Nonsteroidal antiinflammatory drugs (NSAIDs) are used in large amounts for analgesic, antiinflammatory, and antithrombotic indications. This use is not without side effects on the gut and other organs, and some of these side effects may be serious and even lethal. No NSAID has been shown to be without side-effect potential. Use increases with age, and age poses additional risks to patients with side effects. The most serious side effects are perforation of peptic and gut ulcers and gastrointestinal (GI) bleeding, which NSAIDs, and especially acetylsalicylic acid (ASA; aspirin), may promote from both ulcer and nonulcer lesions of both the upper and lower GI tract (i.e., both acid- and nonacid-dependent). Upper GI mucosal lesions range from trivial--petechiae and superficial erosions--to significant and potentially serious deep (chronic) peptic ulcers, esophagitis, and, less commonly, small and large gut ulcers. Symptoms may occur independently of observable lesions, and serious lesions may occur without any prior symptoms. The risk of ulceration due to therapeutic doses of NSAIDs is estimated at 5- to 10-fold. NSAIDs also delay healing of conventional peptic ulcers. Moreover, ASA abuse, often surreptitious and discoverable by serum salicylate level measurement, may cause totally intractable gastric or duodenal ulceration. Surgery is contraindicated because relapse is inevitable and progressively more serious. The rational use of NSAIDs is discussed, and prophylactic and treatment strategies are proposed. None seems entirely satisfactory, and the best prophylaxis would be to avoid the use of NSAIDs except for proven indications. Until the mechanisms whereby NSAIDs both cause injury and provide therapeutic benefits can be separated, the problem of side effects and their prevention or treatment remains unresolvable.