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The adverse effects encountered most often with trimethoprim (trimethoprim (trimethoprim tablet) tablet) were rash and pruritus.

Dermatologic

Rash, pruritus, and phototoxic skin eruptions. At the recommended dosage regimens of 100 mg b.i.d. or 200 mg q.d. each for 10 days, the incidence of rash is 2.9% to 6.7%. In clinical studies which employed high doses of trimethoprim (trimethoprim (trimethoprim tablet) tablet) , an elevated incidence of rash was noted. These rashes were maculopapular, morbilliform, pruritic, and generally mild to moderate, appearing 7 to 14 days after the initiation of therapy.

Hypersensitivity

Rare reports of exfoliative dermatitis, erythema multiforme, Stevens-Johnson syndrome, toxic epidermal necrolysis (Lyell Syndrome), and anaphylaxis have been received.

Gastrointestinal

Epigastric distress, nausea, vomiting, and glossitis. Elevation of serum transaminase and bilirubin has been noted, but the significance of this finding is unknown. Cholestatic jaundice has been rarely reported.

Hematologic

Thrombocytopenia, leukopenia, neutropenia, megaloblastic anemia, and methemoglobinemia.

Metabolic

Hyperkalemia, hyponatremia.

Neurologic

Aseptic meningitis has been rarely reported.

Miscellaneous

Fever, and increases in BUN and serum creatinine levels.

Trimethoprim may inhibit the hepatic metabolism of phenytoin. Trimethoprim (trimethoprim (trimethoprim tablet) tablet) , given at a common clinical dosage, increased the phenytoin half-life by 51% and decreased the phenytoin metabolic clearance rate by 30%. When administering these drugs concurrently, one should be alert for possible excessive phenytoin effect.

Drug/Laboratory Test Interactions

Trimethoprim (trimethoprim (trimethoprim tablet) tablet) can interfere with a serum methotrexate assay as determined by the Competitive Binding Protein Technique (CBPA) when a bacterial dihydrofolate reductase is used as the binding protein. No interference occurs, however, if methotrexate is measured by a radioimmunoassay (RIA). The presence of trimethoprim (trimethoprim (trimethoprim tablet) tablet) may also interfere with the Jaffé alkaline picrate reaction assay for creatinine resulting in over estimations of about 10% in the range of normal values.

Last reviewed on RxList: 9/18/2008
This monograph has been modified to include the generic and brand name in many instances.