excretion
Excretion is how the body shows a drug the door. After a drug has done its job (or been chemically altered by metabolism), it and its breakdown products must be physically removed, mainly in urine and feces. Without excretion, a drug would simply accumulate with every dose until it reached toxic levels.
The kidneys are the main exit. They filter the blood, and water-soluble drugs and metabolites pass into the urine; some compounds are also actively pumped out by transporters in the kidney tubules. The liver provides a second route, dumping drugs and metabolites into bile, which carries them into the gut for loss in the feces. Because the kidney handles water-soluble molecules well but greasy ones poorly, the body often metabolizes a lipophilic drug into a more water-soluble form first, specifically to make it excretable.
Excretion is the final E of ADME and, together with metabolism, makes up the body's total elimination of a drug. A patient with impaired kidney function clears renally excreted drugs more slowly, so doses often must be reduced to avoid dangerous buildup.
Many antibiotics are excreted largely unchanged by the kidneys, which concentrates them in the urine — useful for treating urinary infections but a reason to lower the dose in patients with kidney disease.
Renal excretion both shapes efficacy and demands dose adjustment.
Excretion (physical removal) and metabolism (chemical change) are distinct steps, but both count toward elimination — the net disappearance of active drug from the body.