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.
When drugs enter the body, they eventually undergo irreversible elimination. Drug elimination primarily involves excretion and metabolism or biotransformation.
Drug excretion is the removal of the unchanged drug. Polar, nonvolatile drugs are primarily excreted through the kidneys via the urine.
Lipophilic drugs are not directly excreted by the kidneys. Instead, first, they undergo biotransformation by the hepatic cytochrome P450 enzymes. The polar metabolites formed are then excreted via the urine.
Additional excretory routes may include breast milk, feces, hepatic bile, and through the lungs.
Drugs excreted in breast milk may affect the infant's well-being.
Fecal excretion is primarily associated with unabsorbed oral drugs. The unabsorbed oral drugs are excreted through the hepatic bile or directly into the intestines without reabsorption. In case of renal impairment, medications like digoxin are significantly excreted in feces instead of urine.
Volatile drugs exit through the lungs via exhaled air.
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