A half-life is usually read off a label. Retatrutide has none: Eli Lilly plans to file in 2027, and the retatrutide sold in research vials carries no manufacturer data at all. So the "retatrutide half-life" in every chart and forum post traces back to one sentence in one trial paper. This page gives that sentence its source, sets out how thin the published pharmacology is, and works the figure into what remains after each week and how weekly doses build.
The trial doses and syringe units are on the retatrutide dosage page; the compound itself is covered in Medibact's Retatrutide guide.
Where "about 6 days" comes from
Urva and colleagues (The Lancet 2022, PMID 36354040) ran the phase 1b trial NCT04143802: 72 adults with type 2 diabetes at four US centres, 12 weeks of once-weekly injections of retatrutide (then LY3437943), placebo or dulaglutide. Retatrutide cohorts received 0.5, 1.5, 3, 3→6 or 3→6→9→12 mg. The abstract's pharmacokinetic finding is one line: the drug's pharmacokinetics "were dose proportional and its half-life was approximately 6 days", which the authors say suggests "suitability for once-weekly dosing". Nine of the eleven authors were Lilly employees and the trial was Lilly-funded.
The earlier single-dose study in healthy volunteers (NCT03841630, 45 enrolled, finished July 2019) appears in Lilly's discovery paper (Coskun et al., Cell Metabolism 2022, PMID 35985340) as a summary: the pharmacokinetic profile "supported once-weekly dosing", and a reduction in body weight "persisted up to day 43 after a single dose". The second statement is often read as a drug-level claim. It is not: it describes weight, which falls and recovers on its own timescale.
How much else has been published
| Record | What it contains | Status on 2026-09-25 |
|---|---|---|
| PubMed: retatrutide or LY3437943 with pharmacokinetics or half-life in title or abstract | 9 records | 2 primary Lilly papers (above); 7 reviews or commentaries that repeat them |
| NCT05611957, kidney impairment (29) | PK in impaired and normal kidney function | Completed 2023-09; no results posted |
| NCT05916560, liver impairment (43) | PK in impaired and normal liver function | Completed 2025-03; no results posted |
| NCT05959096, high body-mass index (85) | PK, exposure and peak concentration | Completed 2024-07; no results posted |
| NCT06003465, device versions (57) | Similarity of exposure between injection devices | Completed 2024-02; no results posted |
| NCT03841630, single dose in healthy volunteers (45) | Safety and PK of one injection | Completed 2019-07; no results posted (summary only, in the 2022 paper) |
| NCT05445232 (32) and NCT06808802 (30), drug interactions | Effect on midazolam and on metoprolol exposure | Completed 2023-02 and 2025-04; no results posted |
| NCT05548231 (Chinese, 32) and NCT04823208 (Japanese, 64) | Safety and PK in regional cohorts | Completed 2023 and 2022; no results posted |
Nine completed pharmacology studies by the manufacturer, and the only public half-life is the one-line phase 1b figure. Whether kidney or liver impairment changes the half-life, whether body size does, and what the variation between people is, are all measured and none is public. Every figure below therefore rests on "approximately 6 days".
What remains, week by week
With a 6-day half-life, the fraction of one dose left after d days is ½ raised to d/6:
| Days after one dose | Share remaining |
|---|---|
| 3 | 71% |
| 7 | 44.5% |
| 14 | 19.8% |
| 21 | 8.8% |
| 28 | 3.9% |
| 35 | 1.8% |
| 42 | 0.8% |
This ignores absorption over the first day or two, so the early rows are approximate. Five half-lives, the usual convention for "effectively cleared", is 30 days. Against Lilly's statement that weight reduction persisted to day 43, the arithmetic puts the drug at under 1% of the dose by then: the effect outlasts the drug, which is common for weight and says nothing about the level.
How it builds up with weekly doses
Close to half of each dose is still present at the next injection, so weekly doses accumulate by a factor of 1 ÷ (1 − 0.445), about 1.8. The approach to that plateau:
| Weekly doses at one strength | Share of steady state |
|---|---|
| 1 | 56% |
| 2 | 80% |
| 3 | 91% |
| 4 | 96% |
Lilly's release of 2026-07-23 on TRIUMPH-2 and TRIUMPH-3 states that participants started at 2 mg once weekly and increased every four weeks; its release of 2026-05-21 on TRIUMPH-1 describes the 12 mg arm rising through 2, 4, 6 and 9 mg. By the arithmetic, four weeks at each step reaches about 96% of that step's plateau before the next increase. No publication states that this was the design reason; the fit is this page's arithmetic.
Against tirzepatide and semaglutide
| Molecule | Half-life | Source | Share left at 7 days |
|---|---|---|---|
| Retatrutide | about 6 days | Phase 1b trial (Lancet 2022) | 44.5% |
| Tirzepatide | about 5 days; 5–6 in obesity | MOUNJARO and ZEPBOUND labels, August 2026 | 38–45% |
| Semaglutide | about 1 week | WEGOVY label | about 50% |
The three sit within about two days of each other, and the retatrutide figure is the only one that has not been through a regulator. The tirzepatide figures are worked the same way on tirzepatide's half-life page. What the overlap means for anyone moving from one to the other is set out, with the trial record, on switching from tirzepatide to retatrutide.
Units for the trial doses
Units on a U-100 syringe = mg ÷ mg/mL × 100. At 10 mg/mL (a 10 mg vial with 1 mL, or 20 mg with 2 mL) the TRIUMPH steps of 2, 4, 6, 9 and 12 mg are 20, 40, 60, 90 and 120 units; at 20 mg/mL they are 10, 20, 30, 45 and 60. The half-life is the same whatever the concentration. The retatrutide reconstitution page works each vial size, and the calculator's retatrutide setting any other.
What is not established
The half-life is one reported figure from one 12-week trial in 72 people with type 2 diabetes, with no range published; no regulator has reviewed it; the kidney, liver, body-size and device studies that would qualify it are complete and unpublished. Research-market vials of retatrutide have no pharmacokinetic data of their own, so whether their contents behave like Lilly's molecule is not known. The week-by-week and steady-state figures are arithmetic. Nothing on this page is a dose or a schedule.
Sources and dates
Opened 2026-09-25: abstracts via NCBI E-utilities for Urva et al., The Lancet 2022 (PMID 36354040, with its conflict-of-interest statement) and Coskun et al., Cell Metabolism 2022 (PMID 35985340); the PubMed search described above; ClinicalTrials.gov v2 records for NCT04143802, NCT03841630, NCT06808802, NCT05611957, NCT05916560, NCT05959096, NCT06003465, NCT05445232, NCT05548231 and NCT04823208 (status, completion date, results flag); the tirzepatide and semaglutide half-lives as stated on their labels and cited on this site; Lilly's TRIUMPH escalation as cited on this site's retatrutide dosage page. Remaining shares, accumulation and steady-state figures are arithmetic by this site. Corrections go to the contact page.