Retatrutide dosing: there is no approved dose, only trial schedules

Retatrutide has no approved dose, and no human dose has been established for use outside a trial. It is an investigational drug, so the only doses that exist are the ones written into Lilly’s trial protocols, given under medical supervision with scheduled visits and lab work. What follows describes those protocols for readers who want to understand what the trials did. It is not a dosing guide. For the drug itself, its FDA status and what the trials found, start with the retatrutide hub.

The phase 3 schedule: TRIUMPH-1 to TRIUMPH-4

Lilly describes one escalation pattern across the phase 3 program: “participants randomized to retatrutide initiated treatment with 2 mg once weekly and increased the dose every four weeks until reaching their target dose.” The program uses five dose strengths: 2, 4, 6, 9 and 12 mg.

The steps come from Lilly’s published trial descriptions; the week ranges follow from four weeks per step. All doses are once weekly, injected under the skin. The ClinicalTrials.gov entries for these trials list the arms only as “Dose 1”, “Dose 2” and “Dose 3”; the milligram amounts come from Lilly’s announcements and the published papers.

That calculator converts a milligram amount into syringe units for a given vial and water volume. Its dropdown lists the phase 2 doses (1, 4, 8 and 12 mg); the phase 3 steps of 2, 6 and 9 mg can be entered with “Other.” The trials report doses in milligrams, not syringe units, so the calculator explains arithmetic and says nothing about what the trials did. The sum itself is simple. On a U-100 syringe, units equal milligrams divided by mg per mL, times 100, and 100 units is always 1 mL. Units measure liquid, so the same number of units can hold very different amounts of drug. The mg/mL to units converter covers the general math. Without a verified concentration, which an unregulated vial cannot give you, the answer means nothing.

What happened at each dose in TRIUMPH-1?

Higher doses brought more weight loss, and past 9 mg they mostly brought more people stopping.

Weight figures are the treatment-regimen estimand from NEJM; side-effect figures are from Lilly’s May 21, 2026 release. Lilly highlighted that the 4 mg dose was reached with a single step up from 2 mg, and fewer people stopped on it than on placebo. TRIUMPH-1 also had a 24-week extension for 532 people with a starting BMI of 35 or more, in which every group was moved, still blinded, toward a “maximum tolerated dose” of 9 or 12 mg. The full picture by trial is on the retatrutide side effects page.

Which trial used which doses?

TRANSCEND-T2D-2 used the same start: 2 mg, raised every four weeks to the target.

The phase 2 schedules (2023)

The phase 2 obesity trial (Jastreboff et al., NEJM 2023) ran 48 weeks in 338 adults without diabetes. It tested different starting doses on purpose, to see how the start affected side effects. Doses rose every four weeks, with escalation lasting up to 12 weeks.

The starting dose made a visible difference. Nausea was reported by 18% of the 4 mg group that started at 2 mg versus 36% of the 4 mg group that started at 4 mg, and by 17% versus 60% in the two 8 mg groups. Vomiting followed the same pattern in the 8 mg groups, 6% versus 26%. The authors concluded that gastrointestinal events “were partially mitigated by the use of a lower starting dose (2 mg vs. 4 mg)” and that “further refinement of the dose-escalation scheme for phase 3 trials may improve the side-effect profile.” That is the bridge to the phase 3 ladder above, which always starts at 2 mg and adds a 6 mg step.

The phase 2 trial also built in a floor. Eight participants on retatrutide had protocol-driven dose reductions because their BMI fell to 22 or lower. No participant fell to a BMI of 19 or lower.

The phase 2 diabetes trial

A separate 36-week phase 2 trial in 281 adults with type 2 diabetes (Rosenstock et al., Lancet 2023) used a similar design: 0.5 mg; 4 mg reached by escalation from 2 mg or started directly; 8 mg started at 2 mg (“slow”) or 4 mg (“fast”); and 12 mg started at 2 mg. It compared these with placebo and with dulaglutide 1.5 mg, an approved GLP-1 drug. The authors wrote that the data “informed dose selection for the phase 3 programme.”

Timing and missed doses

The trials gave retatrutide once a week. Beyond that, the public record stops. Retatrutide has no label, and we could not find a published rule on time of day, injection site, food or missed doses from Lilly’s trials. If you are in a trial, those instructions come from your study team. The rules on an approved label, such as the 4-day window in the tirzepatide dosage chart, apply to that drug and should not be borrowed.

Common misreadings of the trial schedules

• Reading a table as a schedule to follow. Trial participants were screened against long exclusion lists, seen at scheduled visits, and had their dose held or reduced by investigators when side effects or weight loss called for it. Nobody outside a trial has that.
• Assuming a vial holds the trial drug. Trial supplies come from the sponsor under an FDA investigational application. A vial bought online is not that drug, whatever its label says.
• Assuming more is better for everyone. In TRIUMPH-1, the step from 9 to 12 mg added little average weight loss (23.7% to 25.0%) but raised the share stopping for side effects from 6.9% to 11.3%.
• Mixing up the two programs. Phase 2 used 8 mg. Phase 3 uses 6 and 9 mg instead. An “8 mg” number comes from the older trial.
• Treating units as the dose. The trials never used units. A number of units means nothing without a verified concentration.
• Assuming the label will match. If retatrutide is approved, the FDA label will set the doses and steps, and they may differ from the trial protocols.

Research retatrutide is not the trial drug

Retatrutide is not an approved drug, and the FDA says it “cannot be used in compounding under federal law,” so every vial sold online is an unapproved new drug. Under 21 CFR 312.160, such products may be shipped only for laboratory animal or test-tube research, labeled “Not for use in humans.” Products labeled “research use only” are not legal to use on people: the FDA’s March 31, 2026 warning letter to one seller said that despite that labeling, “evidence obtained from your website establishes that your products are intended to be drugs for human use.” Lilly’s own statement is that retatrutide “cannot be legally sold or marketed for human use.”

The risk goes beyond the law. A 2026 case report in BMJ Case Reports describes a woman in her mid-20s admitted to hospital with five days of nausea, repeated vomiting and diarrhea that did not respond to standard anti-nausea drugs, after four weekly, dose-escalated injections of an unverified product said to contain retatrutide, given in a wellness setting by someone with no medical qualification. Her symptoms resolved within a week.

Trial dosing happens inside registered clinical trials. Lilly also runs a narrow compassionate use program, applied for by physicians, for adults with refractory obesity and serious complications who cannot enroll in a trial. We found no public dose schedule for it, and anything sold online is not that program. The retatrutide hub explains where access stands and how to search ClinicalTrials.gov.