Where the numbers come fromThe half-life, time to peak and steady-state figures are from the clinical pharmacology sections of the Zepbound and Mounjaro labels.[1][2] Everything else on this page is exponential-decay arithmetic using a 5-day half-life. With 6 days, slightly more would remain at each point. Individual levels vary.
How much of one dose is left
With a half-life of 5 days, the amount left after t days is 0.5t/5 of the peak. For a single dose:
| Time after peak | Remaining | Of a 5 mg dose | Of a 15 mg dose |
|---|---|---|---|
| 1 days | 87.1% | 4.35 mg | 13.06 mg |
| 3 days | 66% | 3.3 mg | 9.9 mg |
| 5 days | 50% | 2.5 mg | 7.5 mg |
| 7 days (1 week) | 37.9% | 1.89 mg | 5.68 mg |
| 10 days | 25% | 1.25 mg | 3.75 mg |
| 14 days (2 weeks) | 14.4% | 0.72 mg | 2.15 mg |
| 21 days (3 weeks) | 5.4% | 0.27 mg | 0.82 mg |
| 28 days (4 weeks) | 2.1% | 0.1 mg | 0.31 mg |
| 35 days (5 weeks) | 0.8% | 0.04 mg | 0.12 mg |
| 42 days (6 weeks) | 0.3% | 0.01 mg | 0.04 mg |
A common rule of thumb is that a drug is mostly gone after about 5 half-lives. For tirzepatide that is about 25 days, when roughly 3% of the last dose remains.
Why it builds up over the first weeks
Because about 38% of each dose is still there when the next one is given, levels rise for several weeks before they level off. The Zepbound label says steady state is reached after 4 weeks of once-weekly dosing.[1] The arithmetic agrees: with the same weekly dose, the level after each injection approaches about 1.61 times the first-dose level.
| Injection number | Level just after injection | Share of steady state |
|---|---|---|
| Week 1 | 1 | 62% |
| Week 2 | 1.38 | 86% |
| Week 3 | 1.52 | 95% |
| Week 4 | 1.58 | 98% |
| Week 5 | 1.6 | 99% |
| Week 6 | 1.61 | 100% |
| Week 8 | 1.61 | 100% |
This is also why the label escalates in 4-week steps: each new dose is given time to reach its own steady level before the next increase.
What the half-life means for a missed dose
Four days after a scheduled dose was missed, about 22% of the previous dose (given 11 days earlier) is still in the body. The label allows a late dose within 4 days and requires at least 3 days between doses.[1] Details are on the missed dose page.
Stopping and restarting
How long side effects tend to last is a separate question, covered on how long do Zepbound side effects last.
- After the last dose, levels fall by half about every 5 days. Roughly 2% of it remains after 4 weeks.
- The labels advise people using oral hormonal contraceptives to switch to a non-oral method, or add a barrier method, for 4 weeks after starting tirzepatide and for 4 weeks after each dose increase.[1][2]
- The labels do not give a restart schedule after a long break. That is a question for your prescriber.
Related
- Missed dose ruleThe 4-day window on a calendar
- Label dose scheduleWhy each step lasts at least 4 weeks
Frequently asked questions
What is the half-life of tirzepatide?
How long does tirzepatide stay in your system?
Using the 5-day half-life, about 2% of the last dose remains after 4 weeks and about 3% after 25 days. Exact levels vary between people.