Retatrutide works by activating three hormone receptors at once: GLP-1, GIP and glucagon. The GLP-1 part slows stomach emptying and signals fullness, the GIP part supports insulin release after meals, and the glucagon receptor is the extra piece that separates retatrutide from semaglutide and tirzepatide. It is still an investigational medicine, so how each part works in people is still being studied.
Retatrutide is still investigational, and the published trial results are summarised below.
What each of the three receptors does
The 2023 phase 2 trial, published in the New England Journal of Medicine, describes retatrutide as an agonist of the GIP, GLP-1 and glucagon receptors. An agonist is a substance that attaches to a receptor and triggers the same action as the body’s own hormone, as the Cleveland Clinic’s GLP-1 agonist guide explains.
GLP-1: appetite and digestion
GLP-1 is a hormone the small intestine makes. Cleveland Clinic lists its roles as triggering insulin release, blocking glucagon secretion, slowing stomach emptying and increasing how full you feel after eating. Harvard Health’s Ozempic explainer notes that these medicines mimic a hormone that helps regulate blood sugar and appetite. Retatrutide activates this same receptor.
GIP: insulin release after meals
GIP stands for glucose-dependent insulinotropic polypeptide. Diabetes UK’s GLP-1 medicines page describes tirzepatide as a dual GLP-1 and GIP receptor agonist, and says these medicines raise incretin hormone levels, which help the body make more insulin when it needs it. Retatrutide adds GIP to the GLP-1 action that semaglutide has alone.
Glucagon: the part that sets it apart
Glucagon is the hormone your body uses to raise blood sugar when it is needed, according to Cleveland Clinic. Retatrutide also activates the glucagon receptor. Tirzepatide acts on GLP-1 and GIP, so the glucagon receptor is the main chemical difference between the two. What that does in people is still being worked out, because the trials tested the whole drug at each dose rather than each receptor on its own.
How is retatrutide different from semaglutide and tirzepatide?
A review of how GLP-1 medicines work describes semaglutide as a modified GLP-1 molecule. In practical terms, semaglutide works on one receptor, tirzepatide works on two, and retatrutide works on three.
| Medicine | Receptors it activates | Brand names |
|---|---|---|
| Semaglutide | GLP-1 | Ozempic (diabetes), Wegovy (weight management) |
| Tirzepatide | GLP-1 and GIP | Mounjaro, Zepbound |
| Retatrutide | GLP-1, GIP and glucagon | None; still investigational |
The extra receptor is a difference in how the drug works, not a ranking of results. The next section shows what the trials measured.
What have the retatrutide trials found on weight loss?
The 2023 phase 2 trial enrolled 338 adults with a BMI of 30 or higher, or a BMI of 27 to under 30 plus at least one weight-related condition. Participants received once-weekly injections or placebo for 48 weeks. Average weight change at 48 weeks is shown below.
| Trial group | Average weight change at 48 weeks |
|---|---|
| Placebo | -2.1% |
| Retatrutide 4 mg (combined groups) | -17.1% |
| Retatrutide 8 mg (combined groups) | -22.8% |
| Retatrutide 12 mg | -24.2% |
These are averages for each group, not a prediction for any one person, and the trial doses are research protocol doses rather than dosing guidance. As an everyday picture, someone starting at 200 pounds would be about 48 pounds lighter on average at the 12 mg group’s 24.2%.
The phase 3 TRIUMPH-2 phase 3 trial, published in The Lancet in 2026, enrolled over 1,100 adults with obesity and type 2 diabetes. Average weight change at 80 weeks was -11.9% on 4 mg, -16.8% on 9 mg and -18.8% on 12 mg, compared with -5.1% on placebo. The people and the length of follow-up differ from the phase 2 trial, so the two sets of numbers are not directly comparable.
A body composition substudy, published in The Lancet Diabetes & Endocrinology in 2025 and funded by Eli Lilly, used DXA body scans in 189 adults with type 2 diabetes. Fat mass fell 26.1% in the pooled 8 mg group and 4.5% on placebo at week 36. The authors wrote that the share of lean mass lost along with weight was similar to other obesity treatments.
A 2025 network analysis in Obesity pooled 19 randomized trials of 29,506 adults with overweight or obesity. Retatrutide and the dual agonists averaged 11.0 kg (about 24 pounds) of mean weight loss, compared with 9.0 kg (about 20 pounds) for GLP-1 medicines. Retatrutide had the highest risk of adverse events in that analysis. Because it pools separate trials rather than comparing the drugs in one head-to-head study, it is a comparison across studies, not a direct test.
What are the side effects of retatrutide?
Gastrointestinal effects were the most common side effects in the trials. In the phase 2 trial they were dose-related, mostly mild to moderate, and partly reduced by a lower starting dose. TRIUMPH-2 reported the following, with placebo for comparison.
| Side effect | 4 mg | 9 mg | 12 mg | Placebo |
|---|---|---|---|---|
| Diarrhea | 27% | 34% | 34% | 13% |
| Nausea | 14% | 21% | 28% | 8% |
| Dysesthesia | 4% | 6% | 7% | 1% |
| Hypotension (low blood pressure) | 1% | 5% | 6% | under 1% |
Permanent discontinuation due to adverse events or death was 12% on 9 mg and 8% on 12 mg, compared with 4% on 4 mg and 5% on placebo. In phase 2, heart rate rose with dose, peaked at 24 weeks and declined afterward. The 2026 Lancet report describes the safety profile as generally consistent with other GLP-1 receptor agonists.
Harvard Health notes that some reports link rapid GLP-1 weight loss to lower muscle mass, which matters most for older adults at higher risk of frailty and falls.
Is retatrutide approved yet, and what is still unknown?
As of October 2026, retatrutide is still investigational. The 2026 Lancet report on TRIUMPH-2 describes it as under investigation for obesity. Check the FDA’s own drug listings before you accept any claim that it is approved.
Several questions remain open:
- Long-term results. The longest trial cited here ran 80 weeks.
- Whether results last. The NAM Health Basics on GLP-1s page says many people regain most of the weight they lost after stopping a GLP-1 medicine.
- How much each receptor contributes on its own.
- How retatrutide compares with semaglutide or tirzepatide in a single trial. The figures above come from separate studies.
What about compounded or online versions?
The NAM Health Basics page says the FDA has not assessed compounded GLP-1 formulations for safety, effectiveness or quality, and that some may be improperly stored or fraudulent. It recommends filling prescriptions at a state-licensed pharmacy. For a side-by-side look at how compounded and approved GLP-1 tablets compare, read our oral GLP-1 pills guide. Harvard Health also notes that GLP-1 drugs are not considered safe during pregnancy or breastfeeding, so raise that with your prescriber before any decision.
The next step is simple: write down your medical history and current medicines, and bring them to a prescriber who can tell you which options fit you.
By HealthDataConsortium.org Research Team
This article is for general information purposes only and does not constitute medical advice. Consult your doctor or qualified healthcare provider before making changes to your health routine.
