What does half-life mean?

What does half-life mean?

In pharmacology, “elimination half-life” is the time it takes for the concentration of a substance that has entered the body to fall by half in the blood (plasma) after distribution is complete, and it is abbreviated as t½. For most drugs this time is independent of dose: a large dose and a small dose both fall by half in the same time. The Summary of Product Characteristics (SmPC; Turkish KÜB) of licensed drugs gives this figure in the “Pharmacokinetic properties” section. The Ozempic SmPC gives about 1 week for semaglutide ^[1], the Mounjaro SmPC about 5 days for tirzepatide ^[2], and the Saxenda SmPC about 13 hours for liraglutide. ^[3]

This site’s name also comes from this concept: knowing how long a substance stays in the body is the first step in understanding how often it is given, when its effect fully settles in, and when it will end once stopped.

How is the half-life number read?

Each half-life halves the amount remaining at that moment. The table below shows this arithmetic; the time elapsed after a single dose is measured in numbers of half-lives.

The sentence in the Ozempic SmPC is this calculation itself: with an elimination half-life of about 1 week, semaglutide remains in the circulation for about 5 weeks after the last dose. ^[1]

What are steady state and accumulation?

When a drug is given at regular intervals, the new dose is added on top of what remains from the previous dose; this is called accumulation. Accumulation does not continue forever: at some point, the amount eliminated between two doses balances the dose given. This equilibrium level is called steady state. The same arithmetic applies here: when doses are continued at equal intervals, about 94% of the steady-state level is reached at 4 half-lives and about 97% at 5 half-lives. That is why pharmacology says “steady state is reached in 4–5 half-lives.”

The numbers in the SmPCs follow this rule. According to the Ozempic SmPC, steady-state exposure is reached in 4–5 weeks with once-weekly administration ^[1]; according to the Mounjaro SmPC, this period is 4 weeks for tirzepatide. ^[2]

Substances with a short half-life show no accumulation or very little. The SmPC of the immediate-release melatonin tablet licensed in Türkiye gives the plasma elimination half-life in healthy adults as about 45 minutes (normal range about 30–60 minutes). ^[11] With such a short half-life, a nighttime dose is largely eliminated by the next evening.

Half-life through examples: from GLP-1 to caffeine

The numbers in the table were measured with different methods and in different people; they are not used for a direct “which is better” comparison between two substances in the same row. The retatrutide and CJC-1295 rows are included to remind that the existence of a measured half-life for unlicensed substances does not mean a licence or safety.

Vitamin B12 is deliberately not placed in this table: according to the NIH, the body stores about 1–5 mg of B12; this is about 1,000–2,000 times the daily intake, and that is why it can take several years for deficiency symptoms to appear. ^[10] What is decisive here is not a blood half-life but a large store. The “filling of muscle stores” discussed for creatine is likewise not a half-life but a separate concept relating to the rate of storage; details are in the Creatine guide.

Why is the half-life of peptides short, and how is it extended?

Native GLP-1 is a 30-amino-acid hormone, and its half-life is 1.5 minutes when given intravenously and 1.5 hours when given subcutaneously. ^[5] There are two main ways that make peptides short-lived: enzymes in the blood (DPP-4 for GLP-1) cutting the peptide, and rapid filtration of small molecules in the kidney. Drug developers have closed these two routes with different engineering solutions.

• Enzyme-resistant sequence: Exendin-4, obtained from the venom of Heloderma lizards, shares 53% similarity with human GLP-1. A single amino acid difference at the second position of the sequence (alanine in GLP-1 — Ala8 in classical numbering — and glycine in exendin-4) makes it resistant to DPP-4; its intravenous half-life is about 30 minutes. ^[5] Exenatide, its synthetic form, has a mean terminal half-life of 2.4 hours when given subcutaneously; according to the Byetta SmPC, nonclinical studies showed that exenatide is eliminated mainly by being broken down after kidney filtration (glomerular filtration). ^[4]
• Fatty acid chain and albumin binding (acylation): Fatty acid chains have been added to liraglutide and semaglutide; these chains allow the molecule to bind reversibly to the albumin protein in the blood. Albumin is a large, stable protein that remains in the circulation for weeks; a peptide clinging to it is protected both from DPP-4 and from filtration in the kidney. ^[5] The Ozempic SmPC also describes the basic mechanism of semaglutide’s long action as albumin binding, with the resulting reduction in renal clearance and protection from metabolic breakdown. ^[1] According to the Mounjaro SmPC, tirzepatide also carries a C20 fatty diacid moiety that enables albumin binding and prolongs half-life. ^[2]
• Permanent attachment to a large protein: Albiglutide was produced by permanently attaching GLP-1 to albumin, and dulaglutide by attaching it to an antibody fragment (IgG Fc region); the aim is again to limit filtration by the kidney. ^[5]
• Slow-release depot: The weekly exenatide formulation was obtained by entrapping the drug in microspheres that dissolve slowly under the skin. ^[5]
• PEG attachment (PEGylation): Adding polyethylene glycol chains is a chemical modification used in protein and peptide drugs to reduce the problems of rapid clearance from the circulation, enzymatic breakdown, and immune reaction. ^[8]

The same logic has been used with insulin. The first approved protein with an added fatty acid is insulin detemir; the prolongation of its effect enough for once-daily administration largely results from a depot formed under the skin, whereas with insulin degludec multi-hexamer structures form under the skin. ^[5]

Extending half-life also has a cost. According to the review by the team that developed liraglutide and semaglutide, very strong binding to albumin reduced the molecule’s potency at the GLP-1 receptor; the suitable fatty acid and linker combination was chosen by balancing these two properties. ^[5] Two authors of the review are employees and shareholders of the company that developed and markets these drugs; one is among the inventors of liraglutide and the other of semaglutide. ^[5]

CJC-1295 is likewise an analog designed to prolong the duration of action of growth hormone-releasing hormone (GHRH), and its estimated half-life in healthy adults has been measured as 5.8–8.1 days. ^[7] This measurement is not evidence of a licence or safety; the substance’s licence status and limited human data are described in the CJC-1295 guide.

How does half-life determine the dosing interval?

A drug is dosed so that it is given again before it falls below the effective level; for this reason the dosing interval is closely related to half-life. The Saxenda SmPC gives an elimination half-life of about 13 hours for liraglutide, and the drug is given once daily. ^[3] The Mounjaro SmPC explicitly writes that tirzepatide’s elimination half-life of about 5 days allows once-weekly administration ^[2]; the Ozempic SmPC also states that the half-life of about 1 week makes semaglutide suitable for once-weekly subcutaneous administration. ^[1]

Why do missed-dose rules differ between drugs?

The missed-dose rule is likewise defined separately in each product’s SmPC and is not transferred between drugs. The rules below are summarized verbatim from the SmPCs; they are product information written for physicians.

The shared logic is that a missed dose should not come too close to the next planned dose; if two doses overlap, the blood level can briefly rise above what was planned. The Mounjaro SmPC also writes that if the day of administration is to be changed, there must be at least 3 days between two doses. ^[2] Product details are on the How is Ozempic used? and How is Mounjaro used? pages.

How long does a drug stay in the body after it is stopped?

The “washout” period after stopping is also calculated from half-life, and its most concrete counterpart is in pregnancy warnings:

• According to the Ozempic SmPC, semaglutide should not be used during pregnancy and, because of its long half-life, should be discontinued at least two months before a planned pregnancy. ^[1]
• According to the Mounjaro SmPC, tirzepatide should be discontinued at least 1 month before a planned pregnancy because of its long half-life. ^[2]

The difference between the two periods runs in the same direction as the difference between the two drugs’ half-lives. In overdose as well, half-life is decisive: the Ozempic SmPC writes that, considering semaglutide’s half-life of about 1 week, a long period of observation and treatment for symptoms may be needed ^[1]; the Mounjaro SmPC gives the same warning for tirzepatide based on its half-life of about 5 days. ^[2]

A drug leaving the blood and its effect ending are not the same thing; what happens to weight after stopping has been measured in separate studies and is covered on the What happens when Ozempic is stopped? page.

Are biological half-life and elimination half-life the same thing?

No. Elimination half-life is the time for the concentration in plasma to fall by half. In everyday language, “biological half-life” is often used to mean the rate of decrease of the total amount of a substance in the body or of its effect, and the two numbers do not have to be the same. Three examples show the difference:

• The effect can last longer than the molecule. In the CJC-1295 study, after a single injection, the mean growth hormone level stayed elevated for 6 days or longer and the IGF-1 level for 9–11 days; after multiple doses, IGF-1 remained above baseline for up to 28 days. ^[7] What is measured here is not the substance itself but the hormone cascade it triggers.
• Two forms of the same vitamin have very different lifespans. According to the NIH, the circulating half-life of 25(OH)D, the form of vitamin D measured in the blood, is 15 days; the half-life of the active form 1,25(OH)₂D is measured in hours, and for this reason it is not a good indicator of vitamin D status. ^[9]
• A store can behave independently of the blood level. For B12, body stores of 1,000–2,000 times the daily intake explain why deficiency can be delayed for several years. ^[10]

The route of administration also changes the measured number. The two half-lives of native GLP-1, 1.5 minutes intravenously and 1.5 hours subcutaneously ^[5], are two different measurements of the same molecule: the time measured subcutaneously also includes the slow passage of the substance from tissue into blood.

What does half-life not show?

• It does not show the size of the effect. Modifications that extend half-life can reduce potency at the receptor; developers look for a balance between the two. ^[5]
• It is not the same in everyone. The half-life of caffeine varies between 2.5 and 10 hours in healthy adults and is affected by factors such as genetic differences. ^[12]
• It is not constant at every dose. For some substances the breakdown pathway becomes saturated and half-life lengthens: according to the same review, in caffeine overdose the liver enzyme system (CYP1A2) becomes saturated and the elimination half-life can lengthen to 27 hours; in a case that took 5.9 g of caffeine, an apparent terminal half-life of 27 hours was measured in the first 2 days. ^[12]
• It does not show safety. The measured half-life of about 6 days for retatrutide ^[6] or the measured half-life of 5.8–8.1 days for CJC-1295 ^[7] does not change the fact that these substances are not licensed. The status of retatrutide is described in the Retatrutide guide.

Where to go after this guide?

The GLP-1 hormone itself and how it became drugs are in the What is GLP-1? guide, the comparison of the drugs is on the GLP-1 agonists page, and the individual drugs are in the Semaglutide, Tirzepatide, and Liraglutide guides. The general definition of peptides is in What are peptides?, their types in Types of peptides, the storage conditions of licensed pens on the label on the How are peptides stored? page, the legal status of unlicensed products in Are peptides legal?, and side effects in the Peptide side effects guide. This guide does not recommend personal doses; decisions on dose, missed doses, and stopping belong to the physician.