Drug excretion involves various organs, including the liver, intestines, skin, and eyes. In the case of drugs or toxins, they can be actively secreted into bile by transporters in the hepatocyte's canalicular membrane. These substances enter the GI tract during digestion and may be reabsorbed into the body from the intestine. This process, known as enterohepatic recycling, can significantly prolong the presence and effects of a substance in the body. To interrupt this cycle, specific substances can be administered orally to bind metabolites excreted in the bile, with unabsorbed drugs and biliary excretions being eliminated in the feces.

Excretion through skin, saliva, and tears is quantitatively unimportant. Similarly, excretion into hair is also insignificant, although detecting drugs in these tissues can have forensic implications. Basic compounds may be slightly concentrated in more acidic fluids like milk, while acidic compounds will have lower concentrations. Non-electrolytes like ethanol and urea readily enter breast milk, reaching the same concentration as in plasma. Drugs administered to breastfeeding women can expose the infant to the medication or its metabolites, necessitating caution.