Definition: Tmax is the time between taking a dose and reaching the highest drug level in the blood. It reflects how fast the drug is absorbed. The FDA describes Tmax as giving important information about the rate of absorption. Its partner, Cmax, is how high that peak is.
Why it matters when you track doses
Tmax tells you when a dose is doing the most, which often lines up with when side effects show up. If nausea always hits the day after a GLP-1 shot, Tmax is part of the reason. Logging the date and time of each shot, plus when symptoms start, makes that pattern visible.
Time to peak after an injection, per the current US label.
| Product | Tmax on the label | Route |
|---|
| Ozempic / Wegovy (semaglutide) | 1 to 3 days after the dose | Under the skin, weekly |
| Mounjaro (tirzepatide) | 8 to 72 hours | Under the skin, weekly |
| Xyosted (testosterone enanthate) | Median 11.9 hours (range 5.8 to 168.7 hours) | Under the skin, weekly |
Worked example: finding your peak day
A Monday semaglutide shot at 8 AM
- The label range is 1 to 3 days after the dose.
- 1 day after Monday 8 AM is Tuesday 8 AM. 3 days after is Thursday 8 AM.
- So the peak for that dose would usually fall between Tuesday and Thursday morning.
- For Xyosted, a median of 11.9 hours after a Monday 8 AM shot is about Monday 8 PM, but the range runs out to 168.7 hours, which is a full week.
These are averages from study groups. Your own peak can land anywhere in the range or outside it.
Common mix-ups
- Tmax is not half-life. Tmax is how fast a dose rises; half-life is how fast it falls.
- Route changes Tmax. The same molecule can peak at a different time under the skin, in a muscle or by mouth.
- Single-dose Tmax is not the whole story. Once doses overlap at steady state, the rise after each shot is smaller relative to the total.
Read more on Dose Track: Wegovy guide · Mounjaro guide
General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
When does semaglutide peak after a shot?+
The Ozempic and Wegovy labels say maximum concentration is reached 1 to 3 days after the dose. Individual timing varies.
When does tirzepatide peak?+
The Mounjaro label says the time to maximum concentration ranges from 8 to 72 hours after the dose.
Is Tmax the same for everyone?+
No. Labels report a median or a range from study groups. Injection site, body size and the product itself all shift it.