Retatrutide Drug Level Calculator

The retatrutide drug level calculator above estimates how much drug is in the body after each weekly shot. It turns Lilly's published half-life of about 6 days into a curve, so you can see why a 4-week step on the trial ladder matters, what a late or skipped dose does, and how an old drug fades after a switch. It is a model, not a blood test, and nobody has published a retatrutide level you could compare it with.

Retatrutide is still investigational. As of October 2026, Lilly has reported six positive Phase 3 trials, no approved version exists, and Lilly's latest release says it "cannot be legally sold or marketed for human use". This page does arithmetic on published averages and can't tell you what dose to take.

How the model works

The model treats the body as one compartment. Each injection enters slowly, peaks, and then clears at a steady rate, and the doses add up. Two published numbers set it.

1. Half-life: Lilly Medical Information gives about 6 days, from the Phase 1b trial. Every 6 days the amount halves, once the peak has passed.
2. Time to peak: Lilly says the highest concentration comes within 12 to 72 hours of dosing. We set it to 42 hours, the middle of that window, by tuning how fast the shot is absorbed.

On those numbers, one 1 mg dose peaks around day 1 to 2. A steady weekly dose settles into a peak about 1.8 times its pre-dose low point, and the level just before each shot ends up about 1.8 times what one shot alone leaves a week later. By the time of the fifth shot, the pre-dose level is within 5% of its final value. That lines up with the trial design, where each dose step lasted 4 weeks.

What the scenarios show

The trial ladder starts at 2 mg and steps up every 4 weeks, to 4, 6, 9 and 12 mg depending on the arm (Lilly's TRIUMPH-1 release). The curve climbs in stair steps and never reaches a new plateau before the next step arrives, so on the ladder you are always catching up to the dose you are on. Lilly's TRANSCEND-T2D-1 release says stomach side effects came mainly during dose escalation.

For a 4 mg weekly dose with the fourth shot on day 21, the model gives these estimated levels:

A late dose shifts the whole curve a few days. A skipped dose leaves a deeper gap, and the next one has to climb out of it. Retatrutide has no published missed-dose rule. Wegovy's label says to take a missed dose if the next one is more than 2 days away, and that is semaglutide, not retatrutide.

Splitting a weekly dose into two smaller ones, 3.5 days apart, trims the peak (about 13% lower at 6 mg a week) and lifts the low point. Same total drug, flatter curve. No trial has tested it for retatrutide, so we can't say it changes side effects or results.

Our read: in the model, one late or skipped shot is mostly recovered within two doses, so the 4-week steps matter more than a single slip. Nobody has tested that against side effects or weight loss.

Switching from tirzepatide or semaglutide

The switching view draws the old drug on its own chart, as a share of its steady-state peak, because mg of one drug can't be added to mg of another. The half-lives come from FDA labels: about 5 to 6 days for tirzepatide (Zepbound; Mounjaro says about 5) and about 1 week for semaglutide (Wegovy). By our model, a week after the last 10 mg tirzepatide shot (day 0 here), about 48% of its peak level remains, and 62% for a 1 mg semaglutide shot. Two weeks after the last shot it is 20% and 31%. Retatrutide is already building by then. The switching guide covers what Lilly has said about changing drugs.

Limits

• It is an average. Weight, kidney function, injection site and the product itself change real levels, and we found no published spread for retatrutide.
• Bioavailability is set to 100%, so the milligrams are model units, not what a lab would measure.
• It doesn't know whether a vial holds what its label says. The TGA tested one vial sold as retatrutide and found none.
• A level on a chart isn't a benefit or a risk. Nobody has published a target level.

For how often the trials dosed and why, see how often to take retatrutide. For the plain half-life numbers, see retatrutide half-life. To turn trial doses into syringe units, use the dose chart or the dose calculator.