Switching From Tirzepatide to Retatrutide: Doses, Washout and What Nobody Has Tested

Switching from tirzepatide to retatrutide has never been tested in a published trial, so there's no proven dose conversion, no proven waiting period and no approved way to do it yet. Retatrutide's numbered obesity trials, TRIUMPH-1 through TRIUMPH-9, all turned away people who'd taken a weight-loss drug in the 90 days before screening, and in the Phase 3 trials that have reported so far, everyone started at 2 mg and stepped up every 4 weeks. Retatrutide is still investigational, and no approved version exists yet.

Any chart that turns your tirzepatide dose into a retatrutide dose can't cite a study for it, because there isn't one. The closest evidence comes from the Zepbound label and from trials in people new to each drug. Zepbound and Mounjaro are both tirzepatide, so everything below applies to a switch from either brand.

Can you switch from tirzepatide to retatrutide?

Not in any way that's been studied. We read the eligibility criteria for TRIUMPH-1 through TRIUMPH-9 on ClinicalTrials.gov, and each one excludes anyone who took a weight-loss drug, over-the-counter ones included, in the 90 days before screening. Each also turns away people whose weight changed by more than 5 kg (11 lb) in those 90 days. Even TRIUMPH-5, the trial pitting retatrutide against tirzepatide, has both rules. Lilly counts more than 5,800 people in its 4 registrational TRIUMPH trials, and none of them started retatrutide with recent tirzepatide on board.

So every retatrutide number describes people who'd been off weight drugs for at least 90 days with a steady weight, not people just off tirzepatide. No trial can tell you how long weight loss takes to restart after a switch, and any timeline you read online is one person's report.

Is there a tirzepatide to retatrutide dose conversion?

No. The two are different molecules, and a milligram of one doesn't equal a milligram of the other. Lilly hasn't published a conversion and no trial has tested one, so the dose-matching tables on other sites are guesses.

What you can compare is outcomes. The table lines up average weight loss from two separate trials, both reported with the treatment-regimen method (everyone randomized, whether they stayed on the drug or not).

Sources: SURMOUNT-1 in NEJM (2022), TRIUMPH-1 in NEJM (September 2026) for weight loss, and Lilly's TRIUMPH-1 release (May 2026) for the stopping rates. The trials enrolled different people for different lengths of time, so treat a gap of a point or two as noise. The Zepbound label pools more trials and gives 6.7% stopping for 15 mg, which is why you may see that figure instead of 6.2%.

Our read: tirzepatide 15 mg's average result sits between retatrutide 4 mg and 9 mg. That compares outcomes in people new to each drug, and it can't tell you where someone coming off 15 mg would land. Our retatrutide vs tirzepatide page covers the full comparison, and retatrutide vs Mounjaro covers the diabetes brand.

Why the trials start everyone at 2 mg

In TRIUMPH-1, everyone on retatrutide began at 2 mg once a week. The 4 mg group took one step, the 9 mg group went through 2, 4 and 6 mg, and the 12 mg group through 2, 4, 6 and 9 mg, each step lasting 4 weeks. That puts 12 mg at week 17 at the earliest. The Zepbound label follows the same idea: 2.5 mg for 4 weeks, then 2.5 mg steps, so 15 mg takes at least 20 weeks to reach.

The best evidence on starting higher comes from retatrutide's Phase 2 obesity trial, which randomized some groups to start at 2 mg and others at 4 mg. ClinicalTrials.gov posts the per-arm results.

In the 8 mg groups, starting at 4 mg bought about 2 extra points of weight loss and more than tripled nausea. The NEJM authors wrote that stomach side effects were partly reduced by the lower starting dose. These groups held 33 to 35 people each, and the trial excluded anyone using a weight-loss medicine. A body used to tirzepatide might tolerate a higher start, but nobody has measured that, and it's the claim the conversion charts rest on. The registered trial built to compare escalation schemes is TRIUMPH-9, a Phase 3b with primary completion estimated for October 2028. Every trial schedule is in the retatrutide dosage guide, and the 2 mg step has its own page on the starting dose.

Tirzepatide washout, week by week

Tirzepatide doesn't stop working on the day you skip your shot. The Zepbound label puts its half-life at about 5 to 6 days, with blood levels peaking a median of 24 hours after the injection. From that half-life, here's our estimate of the share still in your body after the peak.

A worked example: if your last tirzepatide shot was 15 mg and you started something new a week after it peaked (8 days after the shot), roughly 4 in 10 parts of that tirzepatide would still be in your body on day 1 of the new drug. Four weeks after the peak, it's down to 2% to 4%. Real levels vary from person to person, so treat these as ballpark figures.

This is why "no washout needed" advice is shaky. Both drugs act on the GLP-1 and GIP receptors, so the first weeks of anything new sit on top of leftover tirzepatide. The Zepbound label says tirzepatide isn't recommended alongside other tirzepatide products or any GLP-1 receptor agonist. Overlapping on purpose is a separate question nobody has tested; our page on retatrutide and tirzepatide together covers it. The label also advises people on the pill to use a non-oral method or add a barrier method for 4 weeks after starting and after each dose increase. Lilly ran a study of retatrutide with a birth-control pill (NCT06039826), but as of October 31, 2026 no results are posted.

Legal routes for switching from tirzepatide to retatrutide

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. Lilly's October 2026 FAQ says retatrutide "has not been reviewed or approved by the FDA or any regulatory agency in the world, and no one can legally sell it for human use." Our is retatrutide legal page has the rest. There are two ways in.

One is a trial. The TRIUMPH obesity trials have finished enrolling. As of October 31, 2026, ClinicalTrials.gov lists two retatrutide studies as recruiting. One is Lilly's SYNERGY-Outcomes, for adults with fatty liver disease (MASLD) at higher risk of liver complications, and it excludes anyone who lost more than 11 pounds in the 3 months before screening. That rules out many people mid-way down on tirzepatide. The other, NCT07467447, is sponsored by Hudson Biotech, not Lilly, with one site in Shenzhen, China. Our clinical trials page tracks the rest.

The other is expanded access. Lilly's single-patient program (NCT07629401, first posted June 2026) lists these criteria:

• Age 18 or older
• BMI of 35 or more despite sticking with and tolerating the highest available dose of a weight-management drug (for Zepbound, that maximum is 15 mg)
• 2 or more serious or life-threatening obesity-related conditions, already being treated
• Unable to join a retatrutide trial, because of eligibility or distance
• All standard options, including bariatric surgery, talked through with your doctor

Your doctor makes the request through the Lilly Answers Center (1-800-545-5979); Lilly asks patients not to call directly. Whether you get it depends on where you live, supply and Lilly's review of your case.

Outside those routes, what exists is vials labeled as retatrutide from peptide sellers. None is Lilly's product, and Lilly's October 2026 FAQ says illicit products may hold unknown ingredients, contaminants, the wrong amount of active ingredient or the wrong ingredient entirely. In a September 14, 2026 alert, Australia's TGA said a vial sold as retatrutide, tested after a user was hospitalized with uncontrollable vomiting, held no retatrutide and about 8 times the semaglutide found in approved products. If you're looking at listings of retatrutide for sale, that page explains how people read batch test reports and why a clean report still isn't an approved drug.

On cost, tirzepatide has a known price and retatrutide doesn't. On October 31, 2026, LillyDirect listed Zepbound vials and KwikPens at $299 to $449 a month depending on dose, with the $449 doses priced that way only if you refill within 45 days. Lilly says it will price retatrutide "to reflect the overall value to the healthcare system" but hasn't named a figure. If you're on Zepbound by brand, our retatrutide vs Zepbound page covers that angle.

Switching from retatrutide back to tirzepatide

Some people go the other way, and no trial has studied that direction either.

Retatrutide clears on a similar clock. Its half-life is about 6 days, from the Phase 1b study in The Lancet, and Lilly Medical Information puts its peak 12 to 72 hours after a dose. Our retatrutide half-life page has more.

Tirzepatide is approved, so a doctor can prescribe it, and the Zepbound label gives one starting instruction for everybody: 2.5 mg a week for 4 weeks, which the label says is for starting treatment and isn't a maintenance dose. It gives no conversion from any other drug.

What to ask your doctor first

Our read: when nobody can tell you the right dose, the trials' answer was a 2 mg start and 4-week steps, and the Phase 2 table above shows what skipping a step cost in nausea.

• Am I still losing weight on tirzepatide, or have I been at the same weight on 15 mg for months?
• Would I qualify for Lilly's expanded access, given my BMI and my weight-related conditions?
• Could I join a trial if I stopped tirzepatide for 90 days and kept my weight within 5 kg, and how likely is that off the drug?
• Am I on insulin or a sulfonylurea that would need adjusting if my appetite dropped further?
• What's my plan for birth control during the first weeks of a new drug?

TRIUMPH-5 is comparing retatrutide with tirzepatide in about 800 adults with obesity (ClinicalTrials.gov's estimate), measuring weight change at week 80. Its primary completion is estimated for November 2026, and Lilly had announced no results as of October 31, 2026. They will be the first head-to-head numbers for the two drugs; the only head-to-head result so far, TRANSCEND-T2D-2 on October 28, 2026, pitted retatrutide against Ozempic 2 mg in type 2 diabetes. For the full set of retatrutide trial numbers, see retatrutide weight loss results.