GLP-1 Half-Life Calculator: How Long It Stays in Your System

Half-life is the time it takes for half of a medicine to leave your body. If a GLP-1 has a half-life of about a week, then a week after a shot roughly half of that dose's estimate is still around. Another week later about a quarter is left, then an eighth, and so on. It never drops to exactly zero on paper, but after about five half-lives only a few percent remains, so it is treated as mostly gone. This calculator uses that idea and nothing fancier: it takes the published half-life for your medication, applies it to each shot you enter, and adds the leftovers together. That is why it is an estimate for learning, not a reading of what is actually in your blood.

It depends on which one you take. Semaglutide, the medicine in Ozempic and Wegovy, has a half-life of about 1 week, so most of a dose is gone in roughly 5 weeks. Tirzepatide, the medicine in Mounjaro and Zepbound, has a shorter half-life of about 5 days, so most of a dose clears in about 25 days, a little over three weeks. Those windows come from applying five half-lives to the numbers on each medication's FDA label. Pick your medication at the top of this page and scrub the timeline forward to watch your own estimate fade toward zero after your last shot. Keep in mind that when the appetite effect fades is personal and can run on a different clock than the math.

Semaglutide clears more slowly than tirzepatide. Semaglutide's half-life is about 7 days, while tirzepatide's is about 5 days, so a tirzepatide dose loses half of its estimate a couple of days sooner. Over a full washout that difference adds up: semaglutide reaches the five-half-lives mark in about 5 weeks, tirzepatide in about 25 days. Both still build up week to week, because a new weekly shot lands while part of the last one is present, but the tirzepatide curve dips a little lower between shots. See the two side by side by switching the medication in the calculator, or open the dedicated semaglutide half-life calculator or tirzepatide half-life calculator. This is educational math, not a reason to prefer one medication over the other.

By the common rule of thumb, a medicine is mostly cleared after about five half-lives, when roughly 97% has left. For semaglutide (Ozempic, Wegovy) that is about 5 weeks: half of the last dose's estimate is gone in a week, three quarters by two weeks, and about 97% by five weeks. For tirzepatide (Mounjaro, Zepbound) it is about 25 days on the same five-step pattern. The last few percent keeps shrinking but never reaches an exact zero on paper, which is why basically gone is the honest way to say it. Scrub this calculator's timeline past your last shot to see the curve flatten toward the bottom. How you feel as the level drops is individual, so bring any questions about stopping to your prescriber.

Yes. This calculator works with any GLP-1, branded or compounded, wherever it is prescribed. If you take compounded semaglutide, choose semaglutide and the tool uses the about 1-week half-life; if you take compounded tirzepatide, choose tirzepatide and it uses the about 5-day half-life. Enter your real dates and doses just as you would for a branded pen. The only number the math cares about is the half-life of the molecule you are on, so the curve looks the same whether your prescription came from a pharmacy shelf or was mixed for you. As always, this is an estimate for learning, not a blood level, and it is not a substitute for the instructions that came with your medication.

Because each weekly shot lands before the last one has cleared. A week after a semaglutide dose, about half of it is still estimated to be present, so the new shot stacks on top of that leftover. The same thing happens with tirzepatide, just from a slightly lower starting point because it fades a little faster. For the first few weeks the amount added each week is more than the amount that clears, so the peaks climb. Once the two roughly match, the curve settles into a repeating up-and-down pattern, often called a steady level. The calculator draws that climb straight from your own dates. Seeing dose amounts build and hold is expected once-weekly behavior, not a sign that anything is wrong.