By Austin Witherow and Mason Calvert · Last updated · Sources
Label half-lives
Weekly GLP-1 medicines are built to last. Their labels give these elimination half-lives, which Pedplot plots:
| Medicine | Half-life | Time to peak | Label |
|---|---|---|---|
| Semaglutide | 7 days | 2 days | Ozempic FDA label |
| Tirzepatide | 5 days | 2 days | Mounjaro FDA label |
A half-life close to the weekly gap means a large share of each dose is still there when the next one lands. With semaglutide (7 days), half of a dose is left a week later.
Why it takes weeks to level off
Each weekly dose adds to what is left of the earlier ones, so the amount in the body climbs for several weeks on the same dose before it settles.
| Same weekly dose | First peak | Peak once levelled off | Ratio | Levels off around |
|---|---|---|---|---|
| 0.25 mg semaglutide | 0.25 mg | 0.5 mg | 2× | day 42 (week 6) |
| 2.5 mg tirzepatide | 2.5 mg | 4.03 mg | 1.6× | day 28 (week 4) |
On an unchanged weekly dose, semaglutide ends up about 2 times higher at its peak than after the first injection, and tirzepatide about 1.6 times, because its half-life is shorter. These are modeled amounts of the drug, not blood levels or effects.
Dose steps on the label schedule
Both labels start low and step the dose up after at least four weeks at each step. The tables below plot the first three steps at the shortest step length each label allows. They are shown to explain timing, not as a schedule to follow; real schedules often hold a step for longer.
The last column compares the amount at the end of each step with where that dose would level off if it were kept. It shows that four weeks is not quite long enough to level off on semaglutide, and that the leftover from earlier steps carries into the next one.
Semaglutide, from the Ozempic label:
| Step | Weekly dose | Weeks | Amount at week 4 of the step | Share of where that dose levels off |
|---|---|---|---|---|
| 1 | 0.25 mg | 1–4 | 0.286 mg | 94% |
| 2 | 0.5 mg | 5–8 | 0.589 mg | 97% |
| 3 | 1 mg | 9–12 | 1.18 mg | 97% |
- Semaglutide, steps 0.25 → 0.5 → 1 mg
Open the semaglutide steps in the plotter
Tirzepatide, from the Mounjaro label:
| Step | Weekly dose | Weeks | Amount at week 4 of the step | Share of where that dose levels off |
|---|---|---|---|---|
| 1 | 2.5 mg | 1–4 | 1.97 mg | 98% |
| 2 | 5 mg | 5–8 | 3.98 mg | 99% |
| 3 | 7.5 mg | 9–12 | 6 mg | 99% |
- Tirzepatide, steps 2.5 → 5 → 7.5 mg
By the end of step 3, tirzepatide is at 99% of where 7.5 mg levels off, against 97% for semaglutide at 1 mg. The shorter half-life gets closer within the same four weeks.
What this does not show
- Effects, side effects or how a person responds. The chart is an amount of drug, nothing more.
- Blood levels. Labels report exposure differently, and people differ in body weight and clearance.
- Doses. The steps are the label's starting schedule, shown only so the timing can be seen. See the disclaimer.
Sources
- FDA label. Ozempic FDA label, section 2: start 0.25 mg once weekly for 4 weeks, then 0.5 mg once weekly; may be increased to 1 mg and then 2 mg after at least 4 weeks on each dose
- FDA label. Mounjaro FDA label, section 2: start 2.5 mg once weekly; increase in 2.5 mg steps after at least 4 weeks on the current dose; maximum 15 mg once weekly
- FDA label. Ozempic: Ozempic FDA label, 12.3 Pharmacokinetics: elimination half-life approximately 1 week; peak 1 to 3 days post dose
- FDA label. Mounjaro: Mounjaro FDA label, 12.3 Pharmacokinetics: elimination half-life approximately 5 days; Tmax 8 to 72 hours
The model and its limits are on the methodology page.