The liver helps eliminate drugs and metabolites through biliary excretion. The hepatocytes of the liver secrete bile, into which various transporters actively discharge polar drugs or hydrophilic drug metabolites. The bile carries the drugs and metabolites into the small intestine, eventually eliminating them through feces. Sometimes, the parent drug or the drug derived from hydrolysis of metabolites by gut microflora is reabsorbed from the intestine and carried to the liver for re-secretion via bile into the intestine. This cycle is known as enterohepatic circulation. Such recirculation of drugs often prolongs drug action. While this is sometimes important, it increases the chances of drug toxicity. Other drug excretion modes include breast milk, lungs, and saliva. Drugs excreted through breast milk have undesirable effects on suckling infants. Drugs eliminated through the lungs are mainly different types of anesthetic gases, while drugs excreted through saliva are generally lipophilic and nonionized. Such drugs diffuse through the epithelial cells of the salivary gland.