Sulfachloropyridazine is a sulfonamide antibiotic that blocks the synthesis of dihydrofolic acid by inhibiting the enzyme dihydropteroate synthase. Sulfachloropyridazine is a competitive inhibitor of bacterial para-aminobenzoic acid (PABA), which is required for bacterial synthesis of folic acid. It is active against Gram positive bacteria, Gram negative bacteria and Chlamydia. Mode of resistance is via the alteration of dihydropteroate synthase or alternative pathway for folic acid synthesis. Sulfachloropyridazine is rapidly absorbed and rapidly excreted in the urine.