Sulfamethoxazole: May antagonize action w/ p-aminobenzoic acid & its derivative (K aminobenzoate & procaine local anesth). May potentiate effects of oral anticoagulants, methotrexate & phenytoin.
Trimethoprim: May increase serum concentrations & potentiate the effects of phenytoin, digoxin, procainamide, rosiglitazone & repaglinidine. May potentiate effects of warfarin. Reduce the renal excretion & increase blood concentrations of zidovudine, zalcitabine & lamivudine. Increase serum concentrations w/ dapsone & decreased w/ rifampicin.