Trimethoprim Disease Interactions

The use of trimethoprim is contraindicated in patients with documented megaloblastic anemia due to folate deficiency. Trimethoprim inhibits dihydrofolate reductase, an enzyme necessary in the synthesis of tetrahydrofolic acid, or the metabolically active form of folic acid. Thrombocytopenia, neutropenia, megaloblastic anemia, and methemoglobinemia have been reported rarely. However, the risk is increased in the presence of folate deficiency, chronic hemolytic anemia and/or renal impairment, as well as during prolonged therapy (e.g., > 6 months) with high dosages. Therapy with trimethoprim should be administered cautiously under these conditions and in patients with suspected folate depletion (e.g., elderly, alcoholic, malnourished or debilitated patients). Folic acid supplementation, if necessary, may be administered concomitantly without interfering with the antibacterial action of trimethoprim. Patients should be instructed to immediately report any signs or symptoms suggestive of hematologic toxicity such as fever, sore throat, local infection, bleeding, pallor, dizziness, or jaundice. Leucovorin (folinic acid) should be administered if bone marrow depression is detected.

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Trimethoprim is moderately removed by hemodialysis. Doses should either be scheduled for administration after dialysis or supplemental doses be given after dialysis.

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Trimethoprim is primarily eliminated by the kidney. The serum concentration of trimethoprim may be increased and the half-life prolonged in patients with impaired renal function. Dosage adjustments may be necessary and modifications should be based on the degree of renal impairment as well as severity of infection. The manufacturers recommend a dosage of 50 mg every 12 hours in patients with creatinine clearance between 15 to 30 mL/min and not using the drug in patients with creatinine clearance below 15 mL/min.

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