Derived from the dye protonsil, and discovered by Domagk when he was trying to save his daughter from a severe Streptococcal infection.
Protonsil did not produce a zone of inhibition, making it a good example of the fact that not all agents that are effective in vivo are effective in vitro and vice versa.
Inhibits folic acid synthesis in bacteria by acting as a competitive inhibitor to para-aminobenzoic acid. Selectively toxic to bacteria as human beings do not synthesize folic acid.
Some resistance to it has developed.