Retatrutide Half Life: How Long It Stays in Your System

The retatrutide half life is about 6 days: every 6 days, the amount in your blood drops by roughly half. That number comes from Lilly's Phase 1b trial, published in The Lancet in 2022, and Lilly's medical information page adds that levels peak 12 to 72 hours after a shot. By our math, that leaves less than 4% of a dose 4 weeks after your last shot, with traces lingering a little longer. How long the effects last runs on a different clock, and the trials only answer part of that.

Retatrutide is still in Phase 3 trials, and no approved version exists yet. Lilly last said, on September 29, 2026, that it plans to submit retatrutide to the FDA in the first quarter of 2027 as a Biologics License Application (BLA). On October 2, 2026, the FDA decided that retatrutide is a drug, not a biological product, so an application would need to be filed under section 505 of the Federal Food, Drug, and Cosmetic Act. Lilly's appeal on that question is pending in the Seventh Circuit, and we found no ruling as of October 31, 2026.

So there's no prescribing label to quote yet. What follows comes from retatrutide's early trials, Lilly's medical information page, the tirzepatide and semaglutide labels (class evidence, marked as such), and our own arithmetic, which we show.

What is the half-life of retatrutide?

In the Phase 1b trial (Urva and colleagues, The Lancet, 2022), 72 adults with type 2 diabetes took weekly injections for 12 weeks. Of those, 52 got retatrutide, at doses from 0.5 mg up to 12 mg; the rest had placebo or dulaglutide. The authors report that retatrutide's pharmacokinetics were dose proportional and its half-life was approximately 6 days, and they concluded it suits once-weekly dosing. The 2024 Nature Medicine paper on the Phase 2 liver-fat substudy and Lilly's medical information page give the same figure.

Dose proportional matters more than it sounds. A 12 mg dose leaves your body at the same pace as a 2 mg dose; there's just 6 times as much to clear. The half-life doesn't stretch as the dose climbs, at least across the 0.5 to 12 mg range that was tested.

You'll also see the 6-day figure credited to the Phase 2 obesity trial in the New England Journal of Medicine, but that paper's abstract doesn't report pharmacokinetics.

The long half-life is designed in. Lilly's medical information page says retatrutide includes a C20 fatty diacid "that enables albumin binding and prolongs the half-life", and the FDA's substance record puts that 20-carbon chain on the lysine at position 17. Tirzepatide's label uses the same words for its own C20 chain and says tirzepatide is 99% bound to the blood protein albumin. Nobody has published a binding percentage for retatrutide. What the rest of the molecule does at its three receptors is on the how retatrutide works page.

How long does reta stay in your system after the last shot?

Each 6-day stretch halves what's left. After 12 days a quarter remains. After about 20 days it's under a tenth, and after 30 days (5 half-lives) it's about 3%. Falling below 1% takes close to 40 days. Those are our calculations from the Phase 1b half-life, and the chart shows the same curve week by week.

The real curve is messier. A shot soaks in from the fat under the skin and peaks 12 to 72 hours later (Lilly), so the countdown starts half a day to 3 days after the injection. "About 6 days" is also an average, and the abstract doesn't report the spread between people. Everyone in the Phase 1b trial had type 2 diabetes. Phase 1 studies in people with kidney impairment (29 people) and liver impairment (43 people) are listed as completed on ClinicalTrials.gov, with no results posted as of October 2026.

When people quote the 5-half-life rule, "gone" means about 3% left, not zero. With a half-life of about 1 week, semaglutide "will be present in the circulation for about 5 to 7 weeks after the last injectable dose", according to the Wegovy label. That's 5 to 7 half-lives.

Our read: using the same 5 to 7 half-lives for retatrutide gives 30 to 42 days, so 4 to 6 weeks is a fair answer to how long reta stays in your system. It's an analogy with semaglutide, not a measurement, and we found no published detection window for blood or urine tests.

How the half-life shapes weekly dosing

With a 6-day half-life and 7 days between shots, a bit under half of each dose is still around when the next one goes in. So on any fixed dose, levels climb until what you add each week matches what you clear, a balance called steady state. Here's how fast you get there, from our arithmetic with the 6-day half-life (a simple model that leaves out the absorption lag):

At steady state, the level is about 1.8 times what a single dose gives on its own. The Mounjaro and Zepbound labels report the same shape for tirzepatide from measured blood levels: steady state was reached "following 4 weeks of once weekly administration". That's tirzepatide data, not retatrutide. The starting dose page runs this build-up for the first month on 2 mg.

It fits how the Phase 3 trials ran. In TRIUMPH-1, everyone on retatrutide started at 2 mg and stepped up every 4 weeks toward a target of 4, 9 or 12 mg, and TRANSCEND-T2D-2 (the Ozempic head-to-head reported Oct 28, 2026) used the same steps. By our math, each step reaches about 96% of its steady state before the next. Lilly hasn't said that's why it chose 4 weeks, and the Phase 2 obesity trial found a lower starting dose partly eased stomach side effects, so tolerability mattered too. The dosing schedule page lays out the full ramp, including what the labels for weekly drugs say about late shots.

When does retatrutide peak, and does it wear off by day 7?

Retatrutide reaches its highest blood level within 12 to 72 hours of a dose. That's the time to maximum concentration (Tmax) on Lilly's medical information page, which pairs it with the 6-day half-life as the case for weekly dosing. Lilly gives the window only, with no typical (median) time and no breakdown by dose.

In clock terms, a shot at 8 am on Monday peaks somewhere between 8 pm Monday and 8 am Thursday. The two approved relatives peak on a similar clock. Tirzepatide peaks a median of 24 hours after a shot, anywhere from 8 to 72 hours, according to the Zepbound label, and the Wegovy label puts semaglutide's peak 1 to 3 days after an injection. Tirzepatide's 24-hour median sometimes gets quoted for retatrutide, but Lilly hasn't published one. Picking a day and hour around that window is on the best time to take retatrutide page.

It partly wears off before the next shot. In the simple model, the level just before a shot is a bit under half the level after the last one soaked in. The real dip is shallower: with the peak 12 to 72 hours in, there are only 4 to 6.5 days of decline before the next dose. How low levels fall before each shot (the trough) isn't published.

Retatrutide half life vs tirzepatide and semaglutide

All three are weekly shots with a fatty acid chain that binds albumin, and the labels for the two approved drugs fill in what retatrutide's sources leave blank.

By our math, at 5 half-lives tirzepatide (using Mounjaro's 5 days) is down to about 3% in about 25 days, retatrutide in about 30 and semaglutide in about 35. Those come from separate studies in different people; no trial has measured the three side by side. If you're coming off tirzepatide, the switching guide shows how much of it is still on board week by week.

How long for reta to wear off after stopping?

Drug level and effect don't fade on the same schedule. In the Phase 1 single-dose study, the weight reduction persisted up to day 43 after one injection. In the Phase 2 liver-fat substudy, people had regained some weight by the safety check 4 weeks after their last dose (Nature Medicine, 2024), while some retatrutide was still on board.

Because the pharmacokinetics are dose proportional, you can work out how long after the last shot the level takes to fall to where a steady 2 mg weekly dose sits (our math with the 6-day half-life):

Our read: 2 weeks after a last 12 mg shot, you're still carrying a little more retatrutide than someone on the 2 mg starting dose, so a stomach upset or appetite dip that lasts a week or two after stopping fits the numbers. Weight after stopping is on the results page, and how long nausea and other effects hang around is in the side effects timeline.

When the half-life matters for your safety

A drug that takes weeks to clear doesn't stop working the day you skip a shot. That matters most around pregnancy and surgery.

The Wegovy label says to stop at least 2 months before a planned pregnancy because of semaglutide's long half-life. No retatrutide guidance exists, so ask your doctor about timing; the pregnancy and fertility page has what's known.

Before surgery or sedation, the concern is different. The tirzepatide and semaglutide labels report rare cases of stomach contents getting into the lungs (pulmonary aspiration) under anesthesia or deep sedation, and the Zepbound label says to tell the people treating you before any planned procedure. With a tail that lasts weeks, mention your last shot even if it was a couple of weeks ago.

Every figure above assumes the vial holds retatrutide at the stated amount. Lilly says retatrutide "cannot be legally sold or marketed for human use", and in September 2026 Australia's TGA found a vial sold as retatrutide held none, just semaglutide at about 8 times the level in approved products. If you're looking at retatrutide for sale online, a batch-specific lab report showing what's in the vial and how much is the least you'd need before any of this math applies to you.

What we couldn't find published

As of October 2026, none of these is in any retatrutide abstract, Lilly page or trial record we could read:

• a typical (median) time to peak, or peak levels by dose; Lilly gives only the 12 to 72 hour window
• how far levels dip before each weekly shot
• how much the half-life varies from person to person
• results from the completed kidney and liver impairment studies
• results from the completed drug interaction studies, including one with oral contraceptives
• how long retatrutide can be found in blood or urine tests

If you're in a Lilly trial, the study team can answer timing questions before a procedure or a planned pregnancy. Outside a trial, bring the dates and doses of your last few shots to any doctor treating you.