Retatrutide: What We Know So Far

Retatrutide is one of the most anticipated weight-management drugs in development. In 2026, Eli Lilly reported several large phase 3 trials with strong results. But it is still not FDA-approved, and that matters for anyone thinking about it. Here is what the research shows so far, in plain English.

Key takeaways

• Retatrutide is a once-weekly injectable drug that acts on three hormone receptors: GLP-1, GIP and glucagon.
• In the TRIUMPH-1 trial, people on the highest dose tested lost an average of about 28% of their body weight at 80 weeks.
• Side effects were mostly stomach-related and became more common at higher doses.
• It is investigational. Lilly plans to file with the FDA in early 2027.
• Products sold online as "research use only" retatrutide are not legitimate medicine, and the FDA has warned against them.

What retatrutide is

Retatrutide is a lab-made peptide developed by Eli Lilly. It is often called a "triple agonist" or "triple G" because it turns on three receptors:

• GLP-1, the same target as semaglutide, which helps with blood sugar, fullness and appetite.
• GIP, another gut hormone receptor, also targeted by tirzepatide.
• Glucagon, a hormone that helps the body release stored energy. Researchers are studying whether this part increases energy use and lowers liver fat.

It is given as a weekly injection under the skin in trials. Learn more on our retatrutide page.

How it compares to current GLP-1 drugs

Comparing weight-loss numbers across different trials is tricky. The people enrolled, trial length and methods all differ. Head-to-head trials give the clearest answers, and those are still limited.

What the phase 3 trials show

Lilly's phase 3 program is called TRIUMPH. Here are the results reported so far in 2026.

TRIUMPH-1: obesity without diabetes

This trial enrolled 2,339 adults with obesity, or overweight plus at least one weight-related health problem, who did not have diabetes. According to results reported in May 2026:

• At 80 weeks, average weight loss ranged from about 19% at the lowest dose to about 28% at the highest dose tested.
• In the highest-dose group, about 45% of people lost 30% or more of their body weight.
• In a smaller extension group of people with a starting BMI of 35 or higher, the highest dose reached an average loss of about 30% at 104 weeks.

TRIUMPH-2: obesity with type 2 diabetes

In 1,152 adults with type 2 diabetes, the highest dose produced average weight loss of about 21% at 80 weeks, and A1C dropped by up to 1.6 points. People with diabetes often lose less weight on these drugs than people without it.

TRIUMPH-3: obesity with heart disease

In 1,949 adults with a BMI of 35 or higher and established heart disease, average weight loss at the highest dose was about 23% at 80 weeks. Early data showed fewer major heart events in the retatrutide group, but the full heart-outcome results are still pending.

Knee pain and sleep apnea

Lilly has also reported phase 3 results on knee osteoarthritis pain, with headlines describing meaningful pain relief, and has studied obstructive sleep apnea. Full peer-reviewed details on these are still coming out.

Side effects seen in trials

As with other drugs in this family, the most common side effects were in the gut. In TRIUMPH-1, rates of nausea, diarrhea, constipation and vomiting were higher than placebo and rose with dose. Nausea, for example, was reported by roughly 29% to 42% of people across the dose groups, compared with about 15% on placebo.

Other findings included:

• More people stopped treatment due to side effects at higher doses, about 11% at the top dose versus about 5% on placebo.
• Some participants reported dysesthesia, an unusual skin sensation such as tingling or sensitivity. Most cases were mild to moderate and resolved.
• Upper respiratory and urinary tract infections were also observed.

Researchers start these drugs at low doses and step up gradually, the same approach used with approved GLP-1s. See why GLP-1 doses start low.

Questions that remain

Strong trial numbers do not answer everything. Open questions include:

• Long-term safety. Rare side effects often appear only after a drug is used by many people for years.
• Muscle and lean mass. Very large weight loss raises questions about how much comes from muscle. See GLP-1s and muscle loss.
• Heart outcomes. Final cardiovascular results are still pending.
• What happens after stopping. With other GLP-1 drugs, weight often returns after stopping. The same may be true here.
• Cost and coverage. These will not be known until after any approval.

Why "research use" retatrutide is a bad idea

Because of the hype, retatrutide is sold online in vials labeled "for research purposes" or "not for human consumption." The FDA has warned companies selling these products, and its October 2026 safety page urges consumers not to buy them. There is no lawful way for pharmacies to compound it, since it is not an approved drug. Products like these may contain the wrong amount, the wrong ingredient or contaminants, and nobody checks them.

If you are interested in retatrutide, the legitimate options are to ask your doctor about approved treatments now, or look into enrolling in a clinical trial at ClinicalTrials.gov.

The bottom line

Retatrutide is a promising triple-hormone drug that produced large average weight loss in 2026 phase 3 trials, with mostly stomach-related side effects. It is still investigational, with an FDA filing planned for 2027. Until then, it is available only in clinical trials, and online "research" versions are unsafe.

Sources

• Retatrutide achieves up to 30.3% average weight loss in phase 3 TRIUMPH-1 trial (AJMC)
• Lilly prepares for retatrutide FDA filing following additional phase 3 data (Pharmacy Times)
• FDA's concerns with unapproved GLP-1 drugs used for weight loss
• Retatrutide FDA approval status (Drugs.com)

Frequently asked questions