Retatrutide half-life: how long it stays in the body

Half-life is the time it takes for the amount of a drug in the body to fall by half. For retatrutide, an investigational weekly agonist of the GIP, GLP-1 and glucagon receptors, the published figure is about 6 days. This guide works through what that number implies: how fast a single dose clears, how weekly doses build up, and how long a washout takes. Retatrutide is not approved by any regulator (Eli Lilly, Aug 12, 2026); for background see the retatrutide hub.

What the trials reported

The 6-day figure comes from the Phase 1b abstract by Urva and colleagues, which also reported that pharmacokinetics were dose proportional (The Lancet, Oct 27, 2022). The 5-to-7-day range and the 1-to-3-day tmax come from Lilly’s summary of the first single-dose study inside the Phase 2 protocols, not from a peer-reviewed table (Eli Lilly, hosted on ClinicalTrials.gov, Aug 29, 2022) (Eli Lilly, hosted on ClinicalTrials.gov, Aug 29, 2022). The Cell Metabolism paper on that study said the profile “supported once-weekly dosing” and that weight reduction persisted up to day 43 after a single dose (Cell Metabolism, Aug 18, 2022). We found no published absolute bioavailability for subcutaneous retatrutide.

Elimination math

With first-order elimination, the fraction left after t days is 2^(−t ÷ t½). The elimination rate constant is ke = ln 2 ÷ t½ = 0.693 ÷ 6 = 0.1155 per day. In plain terms:

• After 1 day, 89.1% remains, so about 10.9% is cleared per day.
• After 7 days, 44.5% remains: 2^(−7 ÷ 6) = 0.4454.
• After 12 days (2 half-lives), 25% remains.
• After 30 days (5 half-lives), about 3.1% remains.

The table below applies the site’s model to a single 1 mg dose. The “modeled amount” column includes absorption, with a peak placed at 1 day; the “left by decay alone” column is the pure 2^(−t ÷ 6) fraction.

Washout after the last dose

Washout is the time for the amount to fall to a negligible level once dosing stops. Two starting points matter: a single dose, and the end of a long run of weekly doses, where the amount just after the last dose is about 1.8 times one dose.

A single dose falls below 1% after 6 × log2(100) = 39.9 days, about 5.7 weeks. From steady state, the amount is still about 1.4% of one dose at 6 weeks and 0.28% at 8 weeks. These are model outputs. Real clearance varies between people, and the published half-life is itself a range of about 5 to 7 days. At 5 days, 4 weeks would leave 2.1% of a single dose; at 7 days, 6.3%.

Steady state and the weekly accumulation ratio

When the same amount is given every 7 days, each new dose adds to what is left of the earlier ones. The total climbs until the amount cleared each week equals the amount added. The ratio between that plateau and a single dose is the accumulation ratio, R = 1 ÷ (1 − e^(−ke × 7)). At a 6-day half-life, R = 1 ÷ (1 − 0.4454) = 1.8033.

The rule of thumb of 5 half-lives to about 97% of steady state gives 30 days. In the pure-decay picture, the steady-state amount swings between 1.80 dose-equivalents right after a dose and 0.80 just before the next, a peak-to-trough ratio of 2.25.

This matters for the trial ladders. Each Phase 3 step lasted 4 weeks (Eli Lilly, May 21, 2026), so by the end of a step the model is about 96% of the way to that step’s plateau before the next increase. The chart shows the 12 mg arm’s five steps with the modeled amount underneath.

• Weekly dose (published steps)
• Modeled amount (mg-eq)
• Stable window

The retatrutide plotter draws the same curve for any published ladder and start date, and the general plotter overlays other compounds. The dosage chart lists every ladder the trials used.

Retatrutide vs tirzepatide and semaglutide

The tirzepatide value is the label’s approximately 5 days (DailyMed, Aug 28, 2026), and the semaglutide value is the label’s about 1 week (Novo Nordisk, Jun 18, 2026). Retatrutide sits between them. The three values come from different study designs and populations, and retatrutide’s comes from a 12-week Phase 1b trial rather than a label, so read the comparison as approximate. A longer half-life means slower build-up, a higher plateau relative to one dose, and a longer washout. See the tirzepatide half-life and semaglutide half-life guides for the label details.

What half-life does and does not tell you

Half-life describes how fast the drug leaves the body. It does not say how long effects last or how strong they are. The first human study reported weight reduction persisting to day 43 after a single dose (Cell Metabolism, Aug 18, 2022), well beyond the point where most of the dose had cleared. Gastrointestinal events in the Phase 2 obesity trial were dose-related and were partly reduced by a lower starting dose (N Engl J Med, Jun 26, 2023), which reflects how quickly the amount rose rather than the half-life itself.

Our model also has limits. It is a one-compartment model with first-order absorption (a Bateman function), using a 6-day half-life and a 1-day tmax, the low end of the reported 1-to-3-day range. Its output is a relative amount in mg-equivalents with bioavailability set to 1, not a plasma concentration. The model card lists every parameter and its source. Nothing here is a recommendation for anyone’s use; retatrutide is legally available only in Lilly clinical trials.