Drug Interaction Report

MONITOR: Coadministration of pyrimethamine in doses exceeding 25 mg per week with trimethoprim or sulfonamides may increase the risk of megaloblastic anemia and pancytopenia. The proposed mechanism is additive antifolate effects.

MANAGEMENT: Periodic clinical and laboratory monitoring for signs of folic acid deficiency and hematologic changes is advisable.

MONITOR: Coadministration of pyrimethamine in doses exceeding 25 mg per week with trimethoprim or sulfonamides may increase the risk of megaloblastic anemia and pancytopenia. The proposed mechanism is additive antifolate effects.

MANAGEMENT: Periodic clinical and laboratory monitoring for signs of folic acid deficiency and hematologic changes is advisable.

MONITOR: Two cases have been reported in which patients on sulfamethoxazole-trimethoprim therapy, after consuming beer, reported flushing, heart palpitations, dyspnea, headache, and nausea (disulfiram - alcohol type reactions). First-generation sulfonylureas have been reported to cause facial flushing when administered with alcohol by inhibiting acetaldehyde dehydrogenase and subsequently causing acetaldehyde accumulation. Since sulfamethoxazole is chemically related to first-generation sulfonylureas, a disulfiram-like reaction with products containing sulfamethoxazole is theoretically possible. However, pharmacokinetic/pharmacodynamic data are lacking and in addition, the two reported cases cannot be clearly attributed to the concomitant use of sulfamethoxazole-trimethoprim and alcohol.

MANAGEMENT: Patients should be alerted to the potential for this interaction and although the risk for this interaction is minimal, caution is recommended while taking sulfamethoxazole-trimethoprim concomitantly with alcohol.