GLP-1 Half-Life Calculator
Semaglutide and tirzepatide are designed to last about a week in the body, which is why they are injected weekly. This calculator estimates how much of the medication remains in your system after your last dose, week by week, using the approximate half-life values published in FDA prescribing information.
How this estimate works
A drug's half-life is the time it takes for the amount in your body to fall by half. Semaglutide has a half-life of about 7 days and tirzepatide about 5 days, according to their FDA prescribing information. The calculator treats the amount in your body at your last dose as 100 percent, then halves it every half-life: after one half-life about 50 percent remains, after two about 25 percent, after three about 12.5 percent, following the standard exponential decay formula.
Knowing the half-life answers questions people ask all the time: why a missed dose is handled the way it is, why side effects can linger for days after an injection, and why it takes several weeks after stopping before the medication is fully gone and appetite returns. After about five half-lives, roughly 97 percent of the drug has cleared, which is about 5 weeks for semaglutide and about 3 and a half weeks for tirzepatide.
Real bodies are not this tidy. Your kidneys, your dose, how long you have been on treatment and your own metabolism all shift the real curve, and this model also treats your last dose as a single starting point rather than the built up level from months of weekly injections. Use the result to understand the pattern, not as an exact reading. If you want to see the other side of the timeline, how results build up month by month, the GLP-1 Weight Loss Calculator covers that.
Frequently asked questions
What does half-life mean for a medication?
Half-life is the time it takes for the amount of a drug in your body to fall by half. After one half-life about 50 percent remains, after two about 25 percent, after three about 12.5 percent, and so on. It is why these medications are dosed weekly and why their effects fade gradually rather than switching off.
How long until GLP-1 medication is fully out of my system?
A common rule of thumb is that after about five half-lives, roughly 97 percent of a drug has cleared. For semaglutide, with a half-life of about 7 days, that is around 5 weeks. For tirzepatide, with a half-life of about 5 days, it is around 3 and a half weeks. Small traces can remain beyond that, and how you feel does not follow the percentage exactly.
Can this calculator tell me when my appetite will come back after stopping?
No. It models the amount of drug remaining, not how you will feel. Appetite, side effects and blood sugar control change at their own pace as levels fall, and they vary a lot between people. If you are thinking about stopping, pausing for surgery or planning a pregnancy, talk to your own doctor first, because the timing advice for those situations is personal.