Retatrutide is an investigational once-weekly injection from Eli Lilly for obesity and type 2 diabetes. It is a peptide that activates three hormone receptors at once (GIP, GLP-1 and glucagon), which is why it is often called a “triple agonist” or, informally, “triple G.” In Phase 3 trials it produced some of the largest average weight losses ever seen with a medicine. It is not FDA-approved yet.

Key facts

• What it is: a single engineered peptide that acts on GIP, GLP-1 and glucagon receptors.
• Who makes it: Eli Lilly (code name LY3437943), the maker of tirzepatide (Zepbound, Mounjaro).
• Best trial result so far: 25.0% average weight loss at 80 weeks on the 12 mg dose in TRIUMPH-1 (people without diabetes, counting everyone who started), vs 3.9% on placebo, per the New England Journal of Medicine (Sept 29, 2026). Among people who stayed on treatment, Lilly reported 28.3%.
• In type 2 diabetes: up to 20.8% weight loss and A1C down up to 1.6 points in TRIUMPH-2 (presented Sept 30, 2026).
• Status: investigational. Lilly plans to submit to the FDA in Q1 2027.

How it works, in plain English

Your gut and pancreas release hormones after you eat. Medicines in this family copy them.

The glucagon piece is what makes retatrutide different. The idea is to combine eating less (GLP-1 and GIP) with burning more (glucagon). That combination is the leading explanation for why its weight-loss numbers run higher than earlier drugs. If you are new to the idea that these medicines are peptides at all, start with Are GLP-1s actually peptides?.

What the trials show

Lilly’s Phase 3 program is called TRIUMPH, with about 5,800 participants across several studies.

• TRIUMPH-1 (obesity without diabetes, 2,339 adults, published in NEJM Sept 29, 2026): 17.6%, 23.7% and 25.0% average weight loss at 4, 9 and 12 mg over 80 weeks, counting everyone who started, vs 3.9% on placebo. Lilly’s figures for people who stayed on treatment were 19.0%, 25.9% and 28.3%. People with a BMI of 35 or higher who stayed on 12 mg to 104 weeks averaged 30.3%.
• TRIUMPH-2 (obesity with type 2 diabetes, Sept 2026): up to 20.8%, with large A1C improvements. Full breakdown here.
• TRIUMPH-3 (obesity with cardiovascular disease, July 2026): about 23% at the highest doses.
• TRIUMPH-4 (obesity with knee osteoarthritis, Dec 2025): 28.7% at 12 mg over 68 weeks, plus up to a 75.8% reduction in knee pain scores.

Side effects

Most are digestive and the same type seen with semaglutide and tirzepatide: nausea, diarrhea, constipation and vomiting, mainly while the dose is being increased. One effect is more specific to retatrutide: dysesthesia, an unusual or uncomfortable skin sensation, reported more often at higher doses (for example 5.1% to 12.5% vs 0.9% on placebo in TRIUMPH-1). Rates of stopping the drug for side effects ranged from about 4% to 11% in TRIUMPH-1, vs about 5% on placebo.

Can you get it?

Not through a normal prescription yet. There are only two legitimate routes today: enrolling in a clinical trial, or Lilly’s limited early access program (opened in August 2026) for adults with severe obesity and serious complications who cannot join a trial, requested by their physician. Anything sold online as “research retatrutide” is not the medicine tested in these trials. More on timing in When will retatrutide be available?, and we track every FDA step on the legal status tracker.

Sources

• Jastreboff et al. Retatrutide, a triple hormone receptor agonist, for treatment of obesity (TRIUMPH-1). N Engl J Med, Sept 29, 2026
• Eli Lilly: What to know about retatrutide
• TCTMD: Retatrutide achieves large weight decreases in patients without diabetes (TRIUMPH-1)
• Pharmacy Times: TRIUMPH-2 results
• BioPharma Dive: TRIUMPH-2 and TRIUMPH-3, FDA filing plans
• Eli Lilly: TRIUMPH-4 topline results
• AJMC: Lilly expands retatrutide access

Educational content only, not medical advice. Retatrutide is an investigational medicine and is not FDA-approved.

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