In elderly patients concurrently receiving diuretics, mainly thiazide, there appears to be an increased risk of thrombocytopenia with purpura. Occasionally reports suggest that patients receiving pyrimethamine as malarial prophylaxis in doses exceeding 25 mg weekly may develop megaloblastic anaemia if Co-trimoxazole be prescribed concurrently. Cotrimoxazole also has been shown to potentiate activity of strongly serum protein-bound drugs like oral hypoglycaemic or anticoagulant. It has been reported that comazole may displace the anticoagulant warfarin. Careful control of the anticoagulant therapy during treatment with Comazole is advisable.
Comazole also may inhibit the hepatic metabolism of phenytoin. A 39% increase in phenytoin half-life and 27% decrease in the metabolic clearance rate of phenytoin have been observed following administration of Comazole of normal clinical dosages. If these two drugs are given concurrently it is important to remain alert for signs of phenytoin toxicity. Concurrent use of rifampicin and Comazole results in shortening of plasma half life of trimethoprim.