What Is Retatrutide? Clinical Trials, Mechanism & FDA Status

How retatrutide works

Retatrutide is often called a “triple agonist.” An agonist is a molecule that switches a receptor on. Retatrutide switches on three receptors that each play a role in appetite, blood sugar, and energy use:

• GLP-1 receptor. GLP-1 (glucagon-like peptide-1) is a gut hormone released after eating. Activating its receptor increases insulin when blood sugar is high, lowers glucagon, slows stomach emptying, and reduces appetite. Semaglutide works on this receptor alone.
• GIP receptor. GIP (glucose-dependent insulinotropic polypeptide) is another gut hormone. It also boosts insulin after meals and appears to affect how fat tissue handles nutrients. Tirzepatide works on GIP and GLP-1 together.
• Glucagon receptor. Glucagon is made in the pancreas and raises blood sugar by telling the liver to release glucose. It also increases energy expenditure and fat burning. Adding glucagon activity is the new part of retatrutide. Pairing it with GLP-1 and GIP is meant to capture the extra calorie burn while the other two hormones keep blood sugar from rising.

Retatrutide is built on a modified GIP backbone with a fatty acid chain attached, which lets it bind to albumin in the blood and stay active for about a week. In early studies its half-life was roughly six days, which supports once-weekly dosing.

What the clinical trials found

Phase 2 (published 2023)

The phase 2 obesity trial enrolled 338 adults with a BMI of 30 or higher, or 27 or higher with at least one weight-related condition, and no diabetes. At 48 weeks, average weight loss was 24.2% in the 12 mg group compared with 2.1% for placebo. A separate phase 2 trial in type 2 diabetes showed A1c reductions of up to about 2 percentage points at 24 weeks, the trial’s main time point, with similar results at 36 weeks.

Phase 3: the TRIUMPH program

Lilly’s phase 3 program is called TRIUMPH. TRIUMPH-1 and TRIUMPH-2 were published in NEJM and The Lancet on September 29, 2026. Their published treatment-regimen results count people who were randomized, whether or not they stayed on treatment. TRIUMPH-3 and TRIUMPH-4 have so far been reported in press releases, not peer-reviewed papers.

Lilly’s press release also reported 28.3% for TRIUMPH-1 and 20.8% for TRIUMPH-2 at 12 mg, estimates of the effect if everyone had stayed on the drug. Those are different from the published treatment-regimen results above.

Other findings reported so far:

• In the TRIUMPH-1 paper, knee pain scores in people with knee osteoarthritis fell 3.6 points on 12 mg versus 1.9 on placebo. In people with obstructive sleep apnea, breathing pauses or shallow breaths during sleep fell by about 32 per hour on 12 mg versus about 10 on placebo.
• Lilly reported that 45.3% of participants on 12 mg in TRIUMPH-1 lost at least 30% of their body weight. Participants with a BMI of 35 or higher who continued to 104 weeks lost 30.3% on average.
• In TRIUMPH-2, A1c fell by 1.4 to 1.6 percentage points across doses, compared with 0.2 for placebo.
• In TRIUMPH-4, knee osteoarthritis pain scores fell by about 74 to 76% with retatrutide, compared with about 40% on placebo.

Doses used in the trials

The trials tested 4, 9, and 12 mg maintenance doses reached by stepwise increases; this is reported here as trial design, not a schedule for anyone to follow. There is no approved product, strength, or dosing schedule.

Side effects reported in trials

The most common side effects were the same gastrointestinal effects seen with GLP-1 drugs: nausea, diarrhea, vomiting, and constipation. In TRIUMPH-1, Lilly reported in a press release that nausea occurred in 42.4% of participants on 12 mg versus 14.8% on placebo, and 11.3% on 12 mg stopped the drug because of side effects, versus 4.9% on placebo.

Two findings stand out as different from existing drugs:

• Dysesthesia, an abnormal skin sensation such as tingling or increased sensitivity, was reported in 20.9% of participants on 12 mg in TRIUMPH-4 versus 0.7% on placebo, according to a Lilly press release.
• Heart rate increases in the phase 2 trial rose with higher doses, peaked around 24 weeks, then declined. Long-term cardiovascular safety is being studied in TRIUMPH-OUTCOMES.

FDA status

Retatrutide is investigational. That means:

• It is not approved for any use in any country.
• It is available only to people enrolled in clinical trials.
• The FDA has stated that retatrutide cannot be used in compounding under federal law, so a pharmacy cannot legally make it.
• Products sold online as “research” retatrutide are unapproved drugs of unknown quality and are not the trial drug.

Lilly has said it plans to submit a Biologics License Application to the FDA in the first quarter of 2027. Under FDA’s user-fee goals, a standard review aims to finish within 10 months after the 60-day filing period, or about a year from submission. A priority review aims for 6 months after filing. This timeline is an estimate until the FDA accepts the filing.

How retatrutide compares to approved drugs

These are separate trials with different populations, lengths, and analysis methods, so their results cannot be compared directly. With that caveat:

• Semaglutide 2.4 mg (Wegovy): 14.9% average weight loss at 68 weeks in STEP 1.
• Tirzepatide 15 mg (Zepbound): 20.9% at 72 weeks in SURMOUNT-1.
• Retatrutide 12 mg: 25.0% at 80 weeks in the published TRIUMPH-1 treatment-regimen analysis; Lilly’s press release also reported 28.3%, an estimate of the effect if everyone had stayed on the drug.

Only a head-to-head trial can show whether retatrutide outperforms tirzepatide. TRIUMPH-5 is the head-to-head trial; it had not reported results as of September 2026.