Drug elimination involves many complex processes and does not necessarily differentiate between distribution and elimination. It is divided into two primary components: excretion and biotransformation.

Excretion refers to removing a drug from the body, either in its unchanged form or as its metabolites. Nonvolatile and polar drugs are primarily excreted through the kidneys, with other pathways including bile, sweat, saliva, and milk. Volatile drugs such as anesthetic gases are excreted via the lungs.

Biotransformation refers to the chemical conversion of the drug to a metabolite. Lipid-soluble drugs are not readily removed until metabolized into more polar compounds. This occurs primarily in the liver but also in other tissues, such as the kidney, lung, small intestine, and skin.

The kidneys play a crucial role in excreting drugs and their metabolites, accounting for eliminating 25%–30% of drugs administered to humans. Fecal excretion mainly consists of unabsorbed orally ingested drugs or their metabolites. Breast milk excretion, though small in amount, is significant due to potential effects on nursing infants.