Sulfonamide, also known as sulphonamide, is a functional group that serves as the basis for several groups of drugs commonly referred to as sulfa drugs or sulpha drugs.

The discovery and use of sulfonamide drugs marked a significant milestone in the history of medicine as they were the first systemic antibacterials with broad effectiveness, paving the way for the development of antibiotics and revolutionizing the treatment of infectious diseases.

Sulfonamide compounds, which contain the sulfonamide functional group, were the first widely used antibacterial agents, and paved the way for the development of other antibiotics. However, not all sulfonamides possess antibacterial activity, such as the anticonvulsant sultiame. Newer drug groups, including sulfonylureas and thiazide diuretics, were developed based on the structure of antibacterial sulfonamides.

Sulfonamides have a range of medical applications, including the treatment of allergies and coughs, as well as their use as antifungal and antimalarial agents.

Sulfonamides can be synthesized by reacting a sulfonyl chloride with ammonia or an amine.