For the last several years, one acronym has dominated the conversation around obesity treatment: GLP-1.
First came drugs that targeted the GLP-1 receptor. Then tirzepatide added a second target, combining GLP-1 and GIP receptor activity in a single molecule. Now researchers are testing what happens when a third metabolic pathway is added to the equation.
That drug is retatrutide, an investigational once-weekly medication from Eli Lilly that simultaneously activates receptors for GLP-1, GIP, and glucagon. And on September 29, the New England Journal of Medicine published Phase 3 results from TRIUMPH-1, giving us one of the clearest looks yet at what a triple-hormone obesity drug can do.
The results were significant. In the primary analysis, adults receiving the highest 12-mg dose lost an average of 25% of their body weight over 80 weeks, compared with 3.9% with placebo. At 9 mg, average weight loss reached 23.7%.
But the more interesting story may be what retatrutide represents.
Obesity pharmacotherapy is no longer simply about finding another way to activate GLP-1. Drug development is beginning to combine multiple metabolic signals into single molecules, and that could change what the next generation of these medications looks like.
Why Add a Third Hormone?
To understand retatrutide, it helps to look at how we arrived here.
GLP-1 receptor agonists demonstrated that targeting gut-hormone signaling could affect appetite, glucose regulation, and body weight. Tirzepatide expanded that idea by activating both GLP-1 and GIP receptors.
Retatrutide adds glucagon receptor activity.
That might initially sound counterintuitive to pharmacists. Glucagon is usually associated with increasing blood glucose, which is why it is used to treat severe hypoglycemia. But glucagon signaling also affects energy expenditure and lipid metabolism. The idea behind retatrutide is to combine those effects with the appetite and metabolic effects associated with GLP-1 and GIP receptor activation.
Instead of three separate medications, retatrutide is one molecule engineered to activate all three receptors.
This concept first attracted major attention in 2023, when a Phase 2 trial involving 338 adults with obesity reported average weight reductions of up to 24.2% after 48 weeks at the highest dose. Those results raised an obvious question: would that magnitude of weight loss hold up in a much larger Phase 3 trial?
We now have part of the answer.
What Happened in Phase 3
TRIUMPH-1 randomized 2,339 adults with obesity without diabetes to receive once-weekly retatrutide at doses of 4 mg, 9 mg, or 12 mg, or placebo for 80 weeks.
In the intention-to-treat analysis published in NEJM, average body-weight reductions were 17.6% with 4 mg, 23.7% with 9 mg, and 25.0% with 12 mg. Participants receiving placebo lost 3.9%.
There is an important detail here because you may see slightly different retatrutide numbers reported elsewhere. Lilly previously announced an average 28.3% weight reduction with the 12-mg dose using an efficacy estimand, which estimates the treatment effect under different assumptions about treatment adherence. The newly published NEJM paper reports 25.0% for the 12-mg group using the treatment-regimen, or intention-to-treat, estimand. They are different analyses of the same trial, not conflicting trial results.
TRIUMPH-1 also looked beyond the number on the scale. Among 574 participants with knee osteoarthritis, retatrutide significantly reduced knee pain compared with placebo. Among 243 participants with obstructive sleep apnea, it significantly reduced apnea-hypopnea events.
That matters because obesity medications are increasingly being studied not only for how much weight people lose, but for what happens to the diseases associated with excess weight.
The Next Question Is Bigger Than Weight Loss
Retatrutide is not approved by the FDA and is not currently available for routine clinical use. It remains an investigational medication being studied across a broad Phase 3 program.
And there are still important questions to answer.
The most common adverse events in TRIUMPH-1 were gastrointestinal, consistent with what has been seen across incretin-based therapies. Longer-term safety, tolerability, durability of weight loss, cardiovascular outcomes, what happens after treatment is stopped, and how the drug ultimately compares with established therapies will all matter if retatrutide eventually reaches clinical practice.
There is also another important point: retatrutide should not simply be viewed as “a stronger GLP-1.”
It represents a different strategy.
Semaglutide targets one receptor. Tirzepatide targets two. Retatrutide targets three. Other companies are simultaneously developing combinations involving amylin, glucagon, GLP-1, GIP, and additional metabolic pathways.
The obesity pipeline is becoming a lesson in combination physiology.
What Pharmacists Should Be Watching
For pharmacists, the most important takeaway may not be memorizing another investigational drug name. It is understanding where the therapeutic category is heading.
The conversation around obesity medications is likely to become more complicated, not less. Future treatment decisions may involve medications targeting different combinations of metabolic pathways, different dosing strategies, different effects on comorbidities, and potentially different safety and tolerability profiles.
That means pharmacists will increasingly need to understand why one metabolic pathway is being targeted instead of another, how these drugs differ mechanistically, and which patients may benefit most from each approach.
Retatrutide is also a reminder of how quickly this field is moving. Only a few years ago, the conversation centered largely on GLP-1 receptor agonists. Then dual agonism became a major therapeutic strategy. Now a triple receptor agonist has reached Phase 3 with substantial weight-loss results.
The next generation of obesity treatment may not be about finding the next GLP-1. It may be about figuring out which combination of metabolic signals produces the best balance of weight loss, metabolic benefit, tolerability, and long-term outcomes.
Retatrutide is one of the clearest signs yet that we are already entering that next chapter.
Sources
• Jastreboff AM, Kaplan LM, Davies MJ, et al. Retatrutide, a Triple Hormone Receptor Agonist, for Treatment of Obesity. New England Journal of Medicine. Published September 29, 2026. doi:10.1056/NEJMoa2604169.
• Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial. New England Journal of Medicine. 2023;389:514-526. doi:10.1056/NEJMoa2301972.
• Eli Lilly and Company. Lilly’s Triple Agonist, Retatrutide, Delivered Powerful Weight Loss in Pivotal Phase 3 Obesity Trial. May 21, 2026.
• Eli Lilly and Company. What to Know About Retatrutide. Updated September 2026.
• Eli Lilly and Company. Lilly’s Triple Agonist, Retatrutide, Delivered Substantial Weight Loss and A1C Reduction, Underscoring Its Potential Promise for People With Obesity and Type 2 Diabetes. • September 29, 2026.
