Half-Life

Elimination half-life (t1/2) is a pharmacokinetic parameter defined as the duration required for the plasma concentration of an administered substance to decrease by 50% under first-order elimination kinetics. Half-life is governed by the relationship between systemic clearance (CL) and apparent volume of distribution (Vd): t1/2 = (0.693 × Vd) / CL.

Native human GLP-1 has a half-life of only 1.5–2 minutes due to rapid cleavage by DPP-4. In contrast, semaglutide was engineered with albumin-binding fatty diacid chains and DPP-4 resistance, extending its half-life to ~168 hours (one week).

Half-life explains why some peptide drugs last minutes and others last days. Natural GLP-1 is broken down within minutes by the enzyme DPP-4, which makes it useless as a long-acting medicine. Semaglutide was engineered with amino acid substitutions and a fatty acid chain that binds blood albumin, stretching its half-life to around a week. Much of modern peptide drug design is, in effect, the engineering of half-life.

It also explains how long a drug stays in the body after it is stopped. As a rule of thumb, it takes about four to five half-lives for most of a drug to be eliminated. For semaglutide, with a half-life of about a week, that means it remains present for several weeks after the last dose, so side effects or interactions can persist after stopping.

When half-life figures appear in marketing for research peptides, check their source. Many quoted values come from animal studies, from a related compound or from no study at all. Without human pharmacokinetic data, a claimed half-life is not reliable, and neither is any advice built on it. Half-life can differ greatly between species and between closely related molecules.

“After two half-lives, the drug is gone”

Each half-life removes half of what remains, not a fixed amount. After one half-life, about 50% remains; after two, about 25%; after three, about 12.5%. Under typical first-order elimination it takes roughly four to five half-lives for the level to fall to just a few percent of where it started.

“Longer half-life means a stronger drug”

Half-life describes how long a drug stays in circulation, not how strongly it acts. A long-acting drug can be mild and a short-acting one very potent. A long half-life does mean that effects, including side effects, take longer to fade, and that levels build up over the first few weeks of regular use.

“Half-life tells you how long the effect lasts”

Plasma half-life and duration of effect are related but not identical. Some drugs keep acting after blood levels fall, for example because of effects on receptors or downstream processes, while others stop working before they are cleared. The duration of effect is established in clinical studies, not inferred from half-life alone.

What is the half-life of semaglutide?

Semaglutide has an elimination half-life of about one week (around 168 hours) in people. This long half-life comes from its resistance to DPP-4 and its fatty acid side chain, which binds albumin in the blood and slows clearance. Because of it, semaglutide remains in the body for several weeks after the last dose.

What is the half-life of natural GLP-1?

Native GLP-1 has a half-life of only about one and a half to two minutes in the bloodstream. It is rapidly cut by the enzyme DPP-4 into an inactive fragment, a process shown in human studies of administered GLP-1. This short life is the main reason drug developers created longer-acting GLP-1 receptor agonists.

How long does it take for a drug to leave your body?

For drugs eliminated by first-order kinetics, a common rule of thumb is about four to five half-lives for nearly all of the drug to be cleared. A drug with a two-hour half-life is largely gone within a day, while one with a one-week half-life persists for over a month. Individual factors such as kidney or liver function can change this.