The tirzepatide half-life, in short
- ~5 days
- elimination half-life
- ~28 h
- to peak after a single injection
- ~4 weeks
- to reach steady state on a fixed weekly dose
- ~25 days
- to fall below about 3% of a final dose
A half-life is the time it takes for half of what is in you to be eliminated. At five days, tirzepatide takes about 25 days – five half-lives – to fall to roughly three per cent of a dose, which is the point at which pharmacologists usually stop caring. In everyday terms: about three to four weeks after your last injection, with a long, thin tail rather than a hard stop.
What one dose does, day by day
A subcutaneous injection does not appear in your bloodstream instantly. It absorbs over roughly a day, peaks around 28 hours after the shot, then falls away on the half-life. Here is a single dose, with nothing else on board:
| Days since the shot | Roughly what is left of that dose |
|---|---|
| 1 day | 87% |
| 3 days | 66% |
| 5 days (one half-life) | 50% |
| 7 days | 38% |
| 10 days (two half-lives) | 25% |
| 14 days | 14% |
| 20 days (four half-lives) | 6% |
| 25 days (five half-lives) | 3% |
Decay of a single dose at a five-day half-life. Absorption means the first day is a rise, not a fall.
Why the first month feels like it is still building
Look at the seven-day row: 38% of a dose is still there when the next one goes in. Weekly dosing therefore stacks. Each injection lands on top of the remainder of the last, and the running total climbs until the amount you eliminate in a week equals the amount you inject.
For tirzepatide that settles at roughly 1.6 times the level a single dose would reach, and it takes about four weeks – four to five half-lives – to get there. This is the arithmetic behind the four-week steps in the dose ladder. It is not a formality; it is how long the previous step needs to stop rising before you can tell what it actually does.
It also explains one of the most common experiences on this drug: week four of a new dose feels different from week one, without anything having changed. Nothing did change. The average level was still climbing.
The trough, and why you can feel it
At steady state the level is not flat. It rises to a peak about a day after each injection, then falls for six days until the next one. On tirzepatide, with a five-day half-life against a seven-day interval, the difference between the peak and the trough is substantial – noticeably more than on semaglutide, where the half-life is longer than the gap between doses.
That is the mechanism behind the pattern people describe constantly and are often told they are imagining: appetite flattest in the first two or three days, food noise returning towards the end of the week, and side effects clustering near the peak rather than spreading evenly. The curve is doing exactly that.
This is the single most useful thing a tracker can show you, and it is why Penwise+ draws the estimated level rather than only a date. Seeing that you are on the downslope reframes a difficult day as a predictable part of the cycle instead of a personal failure.
What a late dose actually costs
One day late is a small dip: you drop about 13% further before the next dose lands. Three days late and you are near half of where the trough would have been. A week missed entirely and the level has fallen to roughly the same place, then has to climb back – which is why the week after a missed dose so often feels like starting again.
It is not the same on every drug, which is the point of tracking the specific one you are on. Semaglutide, with its seven-day half-life, forgives lateness far more readily. Liraglutide, at thirteen hours, forgives almost nothing. What to do about a missed dose goes product by product.
After the last injection
Stopping is the mirror image of starting. The level does not fall off a cliff; it decays on the same five-day half-life. A week later there is still around a third of your last dose present, at two weeks about an eighth, and by three to four weeks it is effectively gone.
That gradual tail is why appetite typically returns over weeks rather than overnight, and why the fortnight after stopping is not a fair test of what life without the medication feels like. If you have logged the whole run, that period is a lot easier to interpret – and a lot easier to describe accurately at your next appointment.
How Penwise models it
Penwise uses a one-compartment model with first-order absorption: the standard shape for a subcutaneous injection. The elimination half-life comes from published clinical pharmacology, the absorption rate is set so a single dose peaks around 28 hours after injection, and the curve you see is the sum of every dose you have logged.
- It is an estimate from a model, not a measurement of your blood, and Penwise never labels it as one.
- It is tuned per medication. Semaglutide, tirzepatide, retatrutide and liraglutide behave differently enough that one curve for all of them would be wrong.
- It is built from your own doses, including the ones you took late, so it reflects the schedule you actually kept.
- It is never used to suggest a dose. That is a conversation with your prescriber, not an output of an app.
See your own curve, not a textbook one
Penwise+ estimates your level from the doses you logged. Fourteen days included, no card and no account.
Common questions
How long does tirzepatide stay in your system?
How long until Mounjaro reaches steady state?
Why does my appetite come back before my next shot?
Is the half-life the same for Mounjaro and Zepbound?
Does Penwise measure how much medication is in my blood?
How does tirzepatide compare with semaglutide on this?
Sources
Every factual claim on this page traces to the product information a regulator publishes, or to a published trial. Links open on the site of whoever issued the document.
- Mounjaro: EPAR product information European Medicines Agency
- Zepbound (tirzepatide) prescribing information DailyMed, U.S. National Library of Medicine
- Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes (SURPASS-2) New England Journal of Medicine, 2021
Penwise is a tracking and education tool, not a medical device. Nothing on this page is medical advice, a diagnosis or a recommendation to start, stop or change a dose. Dosing decisions belong to you and your clinician. Ozempic, Wegovy, Rybelsus, Saxenda and Victoza are trademarks of Novo Nordisk; Mounjaro, Zepbound and Trulicity are trademarks of Eli Lilly. Penwise is not affiliated with, endorsed by or connected to either company.