Drug Interactions

The salicylates displace a number of drugs from plasma protein binding sites, thereby leading to potential adverse effects by these agents. Since aspirin is an over-the-counter medication, patients may fail to inform the doctor of their aspirin consumption. Anticoagulants are potentiated by aspirin by (1) displacement of the anticoagulants from plasma proteins and (2) the intrinsic anticoagulant effect of aspirin. Thus, the dosage of drugs such as coumarin and heparin should be reduced in patients taking aspirin. A similar effect is observed in patients taking oral sulfonylureas (Orinase, DiaBeta) for non-insulin-dependent diabetes or phenytoin (Dilantin) for seizures. Displacement of the sulfonylureas or phenytoin from plasma binding necessitates a decrease in dosage to prevent an acute hypoglycemic event or sedation, respectively. Aspirin enhances the effects of insulin (leading to hypoglycemia), penicillins and sulfonamides (increasing acute toxicity), and corticosteroids. Aspirin increases the hypotensive effects of the cardiac drug nitroglycerin but decreases the effectiveness of the loop diuretics. In patients taking methotrexate for cancer chemotherapy, aspirin may increase retention of the drug, and severe toxicity may result.

Conversely, certain drugs modify the effectiveness or side effects of aspirin. Phenobarbital, occasionally used for seizures, induces liver enzymes that increase the metabolism and excretion of aspirin, (3-adrenoceptor-blocking drugs, such as propranolol, and decrease the an-tiinflammatory effects of aspirin, whereas reserpine decreases its analgesic effects. Antacids decrease the absorption of aspirin. Alcohol consumption in combination with aspirin increases the latter's ulcerogenic effects.

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