Patients should be instructed to maintain an adequate fluid intake in order to prevent crystalluria and stone formation.
In elderly patients concurrently receiving certain diuretics, primarily thiazides, an increased incidence of thrombopenia with purpura has been reported.
It has been reported that sulfamethoxazole may prolong the prothrombin time in patients who are receiving the anticoagulant warfarin. This interaction should be kept in mind when Gantanol is given to patients already on anticoagulant therapy, and the coagulation time should be reassessed.
Sulfamethoxazole may inhibit the hepatic metabolism of phenytoin. At a 1.6-g dose, sulfamethoxazole produced a slight but significant increase in the half-life of phenytoin but did not produce a corresponding decrease in the metabolic clearance rate. When administering these drugs concurrently, one should be alert for possible excessive phenytoin effect.
Sulfonamides can also displace methotrexate from plasma protein-binding sites, thus increasing free methotrexate concentrations.
The presence of sulfamethoxazole may interfere with the Jaffé alkaline picrate reaction assay for creatinine, resulting in overestimations of about 10% in the range of normal values.
Hypersensitivity to sulfonamides. Pediatric patients less than 2 months of age (except in the treatment of congenital toxoplasmosis as adjunctive therapy with pyrimethamine). Pregnancy at term and during the nursing period because sulfonamides pass the placenta and are excreted in the milk and may cause kernicterus.