Drug elimination, clearance and half-life !

How the body removes a drug, why some drugs last hours and others weeks, and why levels build up when dosing is repeated.

Updated 2026-11-01

Elimination is the sum of metabolism (mainly in the liver) and excretion (mainly by the kidney, also bile, lungs and other routes). Clearance and half-life describe how fast it happens. They explain how often a drug has to be given, how long it takes to reach a stable level and how long effects last after it is stopped.

Clearance

Clearance adds up every organ that removes the drug: renal clearance (filtration, tubular secretion, minus reabsorption), hepatic clearance (metabolism and biliary excretion) and minor routes. Hepatic clearance depends on liver blood flow, enzyme activity and the free fraction. Renal clearance falls with age, dehydration and kidney disease.

Half-life and steady state

Half-life is a consequence, not a cause: it reflects how big the space is that the drug fills (volume of distribution) and how fast it is cleared. After a drug is started at a fixed rate, each half-life closes half of the remaining gap to steady state; after it is stopped, each half-life halves the level. That is why a drug with a long half-life takes long to reach steady state and its effects outlast the last dose. The digoxin label notes that steady state comes later when kidney function is poor.

Long half-life: slow to build up, slow to wash out, often once-daily dosing
Short half-life: rapid on and off, needs frequent dosing or an infusion
A loading dose reaches the target quickly without waiting several half-lives

First-order and zero-order kinetics

In first-order kinetics the rate of elimination is proportional to the concentration, so a fixed fraction leaves per hour and half-life is constant. When the eliminating enzyme is saturated, a fixed amount leaves per hour (zero-order) and half-life lengthens as the level rises. The phenytoin label warns that its hydroxylating enzymes saturate, so a small dose increase can produce a very large rise in serum level.

Renal and hepatic elimination

Kidney: glomerular filtration of free drug, active tubular secretion by transporters, and passive reabsorption that depends on urine pH. Gentamicin is excreted mainly by glomerular filtration, so its dosing interval is lengthened or the dose reduced in kidney impairment and serum levels are measured. Liver: phase I and II metabolism and excretion into bile; some conjugates return from the gut (enterohepatic recycling).

Test yourself

Think of the answer, then tap the question to see it.

Which two factors determine a drug's half-life?

Its volume of distribution and its clearance: half-life lengthens when the volume is larger or the clearance is lower.

Does a higher maintenance dose shorten the time to reach steady state?

No. Time to steady state depends on half-life only; the dose changes the level reached, not how fast it is reached.

Why can a small dose increase of phenytoin cause toxicity?

Its metabolism saturates (zero-order kinetics), so above a certain level elimination cannot keep up and the serum level rises out of proportion.

Why are gentamicin levels monitored more closely in kidney impairment?

Gentamicin is cleared almost entirely by glomerular filtration, so it accumulates when kidney function falls, raising the risk of kidney and ear toxicity.

Add to my cards (4)

Example drugs

← All of pharmacology

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - DIGOXIN tablet (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - GENTAMICIN SULFATE injection, solution (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - EXTENDED PHENYTOIN SODIUM- phenytoin sodium capsule (DailyMed (U.S. National Library of Medicine), 2026); Therapeutic Drug Monitoring (MedlinePlus (U.S. National Library of Medicine), 2026).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.