Eli Lilly has confirmed that seven additional Phase 3 readouts are anticipated through the rest of 2026, following TRIUMPH-1. The breadth of this programme — obesity, diabetes, adolescents, joint disease, sleep apnoea, cardiovascular outcomes, kidney disease — reflects retatrutide’s potential as a treatment for obesity-driven multi-organ disease rather than a standalone weight loss medication.

Cardiometabolic Benefits Beyond Weight Loss

TRIUMPH-1 reported clinically meaningful improvements across multiple cardiometabolic risk markers alongside the weight loss data. These are not secondary curiosities — for patients with obesity who also live with hypertension, dyslipidaemia, or elevated inflammatory markers, systemic risk reduction may be as clinically important as the weight loss itself.

The waist circumference reduction of approximately 24.1 cm at the 12mg dose is particularly striking — this represents a clinically meaningful change in visceral adiposity (fat around the organs) which is the component of obesity most strongly associated with cardiometabolic risk, independent of overall BMI.

Retatrutide vs Mounjaro: The Honest Comparison

Both are Eli Lilly medications. Both use the GLP-1 + GIP dual mechanism. Retatrutide adds the glucagon receptor. Here is the clinical comparison — but with an important caveat: these are not head-to-head trial data. Patient populations, trial durations, and protocols differ between the TRIUMPH and SURMOUNT programmes.

Retatrutide vs Wegovy: All Formulations

Full UK Weight Loss Treatment Comparison 2026

All Slinic prices fixed 2026 per pen + £4.99 delivery. No subscription. Subject to clinician approval. Pipeline timelines are indicative estimates.

Side Effects of Retatrutide: What the Trial Data Shows

Retatrutide’s side effect profile is broadly consistent with other GLP-1 medications, with additional considerations at higher doses. The most commonly reported effects are gastrointestinal and occur primarily during the dose-escalation phase.

Key safety observations from TRIUMPH-1

Dysaesthesia: An abnormal sensation of touch — tingling, numbness, or an altered feeling of normal sensations — was reported at higher rates at 9mg and 12mg than at lower doses or with comparator medications. In most cases these events were mild and rarely led to treatment discontinuation, but it is a safety signal that will be monitored closely across all remaining TRIUMPH readouts. Dysaesthesia was also reported at higher rates with the Wegovy 7.2mg injection (22.9%) and appears to be a class-related phenomenon at higher GLP-1 agonist doses.

Discontinuation rate: The 11.3% discontinuation due to adverse events at the 12mg dose is higher than seen with Mounjaro (approximately 7%) at its highest dose. This is clinically relevant — it means approximately 1 in 9 patients on the highest retatrutide dose in the trial stopped treatment due to side effects. The full safety profile will continue to be characterised as remaining trials report.

Gallbladder events: As with all GLP-1 class medications, gallbladder-related events including gallstones were observed in the retatrutide trials. This is a known class effect, more common with rapid weight loss. When retatrutide is licensed, prescribers will follow standard monitoring guidance.

For side effect management strategies applicable to all GLP-1 medications, see our comprehensive Mounjaro side effects guide.

Lean Muscle and Bone Density: The Concern Most Guides Ignore

This is the section that almost no competitor guide covers — but for patients considering any significant weight loss medication, it is clinically important.

When any person loses a significant amount of weight — whether through medication, surgery, or lifestyle changes — the loss is not exclusively fat. Some proportion of the weight lost is lean mass, including skeletal muscle and, in some cases, bone mineral density. The higher the rate of weight loss, and the greater the total weight lost, the more significant this concern becomes.

What the data shows for GLP-1 medications

Analyses from GLP-1 trial data indicate that approximately 25–40% of weight lost on semaglutide and tirzepatide is lean mass rather than fat mass. This is broadly consistent with other weight loss methods at comparable rates of weight loss. For most patients this is clinically manageable — the fat loss is greater and the absolute muscle loss is proportionally small relative to the significant health benefits of obesity treatment.

However, for retatrutide — which produces 28–30% average weight loss rather than 15–22% — the absolute amount of lean mass potentially lost is proportionally larger. A patient losing 35 kg on retatrutide may lose more lean mass in absolute terms than a patient losing 20 kg on Mounjaro, even if the percentage of lean mass lost is similar.

How to protect lean muscle mass on GLP-1 medications

The evidence-based strategies for preserving lean mass during GLP-1 treatment are well established:

• Adequate protein intake — the most important intervention. Aim for at least 1.2–1.6g of protein per kilogram of target body weight per day. Protein prioritisation signals the body to preserve muscle during a caloric deficit.
• Resistance training — progressive resistance exercise (weights, resistance bands, bodyweight training) provides a powerful stimulus to maintain muscle mass during weight loss. Even 2–3 sessions per week produces meaningful preservation of lean mass.
• Adequate calorie intake — very low calorie intake amplifies lean mass loss. The appetite suppression from retatrutide must not lead to dangerously low calorie consumption. Your monthly clinical check-in at Slinic specifically includes dietary adequacy assessment.
• Vitamin D and calcium — particularly relevant for bone density, which can be affected by rapid weight loss. Supplementation should be discussed with your clinician.

Retatrutide Dosing Schedule

Based on trial protocols, retatrutide is a once-weekly subcutaneous injection administered into the abdomen, thigh, or upper arm — identical in technique to Mounjaro and Wegovy. The dose-escalation protocol in TRIUMPH-1 was as follows:

The final licensed dosing schedule will be defined by the MHRA-approved Summary of Product Characteristics if and when regulatory approval is granted. The above reflects trial protocols and may differ from eventual licensed prescribing guidance.

UK Availability Timeline: When Can You Get Retatrutide?

Urgent Warning: Unlicensed “Retatrutide” Being Sold Online

This section is important and I want to be direct with you.

A significant number of websites — some of which appear in Google search results for “retatrutide UK” — are selling products labelled as retatrutide, claiming they are for “research purposes only.” These are not Eli Lilly’s retatrutide. They cannot be. Eli Lilly has not licensed retatrutide to any third-party manufacturer and the product is not available commercially anywhere in the world.

What these websites are selling is unregulated research-grade peptide compounds of unknown purity, dosing accuracy, and safety profile. Using them carries serious risks:

Retatrutide Beyond Obesity: The Bigger Clinical Picture

Most patients searching for retatrutide are focused on weight loss. But Eli Lilly’s TRIUMPH programme is designed to establish retatrutide as a treatment for obesity-driven multi-organ disease — not just a weight loss injection. Understanding the broader clinical picture helps explain why retatrutide may eventually be prescribed for conditions that currently require separate treatments.

Type 2 Diabetes: TRANSCEND-T2D-1

In March 2026, Eli Lilly published topline results from TRANSCEND-T2D-1 — the Phase 3 trial of retatrutide in adults with type 2 diabetes. The trial met its primary endpoint of HbA1c reduction at 40 weeks, with significant improvements in blood sugar control alongside meaningful weight loss. For patients with both obesity and type 2 diabetes, retatrutide’s dual effect on weight and glycaemic control may represent a clinically superior option to existing diabetes treatments that produce little weight loss. The full implications for diabetes prescribing will be assessed by NICE if and when retatrutide reaches the UK market.

Liver Disease: MASLD and NAFLD

Metabolic dysfunction-associated steatotic liver disease (MASLD — formerly NAFLD) affects approximately 1 in 3 UK adults with obesity. Retatrutide’s glucagon receptor component is thought to be particularly beneficial for liver fat reduction — glucagon activation promotes fat mobilisation from the liver as well as from peripheral adipose tissue. Early-phase data showed significant liver fat reductions in retatrutide-treated participants. A dedicated liver outcomes trial is anticipated as part of the broader clinical programme.

Sleep Apnoea: TRIUMPH-5

Obstructive sleep apnoea affects approximately 1 in 4 adults with obesity and is strongly driven by weight-related anatomical changes in the upper airway. TRIUMPH-5 evaluates retatrutide in adults with obesity and sleep apnoea. Results are expected in 2026–2027. If positive, they would add sleep apnoea to the list of obesity comorbidities in which retatrutide demonstrates clinical benefit — strengthening the NICE cost-effectiveness case for NHS access by demonstrating benefit across multiple conditions simultaneously.

Kidney Disease: TRANSCEND-CKD

Chronic kidney disease (CKD) is a growing comorbidity of obesity and type 2 diabetes. The TRANSCEND-CKD trial is investigating whether retatrutide’s anti-inflammatory and weight-reducing effects translate to preserved kidney function in patients with CKD stage G3. Topline results are expected during 2026. This will be hypothesis-generating rather than a regulatory endpoint trial, but the results will inform the design of a potential dedicated Phase 3 kidney outcomes study.

Cardiovascular Outcomes: TRIUMPH-Outcomes

The cardiovascular outcomes trial — TRIUMPH-Outcomes — is the longest and most commercially significant trial in the programme. It will determine whether retatrutide produces a reduction in major adverse cardiovascular events (MACE) comparable to or greater than the SELECT trial result for semaglutide (20% reduction). If positive, this would give retatrutide a cardiovascular benefit label — one of the key differentiators currently held only by Wegovy. Results are expected 2027–2028 and will be critical for NHS NICE appraisal and reimbursement decisions.

Retatrutide vs Bariatric Surgery: Closing the Gap

One of the most discussed aspects of retatrutide’s trial data is its proximity to bariatric surgery outcomes. This comparison deserves careful clinical framing.

Bariatric surgery — including Roux-en-Y gastric bypass and sleeve gastrectomy — produces average weight loss of 25–35% of starting body weight in published surgical series, with some patients achieving 40% or more. For decades, this was a level of weight loss that no pharmacotherapy could approach.

Retatrutide has changed this significantly. At the 12mg dose in TRIUMPH-1:

• Average weight loss of 28.3% at 80 weeks — within the range of bariatric surgery outcomes
• 45.3% of participants lost 30% or more — the threshold associated with bariatric surgery in published literature
• 65.3% achieved BMI below 30 — no longer classifiable as obese by standard criteria
• Extension data to 104 weeks shows 30.3% average — continuing to approach surgical outcomes with longer treatment

The comparison is not a simple equivalence. Bariatric surgery produces more durable weight loss in some patients, addresses mechanical aspects of obesity (gastric capacity reduction), and does not require continued medication. Retatrutide requires ongoing weekly injection and produces weight regain if discontinued without established lifestyle habits — as shown in the SURMOUNT-4 extension data for Mounjaro.

But for patients who are not candidates for bariatric surgery, who do not want surgery, or who are on NHS waiting lists for procedures that may be years away, the pharmacological achievement represented by retatrutide’s data is genuinely significant. And critically — unlike surgery, pharmacotherapy is reversible. If a patient experiences adverse effects or their clinical circumstances change, they can stop the medication. That option does not exist after surgery.

What to Do While Waiting for Retatrutide

Retatrutide is not available in the UK. The most optimistic realistic timeline for private UK availability is late 2027. For most patients, that means waiting 12–18 months or longer.

My clinical recommendation is the same as for orforglipron: do not wait if you can safely start treatment today. Here is why:

• Every month without treatment is a month without clinical progress — and obesity is a progressive condition
• Mounjaro already produces 22.5% average weight loss — extraordinary results that would have seemed impossible five years ago
• Starting now means you could reach or exceed your weight goal before retatrutide is even available
• If retatrutide is approved and you want to switch, your Slinic clinician will manage that transition

Start with Mounjaro — Eli Lilly’s proven treatment available now

Mounjaro is the strongest licensed weight loss treatment available in the UK today. Made by Eli Lilly — the same company developing retatrutide. Average weight loss of 22.5% at the 15mg dose. Available at Slinic now with no waiting list. See our Mounjaro dosing guide, week 1 and month 1 results, and eligibility guide.

Or start with Wegovy — available from £99.99

Wegovy produces 14.9–20.7% average weight loss depending on dose. Available at Slinic from £99.99/pen. The Wegovy pill (MHRA approved 11 June 2026) will also be available within weeks for patients who prefer a tablet.

Frequently Asked Questions

Clinical References

1. Eli Lilly press release. TRIUMPH-1 Phase 3 topline results: retatrutide in adults with obesity. 21 May 2026. (Topline data — full peer-reviewed publication pending.)
2. Eli Lilly press release. TRIUMPH-4: retatrutide in adults with obesity and knee osteoarthritis. December 2025.
3. Eli Lilly press release. TRANSCEND-T2D-1: retatrutide in type 2 diabetes. March 2026.
4. Jastreboff AM, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — Phase 2 Trial. NEJM. 2023;389(6):514-526.
5. Jastreboff AM, et al. Tirzepatide (Mounjaro) for the Treatment of Obesity (SURMOUNT-1). NEJM. 2022;387(3):205-216.
6. Aronne LJ, et al. Tirzepatide vs Semaglutide for Obesity. NEJM. 2025;393(1):26-36.
7. Wharton S, et al. Once-weekly semaglutide 7.2mg (STEP UP). Lancet Diabetes & Endocrinology. 2025;13(11):949-963.
8. Lincoff AM, et al. Semaglutide and Cardiovascular Outcomes (SELECT). NEJM. 2023;389(24):2221-2232.
9. NICE TA1026 — Tirzepatide for managing overweight and obesity.
10. GPhC guidance for online pharmacies. Updated February 2025. pharmacyregulation.org

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