The last letter of ADME
Every drug's journey through the body is summarised by four letters: ADME — Absorption, Distribution, Metabolism, and Excretion. This track is about that last E. Excretion means the physical removal of a drug or its breakdown products from the body, most often in urine or bile. It is different from metabolism, which chemically changes a drug (usually into something easier to excrete) without itself getting rid of anything.
Together, metabolism and excretion are the body's two ways of getting rid of a drug. We group them under one word: elimination. Keep that distinction in your head — a drug molecule that is metabolised in the liver has not yet been excreted; it has just been turned into a new molecule that the kidney will later hand to the urine.
The exits: kidney, bile, and a few minor doors
The kidney is the main exit for most drugs and almost all water-soluble metabolites — this is renal elimination, measured by renal clearance. A second important route is biliary excretion: the liver dumps certain drugs into bile, which carries them into the gut and out in the faeces (faecal excretion). Minor routes include the lungs (for volatile anaesthetics), sweat, saliva, tears — and breast milk, which matters far more clinically than its small volume suggests.
- Kidney → urine. The dominant route. Best for small, water-soluble, polar molecules.
- Liver → bile → gut → faeces. Favours larger, conjugated molecules; some get reabsorbed and recycled.
- Lungs → exhaled air. Only for volatile/gaseous drugs (e.g. inhaled anaesthetics, alcohol).
- Minor doors. Sweat, saliva, tears, milk — small in amount but sometimes useful (or hazardous to a nursing infant).
Why this matters at the bedside
Knowing a drug's exit route tells you who is at risk. If a drug leaves mostly unchanged through the kidney, a patient with poor kidney function will accumulate it and may overdose. If it leaves through the liver and bile, liver disease is the concern. This single fact — how does this drug get out? — drives dose adjustments in the elderly, in kidney or liver failure, and in newborns whose exit routes are still immature.