Short answer: Each weekly tirzepatide shot peaks in the blood about 24 hours after the injection (Zepbound label), and the Mounjaro and Zepbound labels say steady levels are reached after 4 weeks on a dose. Treatment starts at 2.5 mg, which the Zepbound label says is not a maintenance dose, and rises in 2.5 mg steps at least 4 weeks apart. The trials read their main results at week 40 for blood sugar (Mounjaro) and week 72 for weight (Zepbound).
The Zepbound label gives the median time to the highest blood level as 24 hours after the shot, with a range of 8 to 72 hours. Steady-state levels are reached after 4 weeks of once-weekly dosing, and exposure rises in step with the dose. The half-life is about 5 days (5 to 6 days in people with overweight or obesity, per the Zepbound label). Exposure is similar in the belly, thigh and upper arm.
The Zepbound label also describes an effect on stomach emptying: it was greatest after the first 5 mg dose and became smaller with later doses. That is one reason early weeks can feel different from later ones.
Both labels start at 2.5 mg once a week for 4 weeks. The Zepbound label says the 2.5 mg dose "is for treatment initiation and is not approved as a maintenance dosage". After that, the dose goes up in 2.5 mg steps after at least 4 weeks on each dose, to reduce stomach and gut side effects.
| Weeks | Weekly dose |
|---|---|
| 1 to 4 | 2.5 mg (start only) |
| 5 to 8 | 5 mg (lowest maintenance dose for weight reduction) |
| 9 to 12 | 7.5 mg |
| 13 to 16 | 10 mg |
| 17 to 20 | 12.5 mg |
| 21 onward | 15 mg (the maximum) |
Not everyone goes to the top. The Zepbound maintenance doses for weight reduction are 5, 10 or 15 mg, chosen by response and tolerability. Each step needs at least 4 weeks, so the timeline above is the fastest the label allows.
Weight (Zepbound): weight reduction was assessed after 72 weeks of treatment, with at least 52 weeks at the maintenance dose. In Study 1, average body weight fell by 15.0%, 19.5% and 20.9% on 5, 10 and 15 mg, compared with 3.1% on placebo. At least 5% weight loss was reached by 85.1%, 88.9% and 90.9% of people on those doses.
Blood sugar (Mounjaro): in the label's monotherapy trial of adults with type 2 diabetes, HbA1c fell by 1.8, 1.7 and 1.7 points on 5, 10 and 15 mg at week 40, compared with 0.1 on placebo.
These are endpoints, not the time it takes to see a first change. The Zepbound label charts weight across the 72 weeks in figures, not as week-by-week numbers in the text.
For the first months you are on a dose that is lower than the one you will stay on, and the dose is still climbing to a new steady state after each step. Comparing your weight or blood sugar at week 6 with the week-72 trial averages mixes two different stages.
A steadier comparison is the same measure, at the same time of day, against the dose you were on that week. Dose Track keeps doses, weight and side effects on one timeline, so each change lines up with its dose step. Whether to step up, hold or change dose is a decision for your prescriber.
Read more on Dose Track: Tirzepatide guide · Zepbound dosing schedule · Mounjaro dosing schedule · Missed a Zepbound dose · Semaglutide vs tirzepatide · Steady state (glossary) · How long tirzepatide stays in your system