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Sulphonamides competitively inhibit dihydropteroate synthase, while trimethoprim inhibits dihydrofolate reductase. Each drug is bacteriostatic alone; together, their sequential blockade of the folate pathway produces a synergistic bactericidal effect. Co-trimoxazole combines sulfamethoxazole and trimethoprim. The lesson covers its spectrum and clinical uses, including urinary infections and Pneumocystis pneumonia, alongside selected uses of individual sulphonamides. It also reviews resistance mechanisms, pharmacokinetics, and important safety considerations, such as hypersensitivity, blood-related toxicity, hyperkalaemia, and risks in neonates and pregnancy. Interactions, renal dose considerations, and monitoring are included to support a structured understanding of these medicines.