trade name of the first synthetic drug used in the treatment of general bacterial infections in humans. Prontosil was introduced into medicine in the 1930s.

Prontosil resulted from research, directed by German chemist and pathologist Gerhard Domagk (Domagk, Gerhard), on the antibacterial action of azo dyes (azo dye). A red azo dye of low toxicity, Prontosil was shown by Domagk to prevent mortality in mice infected with Streptococcus bacteria. The dye was also effective in controlling Staphylococcus infections in rabbits. Within a relatively short period, it was demonstrated that Prontosil was effective not only in combating experimental infections in animals but also against streptococcal diseases in humans, including meningitis and puerperal sepsis. Later it was found that Prontosil is disrupted in the tissues to form para-aminobenzenesulfonamide (sulfanilamide).

Prontosil has been replaced in clinical use by newer sulfonamide drugs, including sulfanilamide, sulfathiazole, sulfamethoxazole, and others.