Co-trimoxazole exhibits the synergistic actions of its components (sulfamethoxazole and trimethoprim) by 10-fold. Trimethoprim: Inhibits dihydrofolate reductase, thereby blocking production of tetrahydrofolic acid from dihydrofolic acid. Sulfamethoxazole: Inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid.

Renal failure, nausea, vomiting, diarrhoea, anorexia; skin rashes, urticaria.