TL;DR

Retatrutide, Eli Lilly's triple-hormone agonist, produced 28.3% average body weight loss at 80 weeks in the TRIUMPH-1 Phase 3 trial — roughly double what Wegovy achieves and nearly 8 percentage points ahead of Zepbound. FDA submission is expected later in 2026.

Every few years, a drug result lands that genuinely resets expectations. TRIUMPH-1 is one of those results. Retatrutide didn't just outperform existing GLP-1 medications - it reached weight-loss territory that previously only appeared in bariatric surgery data.

What TRIUMPH-1 actually tested

The TRIUMPH-1 trial (NCT05929066) enrolled 2,339 participants with obesity or overweight, randomised them to retatrutide at 4 mg, 9 mg, or 12 mg weekly, or placebo, and followed them for 80 weeks. A pre-specified extension enrolled 532 participants with BMI of 35 or above for an additional 24 weeks, reaching 104 weeks total.

Results were announced by Eli Lilly on May 21, 2026, making this the freshest major obesity drug data available right now. Additional Phase 3 trials - TRIUMPH-2 and TRIUMPH-3 - are expected to report later in 2026.

The weight loss numbers, dose by dose

Among participants who stayed adherent to treatment through 80 weeks:

  • 4 mg dose: 47.2 lbs lost - 19.0% body weight reduction
  • 9 mg dose: 64.4 lbs lost - 25.9% body weight reduction
  • 12 mg dose: 70.3 lbs lost - 28.3% body weight reduction
  • Placebo: 5.5 lbs lost - 2.2% body weight reduction

The 104-week extension data on the 12 mg dose went further still: participants lost an average of 85.0 lbs - a 30.3% reduction. That means nearly one-third of total body weight lost over roughly two years, while continuing treatment.

To put those numbers in context: Wegovy (semaglutide 2.4 mg) produces roughly 14.9% weight loss in STEP-1 trial conditions. Zepbound (tirzepatide 15 mg) produces around 20.9% in SURMOUNT-1. Retatrutide at 12 mg adds another 7 to 13 percentage points on top of those benchmarks.

Who hits which targets?

The 12 mg dose data at 80 weeks shows just how far some patients went:

  • 62.5% of participants lost 25% or more of their body weight
  • 45.3% lost 30% or more
  • 65.3% fell below a BMI of 30 - moving out of the obese range entirely
  • Among those who started with a BMI of 40 or above, 37.5% reached a BMI under 30

That last figure is striking. Moving someone from severe obesity (BMI ≥40) to a normal-weight BMI in two years has not been achievable with any pill or injection before. Bariatric surgery typically achieves 25 to 35% excess weight loss at two years, depending on the procedure. Retatrutide is entering that range from a non-surgical intervention.

How retatrutide works differently

Existing GLP-1 medications target one or two hormone receptors. Wegovy and Ozempic activate only the GLP-1 receptor. Zepbound and Mounjaro activate both GLP-1 and GIP receptors - which is why tirzepatide outperforms semaglutide head-to-head in SURMOUNT-5 data (20.2% vs 13.7% weight loss).

Retatrutide adds a third receptor: glucagon. Glucagon increases energy expenditure by raising the rate at which your body burns calories at rest. GLP-1 receptor activity suppresses appetite and slows gastric emptying. GIP receptor activity works with GLP-1 to amplify insulin secretion and may improve the body's fat storage and mobilisation. Hitting all three simultaneously appears to produce an additive - possibly synergistic - effect that neither dual nor single agonism can match.

Lilly's internal portfolio now looks like a ladder: Foundayo (GLP-1 only), Zepbound (GLP-1 + GIP), retatrutide (GLP-1 + GIP + glucagon). Each step up adds another mechanism and another 5 to 8 percentage points of average weight loss.

Safety and tolerability

The side-effect profile at the 12 mg dose reflects the triple mechanism - and it is more significant than existing drugs. At 80 weeks:

  • Nausea: 42.4% of retatrutide patients vs 14.8% on placebo
  • Diarrhoea: 32.0% vs 13.5%
  • Constipation: 26.1% vs 10.9%
  • Discontinuation due to adverse events: 11.3% vs 4.9%
  • Dysesthesia (abnormal skin sensations): 12.5% vs 0.9%

That dysesthesia figure is unusual - none of the current approved drugs carry this signal at this frequency. Researchers are investigating whether it relates to glucagon receptor activity. At the lower 4 mg dose, the discontinuation rate due to adverse events was actually 4% - lower than the 5% placebo rate, suggesting the dose matters considerably for tolerability.

What this means for people currently on GLP-1 medications

Retatrutide is not yet approved. Lilly is expected to submit a New Drug Application to the FDA later in 2026, with a decision possibly in 2027. So if you're currently on Ozempic, Wegovy, Mounjaro, or Zepbound, nothing changes today.

But the TRIUMPH-1 data raises several practical questions worth thinking through now. First, for people who respond partially to current GLP-1 medications - losing 10 to 15% rather than the trial-average 20% - retatrutide may eventually offer a meaningful upgrade. Second, the higher GI side effect rate means nutritional support will matter even more: people losing 28% of body weight need to be intentional about protein, micronutrients, and muscle preservation in ways that current GLP-1 users already face but at a lower intensity.

At GLP-1 Shield, we track all of this because the supplement gaps that open up with 15% weight loss open up further with 28% weight loss. The nutrients at risk - vitamin D, iron, vitamin B12, zinc, protein - don't change, but the magnitude of the depletion likely does.

The broader pipeline shift

TRIUMPH-1 is part of a broader inflection point. CagriSema (Novo Nordisk's cagrilintide-semaglutide combination) reported 14.2% weight loss across the REIMAGINE Phase 3 program at ADA 2026 - solid data, though short of retatrutide's ceiling. Zenagamtide, a GLP-1/amylin dual agonist, showed 14.6% weight loss in early data, also at ADA 2026. Retatrutide stands alone in the 28% territory for now.

The practical implication: the obesity drug market of 2028 will look substantially different from today's. Patients who respond well to current drugs may have even more powerful options coming. Patients who struggle with tolerability may find the newer generation harder to manage without robust nutritional support strategies in place.

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Frequently asked questions

What is retatrutide and how does it differ from Ozempic or Zepbound?
Retatrutide is a triple hormone receptor agonist that activates GLP-1, GIP, and glucagon receptors simultaneously. Ozempic and Wegovy activate GLP-1 only. Zepbound and Mounjaro activate GLP-1 and GIP. Adding glucagon receptor activation increases resting energy expenditure, which appears to explain retatrutide's superior weight-loss results in TRIUMPH-1.
How much weight can you lose on retatrutide?
In the TRIUMPH-1 Phase 3 trial, participants on the 12 mg dose lost an average of 28.3% of body weight (70.3 lbs) over 80 weeks. In a 104-week extension, that reached 30.3% (85.0 lbs). Nearly half of participants on the 12 mg dose lost 30% or more of their body weight.
When will retatrutide be approved?
Eli Lilly is expected to file a New Drug Application with the FDA later in 2026, following the TRIUMPH-1 results. Additional Phase 3 trials - TRIUMPH-2 and TRIUMPH-3 - are still reporting. An FDA approval decision could come in 2027, though the timeline is not confirmed.
What are the side effects of retatrutide?
The most common side effects at the highest dose (12 mg) were nausea (42.4%), diarrhoea (32%), and constipation (26.1%) - higher rates than current approved drugs. An unusual finding was dysesthesia (abnormal skin sensations) in 12.5% of patients, which was not seen with placebo. The discontinuation rate due to adverse events was 11.3%, compared to 4.9% on placebo.