May decrease immunological response to concurrent vaccination. May cause severe antigenic reaction w/ live vaccine. May be displaced by salicylates, sulphonamides, diphenylhydantoins, tetracyclines, chloramphenicol, sulfazole, doxorubicin, cyclophosphamide & barbiturates leading to high plasma levels & increased toxicity. Increased plasma levels w/ other drugs subject to active renal secretion. Reduce dose when given concomitantly w/ probenecid. Intracellular conc may be increased w/ vinca alkaloids. Avoid concomitant use of drugs w/ nephrotoxic or hepatotoxic potential (including alcohol). Response may be altered w/ vit or oral Fe prep. Renal clearance may be impaired w/ NSAIDs. Serum levels may be increased by etretinate. Rare cases of acute pancytopenia w/ folate antagonists eg, trimethoprim/sulphamethoxazole.