Phenothiazine, the parent compound of a multitude of present-day drugs, has been employed on an extensive scale for its insecticidal, fungicidal, antibacterial and anthelmintic properties. Phenothiazine was formerly used as an insecticide and as a drug to treat infections with parasitic worms (antihelminthic) in livestock and people. It was introduced as antihelminthic in livestock in 1940 and is considered, with thiabendazole, to be the first modern antihelminthic. Almost a catholicon, its widespread use in animals and man has led to the uncovering of many adverse reactions encompassing effects on blood elements, neuromuscular problems and photosensitization. Its potential side effects have now limited its use. The chemical structure of phenothiazine provides a most valuable molecular template for the development of agents able to interact with a wide variety of biological processes. Synthetic phenothiazines (with aliphatic, methylpiperazine, piperazine-ethanol, piperazine-ethyl, or piperidine side-chain) and/or phenothiazine-derived agents e.g., thioxanthenes, benzepines, imonostilbenes, tricyclic antidepressants, dimetothiazine, and cyproheptadine have been effective in the treatment of a number of medical conditions with widely different etiology. These include various currently clinically used drugs for their significant antihistamic, antipsychotic, anticholinergic (antiparkinson), antipruritic, and/or antiemetic properties.