sulfa drug, any of a class of synthetic chemical substances derived from sulfanilamide, or para-aminobenzenesulfonamide. Sulfa drugs are used to treat bacterial infections, although they have largely been replaced for this purpose by antibiotics; some are also used in the treatment of diabetes. Because sulfa drugs were first used to elucidate ways in which substances can interfere with the metabolism of invading microorganisms, they are of historical interest. The parent compound, para-aminobenzenesulfonamide, was synthesized in 1908 by Paul Gelmo, an Austrian industrial chemist. In 1932 the German chemist Gerhard Domagk discovered that the dye Prontosil had antagonistic properties against a wide range of bacteria, and in 1935 it was found that the sulfanilamide portion of the Prontosil molecule was responsible for its antibacterial effect. In 1940 it was shown that sulfanilamide inhibited the action of the physiological substance para-aminobenzoic acid, which bacteria need to synthesize folic acid. The idea that the two substances were antagonists led to a theory of the mechanism of action of drugs: Many chemotherapeutic substances compete with structurally similar substances that are necessary to the metabolism of invading microorganisms. Since sulfanilamide first came into use, more than 150 different derivatives have appeared on the market, chemically modified to achieve more effective antibacterial activity, wider spectrum of microorganisms affected, or more prolonged action. Because of their low cost they are still used in many parts of the world. However, resistance to sulfa drugs has emerged among many microorganisms, especially streptococci, meningococci, and shigella, making them less effective than formerly (see drug resistance). The substances are still used to treat some urinary tract infections, leprosy, and in combination with other drugs, fungal diseases such as toxoplasmosis.