This free GLP-1 medication level 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. Pick your medication, enter your dose and last shot, and it plots the curve. It uses each drug’s published half-life, not a blood test, and it is not a dosing tool.
How the GLP-1 Medication Level 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.
Medication Level Calculator FAQ
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 August 2026.