Trimethoprim in combination with sulfamethoxazole (an antibiotic belonging to the sulfonamide group) acts synergistically by blocking successive steps in bacterial folate metabolism. Trimethoprim prevents the reduction of dihydrofolate to tetrahydrofolate, while sulfamethoxazole inhibits the conversion of para-aminobenzoic acid (PABA) to dihydropteroate. This synergistic effect produces maximum antibacterial activity, which is usually bactericidal.