Excretion

Excretion is the process by which the body removes a drug and its metabolites from circulation, ending the drug's pharmacological activity. It forms one of four core pharmacokinetic steps — alongside absorption, distribution, and metabolism — that together govern how a substance behaves in the body over time.

The primary route is renal excretion: the kidneys filter drug molecules and metabolites out of the blood and into urine. Secondary routes include biliary excretion through the liver into bile and feces, pulmonary excretion through the lungs for volatile compounds, and minor pathways via sweat, saliva, and breast milk.

How it works · its role

In renal excretion, the kidneys pass small molecules through the glomerular filter into forming urine. Tubular transporters can then actively secrete additional molecules into the urine or pull them back into the bloodstream — a balance that depends partly on a molecule's charge at the prevailing urine pH. Ionised molecules tend to remain in urine and be excreted; unionised ones are more readily reabsorbed.

Biliary excretion carries metabolites through the liver into bile, which drains into the intestine. Some of those compounds are then reabsorbed from the gut back into the bloodstream — a cycle called enterohepatic recirculation — which can meaningfully extend a drug's effective duration.

For volatile compounds, the lungs provide a third route: ethanol and inhaled gases leave the body in exhaled air, the principle behind breath alcohol testing.

The overall pace of removal is captured by a substance's half-life: the time for plasma concentration to fall by half. Complete elimination takes roughly five half-lives, so a drug with a twenty-hour half-life is not fully cleared for about four days.

Relevance to substances & effects

Excretion rate governs two timelines that often diverge sharply: how long a substance remains pharmacologically active, and how long it stays detectable. Cannabis produces psychoactive effects lasting a few hours, but its lipid-soluble metabolites accumulate in fat tissue and are excreted gradually; a single session may remain detectable in urine for a week or more in occasional users, and longer in regular ones.

Substances with long half-lives — many benzodiazepines, methadone, certain antipsychotics — can accumulate with repeated dosing, raising the risk of toxicity if the dosing interval is not adjusted accordingly.

Impaired kidney or liver function slows excretion and prolongs exposure, which is why dose adjustments are standard in renal or hepatic disease. Drug interactions can also alter excretion: compounds that compete for the same renal transporters, or that shift urine pH, can raise or lower plasma levels in ways that are difficult to anticipate.

Understanding excretion clarifies why a drug test can detect past use well after effects have resolved — detection windows reflect excretion kinetics, not ongoing intoxication.