A drug half-life is the time it takes for the level of a medicine in your blood to fall by half. If the half-life is one week, the level in your blood is about half as high seven days later, a quarter after two weeks, an eighth after three. The level fades on a curve. It doesn't switch off.
That one number explains why semaglutide can be a weekly shot, why sermorelin's blood level is counted in minutes, and why a new dose of a weekly medicine takes about a month to settle. It doesn't tell you how long a medicine works, or when a test stops finding it.
What does drug half-life mean in medicine?
In medicine, "half-life" nearly always means elimination half-life. A National Library of Medicine StatPearls chapter defines it as "the time required for the concentration of a specific substance, typically a drug, to decrease to half of its initial amount in the body" (Hallare and Gerriets, StatPearls, updated 2025). Most medicines are cleared in proportion to how much is there, so the body removes the same fraction, not the same amount, in each stretch of time. That's why the halving repeats.
On a U.S. prescription label, the number sits in section 12.3, Pharmacokinetics (what the body does to a drug over time), under "Elimination." FDA's labeling guidance says the figure reported there should usually be the one that predicts how long a drug takes to reach steady state.
The term exists because it predicts two practical things: how often a drug needs to be given, and how long it lingers after the last dose.
How many half-lives does it take to leave your system?
The usual answer is four to five. A short teaching paper in Paediatrics & Child Health lays out the arithmetic: 50% of the peak level remains after one half-life, then 25%, 12.5%, 6.25%, and about 3.1% after five. By four to five half-lives, the level has dropped by "almost 94% to 97%" (Ito, 2011).
So "gone" really means "down to a few percent," not zero. For a medicine with a one-week half-life, that's about five weeks. For one with a four-minute half-life, it's about twenty minutes. Same rule, very different calendars.
A label's figure is an average, and individual factors such as kidney or liver function can shift it. It also describes blood levels, not how you feel.
Steady state: why a new dose takes weeks to settle
Steady state is the point where the amount arriving with each dose matches the amount leaving between doses, so the level rises and falls in the same pattern every time. It takes the same four to five half-lives to get there, because by then the earliest doses have faded to almost nothing.
The labels line up with the rule. As of September 23, 2026, the day this article was written, the current Ozempic label (revised May 2026) gives semaglutide a half-life of about one week and steady state after 4 to 5 weeks of weekly doses. The Mounjaro label (revised August 2026) gives tirzepatide about 5 days and steady state after 4 weeks. The daily pill orforglipron, with a half-life of roughly 29 to 49 hours, gets there in about a week, per the Foundayo label (revised July 2026).
In everyday terms, the first weeks on a weekly medicine, or after a prescribed dose change, don't yet show the full exposure at that dose. Each drug's own timeline is laid out in how long semaglutide stays in your system and how long tirzepatide stays in your system.
Elimination half-life vs terminal half-life
After a dose, the blood level doesn't always fall in one smooth line. There can be an early, faster drop while the drug spreads into tissues, then a slower final decline. The half-life measured in that last stretch is the terminal half-life. The Vyleesi label (revised March 2024) gives bremelanotide (PT-141) a "mean terminal half-life" of about 2.7 hours.
A 2004 review adds a subtlety: the terminal half-life is the time for the plasma concentration (the level in the liquid part of blood) to halve, "and not the time required to eliminate half the administered dose." When absorption is the slow step, the terminal half-life reflects absorption rather than elimination (Toutain and Bousquet-Mélou, Journal of Veterinary Pharmacology and Therapeutics).
The body's own GLP-1 shows how much the route matters. Novo Nordisk scientists report its half-life as about 1.5 minutes when given into a vein and about 1.5 hours when injected under the skin (Knudsen and Lau, Frontiers in Endocrinology, 2019). Same molecule; what changed is how quickly it reaches the blood.
Why some drugs last a week and others minutes
Small peptides (short chains of amino acids) tend to be cleared fast. Sermorelin, a copy of the first 29 amino acids of growth hormone-releasing hormone, disappeared from the blood with a half-time of 4.3 minutes during an intravenous infusion in 10 healthy men. Swapping one amino acid for its mirror-image form stretched that to 6.7 minutes (Soule et al., Journal of Clinical Endocrinology & Metabolism, 1994).
Semaglutide and tirzepatide were built to last. Both carry a fatty-acid chain that lets them ride on albumin, a long-lived protein in the blood. The Ozempic label says albumin binding is the main reason for semaglutide's long half-life, cutting clearance by the kidneys and protecting it from breakdown. The Mounjaro label says tirzepatide's fatty acid "enables albumin binding and prolongs the half-life."
| Medicine | Half-life | Source |
|---|---|---|
| The body's own GLP-1 | About 1.5 min into a vein; about 1.5 h under the skin | Knudsen and Lau, 2019 |
| Sermorelin | 4.3 min (intravenous infusion) | Soule et al., 1994 |
| PT-141 (bremelanotide) | About 2.7 h (range 1.9–4.0) | Vyleesi label |
| Orforglipron (daily tablet) | About 29–49 h | Foundayo label |
| Tirzepatide | About 5 days | Mounjaro label |
| Semaglutide | About 1 week | Ozempic label |
The details live on each compound's page, such as sermorelin's half-life and PT-141's.
Mistakes people make with half-life
"One half-life and it's gone." Half is still there; it takes four to five to reach a few percent.
"Short half-life means a short effect." A drug can leave the blood while the signal it started carries on. In a study of 30 healthy men, sermorelin was rapidly eliminated after an intravenous injection, yet growth hormone stayed raised for about three hours (Wilton et al., Acta Paediatrica, 1993).
"A number from one study applies everywhere." A half-life belongs to one molecule, given one way, in one species. BPC-157 has no measured human half-life at all, only figures from rats and dogs (BPC-157 half-life), and the figure often quoted for TB-500 comes from a different, larger molecule (TB-500 half-life).
"Half-life tells me when a test will come back clean." Detection depends on the test, the lab and what it looks for, sometimes breakdown products rather than the drug itself.
"Longer is always better." Fewer doses, yes. But a 2018 review by drug-design scientists notes that a very long half-life stretches out both the build-up and the washout, which can make side effects harder to manage (Smith et al., Journal of Medicinal Chemistry).
How half-life shows up in a prescription
Half-life shapes the schedule: the Mounjaro label says tirzepatide's roughly five-day half-life is what enables "once-weekly dosing." It shapes the exit, too. The Wegovy label (revised June 2026) says semaglutide stays in circulation for about five to seven weeks after the last dose, and says to stop at least two months before a planned pregnancy "to account for the long half-life of semaglutide." Half-life describes how fast the level in the blood falls, and when a drug is absorbed slowly it can reflect that absorption too; how much of a dose reaches the blood at all is bioavailability.
What half-life means for a compounded prescription
Through Promise, semaglutide and tirzepatide are dispensed as compounded medications, which is different from an FDA-approved product: the formulations offered here are not FDA-approved. Half-life is a property of the molecule, but the concentration, volume and directions on a compounded prescription can differ from a branded pen's, so the pharmacy label and the reviewing provider set the actual plan. A licensed provider may still prescribe a compounded formulation; that decision is between the patient and the doctor. At Promise, a licensed provider reviews every request, and not everyone qualifies.
Related terms
Bioavailability: how much of a dose reaches the blood, and how fast. Peak concentration: how high the level rises after a dose; the Ozempic label puts semaglutide's peak 1 to 3 days after an injection. Clearance: how quickly the body removes a drug from the blood; together with how widely the drug spreads through the body, it sets the half-life. Washout: the time a drug takes to fall to negligible levels, often taken as about five half-lives.
