This free GLP-1 half-life calculator estimates how much semaglutide or tirzepatide is still active in your body right now, and how that level rises and falls across the week between shots. Use it as an Ozempic, Wegovy, Mounjaro, or Zepbound half-life calculator: pick your medication, enter your dose and last shot, and it plots the curve. It applies each drug’s published half-life, not a blood test, and it is not a dosing tool.
How This GLP-1 Half-Life Calculator Works
Semaglutide and tirzepatide are cleared from the body slowly, which is why they are dosed once a week. Semaglutide’s elimination half-life is about 7 days; tirzepatide’s is about 5 days. That means a week after a shot, roughly half (semaglutide) or a little more than a third (tirzepatide) of that dose is still active, and the next shot lands on top of it. The calculator sums what is left of each recent dose, remaining = dose × 0.5(days since shot ÷ half-life), across your last several shots, and plots the total across the weekly cycle.
Two things fall out of that math. First, levels keep climbing for four to five weeks after you start or increase a dose, until what is added each week equals what is cleared. That is the “steady state” the labels are built around, and it is why the dose schedule holds each step for at least four weeks. Second, the swing between the post-shot peak and the pre-shot trough is real but modest, roughly 30 to 40 percent, which is why many people notice appetite creeping back on days 5 to 7. If that pattern is strong for you, it is worth logging in Pep and mentioning to your prescriber; it is not a reason to move your shot day on your own.
Semaglutide vs Tirzepatide Half-Life
The published half-lives come from the prescribing information: about one week for semaglutide (Ozempic, Wegovy, and compounded semaglutide) and about five days for tirzepatide (Mounjaro, Zepbound, and compounded tirzepatide). The shorter half-life is one reason tirzepatide’s missed-dose window on the label is four days rather than five. Daily pills like Rybelsus, the Wegovy pill, and Foundayo follow different curves and are not modeled here.
GLP-1 Half-Life by Medication
Every number in this GLP-1 half-life calculator comes from the FDA prescribing information for each drug. Here is how the common GLP-1 medications compare, and roughly how long each one takes to clear (about five half-lives):
- Ozempic and Wegovy (semaglutide injection): half-life about 7 days (roughly 160 hours); mostly cleared about 5 weeks after the last shot.
- Mounjaro and Zepbound (tirzepatide): half-life about 5 days; mostly cleared about 3.5 to 4 weeks after the last shot.
- Rybelsus and the Wegovy pill (oral semaglutide): the same roughly one-week half-life as the injection, but a daily dose builds up differently, so the weekly curve above does not apply.
- Trulicity (dulaglutide): half-life about 5 days, similar to tirzepatide.
- Saxenda and Victoza (liraglutide): half-life about 13 hours, which is why they are injected daily and clear within a few days.
Sources: the Ozempic prescribing information and Ro’s clinical review of how long tirzepatide stays in your system.
When a Semaglutide or Tirzepatide Half-Life Calculator Is Useful
Most people open a half-life calculator for one of four reasons:
- The pre-shot dip. If hunger returns on days 5 to 7, the curve shows why: you are at the weekly trough. Logging those days in Pep gives your prescriber a real pattern to work with instead of a vague “I feel hungrier.”
- Starting or stepping up. Levels keep climbing for four to five weeks after each increase, which is why side effects often peak in weeks 2 to 4. Our guides to easing nausea and GLP-1 fatigue cover the rough weeks.
- Stopping or switching. Because semaglutide lingers for about five weeks and tirzepatide for about a month, appetite does not return overnight. See what happens when you stop a GLP-1 and, if you are moving between brands, Zepbound vs Wegovy. Your prescriber sets the timing of any switch, not the chart.
- A late shot. The curve shows how far levels have fallen, but the label rules decide what to do next: our missed dose calculator walks through them medication by medication.
GLP-1 Half-Life Calculator FAQ
Is this an Ozempic half-life calculator or a Mounjaro half-life calculator?
Both. Choose semaglutide for Ozempic, Wegovy, or compounded semaglutide, and tirzepatide for Mounjaro, Zepbound, or compounded tirzepatide. The brand does not change the half-life; the molecule does.
How many half-lives until a GLP-1 is out of your system?
After one half-life, 50% remains; after two, 25%; after three, 12.5%; after four, about 6%; and after five, about 3%. Five half-lives is the usual rule of thumb for “essentially cleared,” which works out to about 5 weeks for semaglutide and about 25 days for tirzepatide.
How long does semaglutide stay in your system?
With a half-life of about 7 days, semaglutide takes roughly five weeks (about five half-lives) to fall to a few percent of steady state after the last dose. Appetite effects usually fade over the first two to three weeks.
How long does tirzepatide stay in your system?
With a half-life of about 5 days, tirzepatide is mostly cleared about three and a half to four weeks after the last dose.
Why do I get hungrier the day before my shot?
Because the level is at its weekly trough. The dip is normal and usually modest. Log it in Pep for a few weeks; if it is strong and consistent, raise it with your prescriber rather than changing your shot day yourself.
Can I use this to decide when to take a late dose?
No. Use the label rules, which our missed-dose calculator walks through by medication, and confirm with your prescriber or pharmacist.
Does the calculator work for compounded GLP-1s?
It uses the same half-lives, so the shape of the curve is the same. Compounded products vary more in absorption, so treat the numbers as a rougher guide. Our units calculator handles the mg-to-units conversion for vials.
Download Pep free — see your medication level in the app, tied to every shot you log.
This calculator is for educational purposes only and is not medical advice. It estimates drug levels from published half-lives and does not measure anything. Never change a dose or schedule based on it; always follow your prescriber’s instructions and the prescribing information for your medication. Last reviewed September 2026.