The short answer: About a month, by arithmetic. A phase 1b trial reported a half-life of about 6 days for retatrutide. A drug is mostly cleared after about 5 half-lives, which works out to roughly 30 days after the last dose. That figure is a calculation and not a measured washout time. Retatrutide is investigational and not FDA approved as of October 2026, so no label states an official number.
What is the half-life of retatrutide?
About 6 days. The figure comes from a phase 1b trial published in The Lancet in 2022. The trial gave weekly injections of retatrutide (then called LY3437943) for 12 weeks to adults with type 2 diabetes. It enrolled 72 people, and some of them received placebo or dulaglutide instead.
The abstract says the drug's pharmacokinetics "were dose proportional and its half-life was approximately 6 days." The authors wrote that this profile suggests the drug suits once-weekly dosing.
A half-life is the time the body takes to remove half of the drug in the blood. After one half-life, half is left. After two, a quarter is left.
An earlier paper, the first-in-human single-dose study in Cell Metabolism, is often cited for the same number. Its abstract does not give a half-life. It says only that the pharmacokinetic profile "supported once-weekly dosing."
Three limits apply to the 6-day figure:
- It comes from one small, early trial.
- Everyone in that trial had type 2 diabetes.
- "Approximately 6 days" is an average. The abstract gives no range between people.
How long until retatrutide is mostly gone?
Pharmacologists use a rule of thumb: a drug is mostly cleared after about 5 half-lives. For a 6-day half-life, the arithmetic looks like this:
- Day 6: about 50% of the drug is left
- Day 12: about 25%
- Day 18: about 12.5%
- Day 24: about 6%
- Day 30: about 3%
These percentages are arithmetic. No trial abstract we read measured retatrutide levels week by week after the last dose.
The rule is not invented for this article. The Ozempic label applies the same logic to semaglutide. It says that with a half-life of about 1 week, semaglutide "will be present in the circulation for about 5 weeks after the last dose."
Dose matters too. The Lancet abstract calls the pharmacokinetics dose proportional, which means a higher dose gives a proportionally higher blood level. So 3% of a high dose is more drug than 3% of a low one. Our post on retatrutide trial doses lists the doses studied.
The phase 3 trials did keep watching people after treatment ended. Lilly's September 29, 2026 release says TRIUMPH-2 "included a post-treatment follow-up period of 4 weeks." The release does not report drug levels from that period.
How does it compare with semaglutide and tirzepatide?
The three half-lives sit close together. The figures for the approved drugs come from their FDA labels.
- Semaglutide (Ozempic, Wegovy): about 1 week. The Wegovy label says semaglutide stays in the circulation for about 5 to 7 weeks after the last injection.
- Tirzepatide (Mounjaro, Zepbound): about 5 days, per the Mounjaro label. The Zepbound label gives 5 to 6 days in people with overweight or obesity.
- Retatrutide: about 6 days, from one phase 1b trial.
These numbers come from separate studies in different groups of people. No study we read compared the three drugs head to head. The safe reading is that all three linger for about a month or longer after the last dose.
There is one large difference. The semaglutide and tirzepatide figures appear in labels that the FDA reviewed. The retatrutide figure has had no such review. For the approved drug with the longest label half-life, see our sister site ozempic.md.
Do the effects wear off as fast as the drug?
No. Drug level and drug effect run on different clocks.
Some effects can outlast a single dose by weeks. In the single-dose study, a reduction in body weight "persisted up to day 43 after a single dose." That is about seven half-lives.
Other effects reverse slowly over months. This article found no published retatrutide weight data from an off-treatment period, so the clearest evidence comes from a related drug. In the SURMOUNT-4 trial of tirzepatide, people lost a mean 20.9% of body weight over 36 weeks. In those who were then switched to placebo, weight rose a mean 14.0% over the next 52 weeks. Tirzepatide's 5-day half-life means the drug itself was gone long before that regain finished.
Tirzepatide is a different molecule, so the result is a guide and not proof. Our post on weight regain after stopping retatrutide covers that evidence in detail.
Side effects can also outlast the last dose. The Mounjaro label says that after an overdose, a period of observation may be necessary, "taking into account the half-life of tirzepatide of approximately 5 days." A drug with a similar half-life would be expected to behave in a similar way. Retatrutide has no label that says so.
Is there an official washout period for retatrutide?
No. A washout period is official advice on how long to wait after the last dose. That advice comes from a drug label, and retatrutide has none.
On September 29, 2026, Lilly described retatrutide as "an investigational molecule that cannot be legally sold or marketed for human use." Lilly said it plans to submit a Biologics License Application to the FDA in the first quarter of 2027.
The approved drugs show what label advice looks like. The Wegovy label says to stop semaglutide at least 2 months before a planned pregnancy "to account for the long half-life of semaglutide." The Ozempic label gives the same 2 months. That advice belongs to semaglutide. It cannot be copied to a different molecule, and this article does not give a stopping time for retatrutide.
The same gap applies to surgery, to switching to another drug, and to pregnancy. Our post on retatrutide and trying to conceive covers what is and is not known there. People in a trial should ask their study team, because trial protocols set their own follow-up rules.
One more caution applies outside trials. The 6-day half-life describes Lilly's trial drug. The FDA page on unapproved GLP-1 drugs, current as of October 1, 2026, says retatrutide "cannot be used in compounding under federal law." It also says the agency has warned companies that sold retatrutide falsely labeled "for research purposes." Nobody has checked what those vials contain, so no half-life can be assumed for them.
The bottom line
Retatrutide has a half-life of about 6 days, according to one phase 1b trial. By the 5 half-lives rule, most of the drug is gone about 30 days after the last dose. That is arithmetic, and nobody has published a measured washout time.
The half-life is close to those of semaglutide (about 1 week) and tirzepatide (about 5 days). Effects on weight follow a slower clock than the drug level. Retatrutide is not FDA approved, so it has no label and no official washout period.
Last updated: October 2026. This article is for informational purposes only and does not constitute medical advice. Retatrutide is investigational; talk with your clinician or study team before you stop it, switch from it, or plan a pregnancy or surgery.