Eli Lilly’s Retatrutide Yields 28% Weight Loss in Trial

Eli Lilly announced on Thursday that its experimental drug retatrutide led to an average weight loss of 28.3% — roughly 70 pounds — over 80 weeks in a pivotal Phase 3 clinical trial, marking one of the most significant pharmaceutical weight loss results ever recorded. Nearly half of participants on the highest dose lost 30% or more of their body weight, a threshold long associated with bariatric surgery.

“This is the largest weight loss I’ve ever seen in any medication trial,” Dr. Susan Spratt, an endocrinologist and senior medical director at Duke Health, told NBC News. “This is huge.”

What Makes Retatrutide Different?

Retatrutide is a first-in-class “triple agonist” — a once-weekly injectable that simultaneously activates receptors for three gut hormones: GIP, GLP-1, and glucagon. This triple mechanism sets it apart from existing weight-loss drugs:

• Wegovy (semaglutide) — Novo Nordisk — activates only GLP-1, producing ~15% average weight loss
• Zepbound (tirzepatide) — Eli Lilly — activates GLP-1 and GIP, producing ~21% average weight loss
• Retatrutide — Eli Lilly — activates GLP-1, GIP, and glucagon, producing ~28% average weight loss

The addition of glucagon is believed to boost energy expenditure and fat metabolism, according to the company’s press release.

The TRIUMPH-1 Trial Results

The TRIUMPH-1 study enrolled 2,339 adults with obesity or overweight who had at least one weight-related comorbidity but no diabetes. Participants were randomized to receive retatrutide at 4 mg, 9 mg, or 12 mg doses, or a placebo.

Key findings at 80 weeks included:

• 12 mg dose: 28.3% average weight loss (-70.3 lbs)
• 9 mg dose: 25.9% average weight loss (-64.4 lbs)
• 4 mg dose: 19.0% average weight loss (-47.2 lbs) with only a single dose-escalation step
• Placebo: 2.2% average weight loss (-5.5 lbs)

A pre-specified extension for participants with severe obesity (BMI ≥35) who continued on the 12 mg dose for 104 weeks showed even more dramatic results: an average weight loss of 30.3%, or 85 pounds. As STAT News reported, the drug’s effectiveness approaches that of bariatric surgery, which typically produces 25% to 35% total body weight loss within one to two years.

Notably, 65.3% of participants on the 12 mg dose achieved a BMI below 30 — the clinical threshold for obesity — including 37.5% of those who started with class 3 obesity (BMI ≥40).

The Side Effect Challenge

While the efficacy data is striking, the trial also revealed significant tolerability concerns. The discontinuation rate due to adverse events at the 12 mg dose was 11.3%, notably higher than the up to 7% seen in pivotal trials of Wegovy and Zepbound.

Gastrointestinal side effects were common, as is typical with GLP-1 drugs:

• Nausea: 42.4% at 12 mg (vs. 14.8% placebo)
• Diarrhea: 32.0% at 12 mg (vs. 13.5% placebo)
• Vomiting: 25.3% at 12 mg (vs. 4.8% placebo)

The trial also reported higher rates of dysesthesia (abnormal skin sensations) and urinary tract infections compared to placebo — side effects less commonly associated with existing GLP-1 medications.

“It was impressive to see that every dose of retatrutide resulted in clinically meaningful weight reduction for nearly all participants,” said Dr. Ania Jastreboff, lead investigator and professor at Yale School of Medicine, in the company’s announcement. “Importantly, treatment with retatrutide not only resulted in robust weight reduction, but also in clear improvements in assessed cardiometabolic health measures.”

Market and Regulatory Outlook

Eli Lilly shares rose approximately 2% in premarket trading following the announcement, reflecting investor confidence in the drug’s commercial potential. If approved, retatrutide would be the most potent obesity medication on the market, joining Lilly’s existing portfolio that includes Zepbound (injectable tirzepatide) and Foundayo (oral orforglipron).

Lilly has indicated it plans to file for FDA approval as early as 2026. As The Guardian noted, the drug appears to outperform other obesity drugs currently on the market, though no head-to-head comparative trials have been conducted.

What This Means for Patients

For patients with severe obesity who need substantial weight loss, retatrutide could represent a transformative option. Dr. Shauna Levy, medical director of the Tulane Weight Loss Center, told NBC News that while bariatric surgery can provide surgery-level results, “it seems like retatrutide is also going to be an effective tool to help patients with a higher BMI achieve a healthy weight.”

However, experts caution that the drug may be “overpowering” for individuals with mild obesity who need more modest weight loss. The higher side effect profile at the 12 mg dose may also limit real-world adoption compared to Zepbound, which offers a more favorable tolerability profile with 21% average weight loss.

Dr. Natalie Azar, NBC News medical contributor, noted on TODAY that the drug “basically starts to approximate what we see in bariatric surgery,” while also improving cardiac risk factors such as non-HDL cholesterol, waist circumference, and blood pressure.

The Road Ahead

Several important questions remain unanswered. The full results have not yet been published in a peer-reviewed medical journal, and the FDA approval timeline remains uncertain. Additional data from the TRIUMPH program — including results for patients with type 2 diabetes (TRIUMPH-2) and cardiovascular disease (TRIUMPH-3) — are expected later this year.

Pricing and insurance coverage decisions will also be critical. As the weight-loss drug market continues to expand rapidly — with global sales projected to reach $100 billion annually by decade’s end — the balance between efficacy, tolerability, and cost will determine which drugs ultimately reach the most patients.

For now, retatrutide represents a remarkable scientific achievement: a medication that can produce weight loss once thought achievable only through surgery. Whether it becomes a blockbuster or a niche therapy will depend on how well patients tolerate it in the real world.

---

This article was compiled from multiple sources including Eli Lilly’s official press release, STAT News, NBC News, The Guardian, and TODAY.