Phenothiazine or thiodiphenylamine as it is known chemically, has been a recognized chemical entity since 1885. It was not until 1935, however, that it was first used as an insecticide. In 1938, research was begun testing it as an anthelmintic. Phenothiazine is a coal tar derivative related chemically to the miracle drug sulfanilamide and is made by combining diphenylamine with sulfur. Phenothiazine is the mother substance of the two well-known dyes, Lauth's violet and methylene blue.