Nonsteroidal anti-inflammatory drugs (NSAIDs) are among the most widely used drugs. Drug interactions with this class of compounds are frequently reported and can be pharmacokinetic and/or pharmacodynamic in nature. The pharmacokinetic interactions can be divided into 3 classes: (1) drugs affecting the pharmacokinetics of an NSAID. (2) an NSAID interfering with the pharmacokinetics of another NSAID and (3) NSAIDs altering the pharmacokinetics of another drug. Although the pharmacokinetics of some NSAIDs may be significantly affected by the concurrent administration of certain other drugs (including other NSAIDs), this type of interaction only occasionally leads to serious complications. Concurrent administration of antacids or sucralfate may delay the rate of oral absorption of NSAIDs but generally has little effect on the extent. Use of antacids increases urinary pH, leading to increased renal excretion of unchanged salicylic acid and decreased plasma concentrations of this antirheumatic agent. The H2-receptor blocking agent cimetidine inhibits the oxidative metabolism of many concurrently administered drugs, including certain NSAIDs. Probenecid inhibits the renal secretion of drug glucuronides and this will lead to accumulation in plasma of those NSAIDs eliminated primarily by the formation of labile acyl glucuronides such as naproxen, ketoprofen, indomethacin, carprofen. Cholestyramine decreases the oral absorption of many concurrently administered drugs, including NSAIDs. It may also decrease plasma concentrations of those NSAIDs undergoing enterohepatic circulation (e.g. piroxicam, tenoxicam) by interrupting the enterohepatic cycle. Corticosteroids stimulate the clearance of salicylic acid, leading to low plasma salicylate concentrations. Plasma concentrations of many NSAIDs are significantly reduced when the NSAID is coadministered with aspirin. The clinical relevance of most of these interactions is not well established. However, in those cases where the interaction results in elevated plasma concentrations of the NSAID, special caution should be exercised to avoid excessive accumulation of the NSAID especially in elderly and/or very sick patients who may be more sensitive to the more serious gastroduodenal and renal side-effects of these agents. By virtue of their pharmacokinetic and pharmacodynamic properties, NSAIDs may significantly affect the disposition kinetics of a number of other drugs. They can displace other drugs from their plasma protein binding sites, inhibit their metabolism or interfere with their renal excretion. If the affected drug has a narrow therapeutic index, the interaction may be clinically significant. The pyrazole NSAIDs (phenylbutazone, oxyphenbutazone, azapropazone) inhibit the metabolism of many drugs such as the coumarin anticoagulants, oral antidiabetics and anticonvulsants such as phenytoin. Salicylates displace oral anticoagulants from their plasma protein binding sites.(ABSTRACT TRUNCATED AT 400 WORDS)