Simple math, not a lab measurementThe percentages on this page apply the published 6-day half-life to a simple model. Real blood levels also depend on how fast the injection is absorbed and differ between people. This page explains a property of the drug; it is not dosing advice.
What the trials report
The retatrutide half-life is about 6 days. The phase 1b study in people with type 2 diabetes reported that "its half-life was approximately 6 days".[1] The phase 2 obesity paper in the New England Journal of Medicine repeated the same figure.[2] Retatrutide carries a fatty-acid chain, a design used to keep peptide drugs in the blood longer.[2]
| Source | Finding |
|---|---|
| Phase 1b, The Lancet, 2022[1] | Half-life approximately 6 days |
| Phase 2, NEJM, 2023[2] | Half-life about 6 days; given once weekly |
How much of a single dose remains
With a 6-day half-life, about 44.5% of a single dose is left after one week and less than 1% after six weeks. Each 6-day period halves what remains:
| Time after the dose | Share remaining |
|---|---|
| 1 week (7 days) | 44.5% |
| 2 weeks (14 days) | 19.8% |
| 3 weeks (21 days) | 8.8% |
| 4 weeks (28 days) | 3.9% |
| 5 weeks (35 days) | 1.8% |
| 6 weeks (42 days) | 0.8% |
Rule of thumb: after about five half-lives (30 days) roughly 3% is left, which pharmacologists treat as close to fully cleared.
Why retatrutide is given once a week
Because nearly half of each dose is still present a week later, one injection a week keeps a fairly steady level in the blood. Every published retatrutide trial used once-weekly injections under the skin.[2][3] Drugs with half-lives of hours have to be given daily or more often to do the same.
Build-up with weekly doses
With repeated weekly doses, each new dose adds to what is left of the earlier ones until intake and clearance balance out, called steady state. Using the 6-day half-life, the simple model gives:
| Week | Level just after the dose, relative to one dose |
|---|---|
| Dose 1 | 1.00× |
| Dose 2 | 1.45× |
| Dose 3 | 1.64× |
| Dose 4 | 1.73× |
| Dose 5 | 1.77× |
| Dose 6 | 1.79× |
The level levels off near 1.8× a single dose, and most of that build-up happens within about four weeks. In the trials, each dose step also lasted four weeks before the next increase.[3] For tirzepatide, a drug with a similar half-life, the FDA label states that steady state is reached after 4 weeks of weekly dosing.[4]
Retatrutide vs tirzepatide half-life
| Drug | Half-life | Dosing | Source |
|---|---|---|---|
| Retatrutide | About 6 days | Once weekly (trials) | Phase 1b and phase 2[1][2] |
| Tirzepatide (Zepbound) | 5 to 6 days in people with overweight or obesity | Once weekly | FDA label[4] |
The two drugs clear from the body at about the same rate. The differences between them are in which receptors they act on and how much weight people lost, covered in retatrutide vs tirzepatide.
How long retatrutide stays in the body after stopping
By the five-half-lives rule, most of the drug is gone about a month after the last dose. Whether the effects on weight last is a different question. TRIUMPH-6, a trial of keeping weight off, is still running, with its main phase due in April 2028.[5]
To see how the weekly doses used in trials translate into syringe units, use the retatrutide dosage calculator or the retatrutide dosing chart.
Frequently asked questions
What is the half-life of retatrutide?
How long does retatrutide stay in your system?
By simple half-life math, about 3% of a dose remains after 30 days (five half-lives). Individual levels vary.[1]
Does retatrutide build up in the body?
Yes, somewhat. With weekly doses and a 6-day half-life, levels level off at roughly 1.8 times a single dose in a simple model.
Sources
- Urva S et al. LY3437943, a GIP, GLP-1 and glucagon receptor agonist, in type 2 diabetes: phase 1b
- Jastreboff AM et al. Triple-hormone-receptor agonist retatrutide for obesity: a phase 2 trial
- Jastreboff AM et al. Retatrutide for obesity (TRIUMPH-1)
- Zepbound (tirzepatide) prescribing information
- Retatrutide studies (NCT04881760, NCT05929066, NCT05931367, NCT06662383, NCT07629401 and others)