What half-life means

One number, one meaning

The elimination half-life describes how fast a medicine leaves the blood once it is there. If the level is 100 at some point, one half-life later it is about 50, then about 25, and so on. Labels quote it as “approximately”, because it comes from groups of people in clinical pharmacology studies.

It is a property of the molecule in an average body, not a prediction for you. Labels also report which factors were studied: the Wegovy® label, for example, found no clinically significant differences by age, sex, body weight, reduced kidney function within the range it tested, or liver impairment ^[3]. How a medicine fits you is still your prescriber’s call, not a formula’s.

Halving, not emptying

• After 1 half-life, about half of the amount is still there.
• After 3 half-lives, about one eighth (12.5%).
• After 5 half-lives, roughly 3%.
• This is why the Ozempic® label pairs a half-life of about one week with semaglutide still circulating roughly five weeks after the final injection: five weeks is about five half-lives ^[2]. The Wegovy® label gives 5 to 7 weeks after the last dose ^[3].

Why peptide half-lives differ so much

Natural GLP-1 lasts only 1.5 to 2 minutes in the blood, according to the Saxenda® label, because enzymes called DPP-4 and neutral endopeptidases break it down ^[1]. The medicines built on it were changed to resist that, and each label says how. The table puts them side by side, from shortest to longest.

Animal numbers are not human numbers

Half-lives are first measured in animals, and the gap can be large. When Novo Nordisk scientists described how they designed semaglutide, they reported a plasma half-life of 46.1 hours after an intravenous dose in mini-pigs ^[13]. In people, the labels give about one week ^[2].

This matters for compounds that have only been studied in animals, such as many products sold as research peptides. A half-life from a rat or a pig is a clue about the molecule, not a figure for a human body.

Half-life is not time to peak

How fast a medicine leaves the blood is a different question from how fast it gets there. The labels list the time to maximum concentration separately: about 30 minutes for teriparatide, 11 hours for liraglutide, a median of 24 hours (range 8 to 72) for tirzepatide in the Zepbound® label, and 1 to 3 days for injected semaglutide ^[6][1][5][2].

So a long half-life does not mean a slow start, and a short one does not mean a fast one. The two numbers describe opposite ends of the same curve.

Half-life and the dosing interval

Labels often connect the two. The Mounjaro® label says a half-life of about 5 days enables once-weekly dosing, the Saxenda® label says a 13-hour half-life makes liraglutide suitable for once-daily use, and the European information for Trulicity® says a half-life of 4.7 days suits weekly injections ^[4][1][10].

But half-life is not the only factor. Teriparatide leaves the blood within hours, yet the Forteo label describes once-daily injections and explains that its effects on bone depend on the pattern of exposure ^[6]. The interval on a label comes from clinical studies of that product, not from the half-life alone.

Same molecule, different numbers

Read two documents about the same medicine and the half-life may not match exactly. The Mounjaro® label gives about 5 days for tirzepatide, while the Zepbound® label gives 5 to 6 days and says this applies to people with overweight or obesity, with or without sleep apnoea ^[4][5]. The European information for Trulicity® even uses two figures in one document: 4.7 days in its pharmacodynamic section and about 5 days in its pharmacokinetic section ^[10].

These are estimates from different study groups and models, not contradictions. Some conditions change the number more clearly. In five people with severe kidney impairment, the half-life of teriparatide was 77% longer than usual ^[6]. For semaglutide, the Wegovy® label found no clinically significant difference in mild to moderate kidney impairment or any degree of liver impairment ^[3].

Half-life is not how long the effect lasts

Blood levels and effects do not always move together. The Zepbound® and Foundayo™ labels both say that stomach emptying slows most after the first dose and that this slowing fades with time, even while the medicine is still being taken and the level is building up ^[5][12].

So the half-life describes the molecule in the blood, nothing more. What a medicine does is reported separately, in the pharmacodynamics section (12.2) and in the clinical studies.

Where the documents put the number to use

The labels mostly use the half-life to explain a waiting period. These are decisions for your prescriber; the half-life is the reason the documents give for them.

Seeing the arithmetic for yourself

The free PepLabo half-life calculator plots how amounts add up and taper using a half-life you enter. It shows the arithmetic described here as an illustration, not as a measured blood level, and it never suggests an amount.

The same arithmetic explains why levels of a weekly medicine rise over the first weeks until they settle, which the labels call steady state; our article on steady state covers each label’s figures.

Ozempic®, Wegovy® and Saxenda® are registered trademarks of Novo Nordisk A/S. Mounjaro®, Zepbound® and Trulicity® are registered trademarks of Eli Lilly and Company, and Foundayo™ is a trademark of Eli Lilly and Company. Symlin® is a registered trademark of the AstraZeneca group of companies; the FDA lists its US presentations as discontinued ^[14]. PepLabo is not affiliated with or endorsed by Novo Nordisk, Eli Lilly and Company or AstraZeneca.

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