Retatrutide

What is retatrutide?

Retatrutide is an experimental weight-loss and diabetes drug developed by Eli Lilly under the code LY3437943. It is a once-weekly injectable peptide that activates three hormone receptors at once: GLP-1, GIP and glucagon. That has earned it the nickname 'triple G' and puts it one step beyond tirzepatide, Lilly's approved dual agonist sold as Mounjaro and Zepbound.

Retatrutide drew attention in 2023 when a phase 2 obesity trial reported average weight loss of about 24% after 48 weeks on the highest dose, a larger figure than earlier incretin drugs had shown in trials of similar length¹. It is being studied for obesity, type 2 diabetes and fatty liver disease. It is not approved anywhere and cannot be legally prescribed outside a clinical trial. Despite that, it is heavily marketed online as a research chemical.

How retatrutide works

After a meal, the gut releases hormones that tell the pancreas to release insulin and tell the brain that you are full. GLP-1 and GIP are two of these hormones. Drugs like semaglutide copy GLP-1, and tirzepatide copies both. Retatrutide adds a third signal, glucagon. Glucagon is usually thought of as a blood-sugar-raising hormone, but it also pushes the liver to burn fat and may raise the number of calories the body uses. The idea is that appetite reduction from GLP-1 and GIP, plus extra fat burning from glucagon, adds up to more weight loss than either alone, while the GLP-1 and GIP effects keep blood sugar in check.

In more precise terms, retatrutide is a single peptide engineered to act as an agonist at the glucose-dependent insulinotropic polypeptide receptor (GIPR), the glucagon-like peptide-1 receptor (GLP-1R) and the glucagon receptor (GCGR)⁴. A fatty acid modification extends its half-life to about six days, which supports weekly dosing. Glucagon receptor agonism increases hepatic lipid oxidation and energy expenditure, which is the proposed reason for the very large drops in liver fat seen in trials. Animal studies show delayed gastric emptying and reduced food intake⁴. Retatrutide and survodutide are the two most advanced molecules that pair glucagon and GLP-1 receptor activation⁸.

What the research shows on retatrutide

The key published trial is a 48-week phase 2 study in 338 adults with obesity or overweight plus a weight-related condition, funded by Lilly. At 48 weeks, mean weight change was -8.7% at 1 mg, -17.1% at 4 mg, -22.8% at 8 mg and -24.2% at 12 mg, compared with -2.1% on placebo¹. On the 12 mg dose, 83% of participants lost at least 15% of their body weight¹. Weight had not clearly plateaued by the end of the trial.

A substudy of 98 participants with fatty liver disease found that liver fat fell by about 82% at 24 weeks on the 8 mg and 12 mg doses, and 86% of those on 12 mg reached normal liver fat levels². In people with type 2 diabetes, trials have reported HbA1c reductions of up to about 2.2 percentage points and weight loss of up to about 17%⁷.

A 2025 systematic review of placebo-controlled trials in adults without diabetes estimated weight loss of up to 22.1% versus placebo for retatrutide at 48 weeks, compared with 17.8% for tirzepatide at 72 weeks and 13.9% for semaglutide at 68 weeks⁶. The authors stressed that no head-to-head trials existed, so these are cross-trial comparisons, not direct proof of superiority⁶. Another systematic review drew on only three published articles covering 691 randomized participants³.

These are the important limits. The phase 2 trials were relatively small and short, industry funded, and not designed to measure heart attacks, strokes or deaths. Phase 3 trials, known as the TRIUMPH program, are designed to test long-term efficacy and safety in larger and more varied populations⁴. Until regulators have reviewed the full phase 3 data, retatrutide's real-world effect size and safety profile are not settled.

Studied for: Investigational obesity treatment, Type 2 diabetes research, NAFLD research.

Retatrutide reference data

Retatrutide dosage in published research

How retatrutide is used in practice

Legitimately, retatrutide is used only inside clinical trials. Trial protocols start at a low weekly dose and step up over weeks to reduce nausea. The phase 2 study found that starting at 2 mg rather than 4 mg partly reduced gastrointestinal side effects¹.

Outside trials, retatrutide is one of the most popular gray-market peptides. Sellers market it as a 'research chemical not for human use' while customers inject it for weight loss, often copying trial doses from social media. It is not a compounding option: US pharmacies cannot lawfully compound an unapproved drug substance, so any clinic or telehealth service offering retatrutide is operating outside the rules. Products bought this way have not been checked for identity, purity, dose or sterility. Because the drug is long-acting, a mis-dosed or contaminated vial can cause problems that last a week or more.

Retatrutide risks and side effects

The most common side effects in trials were gastrointestinal: nausea, vomiting, diarrhea and constipation. They were dose-related and mostly mild to moderate¹. Heart rate rose in a dose-dependent way, peaking at about 24 weeks before easing¹. A 2026 review linked the higher rates of gastrointestinal events at the top doses partly to rapid dose escalation and higher starting doses⁷.

Like other incretin drugs, it is expected to carry warnings about pancreatitis, gallbladder disease, and thyroid C-cell tumors seen in rodents, which is why people with a history of medullary thyroid cancer or MEN2 are excluded. Slowed stomach emptying affects oral drug absorption and raises concern about retained stomach contents during anesthesia⁸. Combined with insulin or sulfonylureas, it can raise the risk of low blood sugar. It should not be used in pregnancy.

Rapid, large weight loss also removes muscle. Reviews of incretin drugs estimate lean mass losses of roughly 10%, or about 6 kg, and recommend resistance exercise to limit this⁵. Long-term effects of sustained glucagon receptor activation in humans are not yet known.

Reported side effects

Alternatives to retatrutide

Several approved drugs target the same goal with far more data. Tirzepatide (Zepbound for obesity, Mounjaro for diabetes) is the closest relative and produced about 18% weight loss versus placebo in trials⁶. Semaglutide (Wegovy, Ozempic) has the longest safety record of the newer drugs and proven cardiovascular benefit. Liraglutide (Saxenda) is older and less effective but well characterized.

Other options include oral medicines such as phentermine-topiramate and naltrexone-bupropion, and metabolic and bariatric surgery for people with severe obesity. For fatty liver disease, resmetirom (Rezdiffra) and semaglutide are FDA approved for MASH with moderate to advanced fibrosis. Whatever the drug, structured nutrition and resistance training help protect muscle during weight loss⁵.

Questions for a clinician

• What evidence applies to my situation, and at what level?
• What established alternatives should I consider first?
• What monitoring would be necessary, and how often?
• Can this be prescribed at all where I live, or only sold for research?