[4] For the next several decades, research on derivatives lapsed until phenothiazine itself came to market as an insecticide and deworming drug.

In the 1940s, chemists working with Paul Charpentier at Rhone-Poulenc Laboratories in Paris (a precursor company to Sanofi), began making derivatives.

[4] At the end of the 1940s the same lab produced chlorpromazine which had an even stronger sedative and soothing effect, and Jean Delay and Pierre Deniker attempted to use it on their psychiatric patients, publishing their results in the early 1950s.

The strong effects they found opened the door of the modern field of psychiatry and led to a proliferation of work on phenothiazine derivatives.

These drugs have antipsychotic and, often, antiemetic properties, although they may also cause severe side effects such as extrapyramidal symptoms (including akathisia and tardive dyskinesia), hyperprolactinaemia, and the rare but potentially fatal neuroleptic malignant syndrome, as well as substantial weight gain.

[13] Like many commercially significant compounds, phenothiazine has numerous trade names, including AFI-Tiazin, Agrazine, Antiverm, Biverm, Dibenzothiazine, Orimon, Lethelmin, Souframine, Nemazene, Vermitin, Padophene, Fenoverm, Fentiazine, Contaverm, Fenothiazine, Phenovarm, Ieeno, ENT 38, Helmetina, Helmetine, Penthazine, XL-50, Wurm-thional, Phenegic, Phenovis, Phenoxur, and Reconox.

[14] Phenothiazine was formerly used as an insecticide and as a drug to treat infections with parasitic worms (anthelminthic) in livestock and people, but its use for those purposes has been superseded by other chemicals.

[16] However, because it was degraded by sunlight and air, it was difficult to determine how much to use in the field, and its use waned in the 1940s with the arrival of new pesticides like DDT that were more durable.

[24] The compound was originally prepared by Bernthsen in 1883 via the reaction of diphenylamine with sulfur, but more recent syntheses rely on the cyclization of 2-substituted diphenyl sulfides.