IT IS A SHORT ACTING SULPHONAMIDE. SULFAMETHOXAZOLE INHIBITS BACTERIAL SYNTHESIS OF DIHYDROFOLIC
ACID BY COMPETING WITH PARA-AMINOBENZOIC ACID (PABA). TRIMETHOPRIM BLOCKS THE PRODUCTION OF TETRAHYDROFOLIC ACID FROM DIHYDROFOLIC ACID BY BINDING TO AND REVERSIBLY INHIBITING THE REQUIRED ENZYME, DIHYDROFOLATE REDUCTASE. THUS,BACTRIM BLOCKS TWO CONSECUTIVE STEPS IN THE BIOSYNTHESIS OF NUCLEIC ACIDS AND PROTEINS ESSENTIAL TO MANY BACTERIA.