Dosage · Pharmacokinetics
GLP-1 Half-Life Calculator
See exactly how long semaglutide or tirzepatide stays active in your system — and what percentage of your last dose is still working right now.
💉 Semaglutide & Tirzepatide
📅 Updated May 2026
📊 FDA pharmacokinetic data
Ready to start?
Get GLP-1 medication from $146/month
Licensed telehealth providers — medical evaluation, prescription, and 2-day shipping included.
We may earn a commission from provider links. This doesn't influence our calculators.
What is the half-life of GLP-1 medications?
The half-life is the time it takes for the active concentration in your bloodstream to fall to half its peak level. Both semaglutide and tirzepatide are engineered with unusually long half-lives so they only need to be injected once per week.
| Property | Semaglutide | Tirzepatide |
|---|
| Half-life | ~7 days (165–184 hrs) | ~5 days (116–123 hrs) |
| Time to peak (Tmax) | 24–72 hours post-injection | 8–72 hours post-injection |
| Time to steady state | 4–5 weeks | 4–5 weeks |
| Falls below 10% by | ~Day 23 after injection | ~Day 17 after injection |
| Essentially cleared by | ~5–6 weeks post-injection | ~3–4 weeks post-injection |
| PK data source | FDA label; Marbury et al. 2017 | FDA label; SURPASS-1 PK data |
Steady state vs. single dose: After 4–5 weekly injections, drug concentration builds to a stable plateau where the amount added each week equals the amount eliminated. Most clinical weight loss results reflect outcomes at steady state, not from a single dose.
Why half-life matters practically
Missing a dose within 5 days still leaves meaningful drug activity from the prior week's injection. Side effects like nausea peak in the 24–48 hours after injection when concentration is rising, then ease as it falls. After stopping, appetite suppression persists for 2–3 weeks as the drug clears — which is why weight regain builds gradually rather than immediately.
Frequently asked questions
What is the half-life of tirzepatide?+
Tirzepatide has a half-life of approximately 5 days (116–123 hours), based on FDA prescribing information and SURPASS pharmacokinetic data. This means 5 days after your injection, roughly 50% of the injected dose is still active. Full clearance below 10% of peak occurs around day 17 after a single injection.
How long does semaglutide stay in your system?+
Semaglutide stays in your system for approximately 5–6 weeks after your last injection. Its ~7-day half-life means concentration falls to ~50% at day 7, ~25% at day 14, and below 10% by around day 23. Most patients report noticeable return of hunger within 2–3 weeks of their last injection as drug activity diminishes.
Is there a difference in half-life between branded and compounded GLP-1?+
No — the half-life is determined by the active molecule, not the manufacturer. Branded Wegovy, Ozempic, and compounded semaglutide all contain the same molecule with the same ~7-day half-life. The same applies to Zepbound, Mounjaro, and compounded tirzepatide. Oral semaglutide (Rybelsus) has a different profile due to lower GI bioavailability and should not be modeled using these values.
Can I change my injection day?+
Yes — both medications can be shifted to a different day of the week. The long half-lives mean shifting by 1–2 days has minimal effect on therapeutic drug levels or side effects. Larger shifts should be discussed with your provider. The standard guideline is that at least 2 days must have passed since your last injection before the new injection day.