Sulfaphenazole is a sulfonamide derivative primarily utilized as a selective inhibitor of cytochrome P450 2C9 (CYP2C9) in biochemical and pharmacological research. It is widely employed in studies investigating drug metabolism, enzyme inhibition, and pharmacokinetic interactions, helping to assess the role of CYP2C9 in the biotransformation of various xenobiotics and therapeutic agents. Historically, sulfaphenazole has been used as an antibacterial agent effective against susceptible bacteria by inhibiting folic acid synthesis, which is crucial for bacterial growth and reproduction. This makes it suitable for treating infections in the respiratory tract, urinary tract, and skin. However, its clinical use is now limited due to bacterial resistance and the availability of broader-spectrum antibiotics with fewer side effects. In research settings, it remains a valuable tool for understanding resistance mechanisms and developing new antibacterial strategies.