Adverse drug reactions are among the most frequent problems encountered clinically and represent a common cause for hospitalization. They occur most frequently in pts receiving multiple drugs and are caused by:

• Errors in self-administration of prescribed drugs (quite common in the elderly);
• Exaggeration of intended pharmacologic effect (e.g., hypotension in a pt given antihypertensive drugs);
• Concomitant administration of drugs with synergistic effects (e.g., aspirin and warfarin);
• Cytotoxic reactions (e.g., hepatic necrosis due to acetaminophen);
• Immunologic mechanisms (e.g., quinidine-induced thrombocytopenia, hydralazine-induced SLE);
• Genetically determined enzymatic defects (e.g., primaquine-induced hemolytic anemia in G6PD deficiency); or
• Idiosyncratic reactions (e.g., chloramphenicol-induced aplastic anemia).