Many patients require multiple-drug therapy, often under the supervision of several physicians, this increasing the risk of adverse drug effects. While drug interactions cannot always be conveniently categorized, familiarity with basic pharmacologic and pharmacokinetic principles permits the clinician to anticipate and avoid many of the more common and serious drug interactions. Most serious drug interactions involve one of the four pharmacokinetic processes governing drug behavior within the body: absorption, distribution, metabolism, and renal clearance. Knowledge of whether a drug is a weak acid or a weak base is clinically useful because it can alert the physician to the propensity for certain types of drug interactions.