any of three organic compounds of the heterocyclic series, having molecular structures that include a ring of four atoms of carbon and one each of nitrogen and sulfur.

Many compounds of 1,4-thiazine are known, most of them derivatives of phenothiazine (C₁₂H₉NS), which was discovered in 1883. Phenothiazine has been used as a vermifuge for livestock and also as an insecticide. Drugs of the phenothiazine type include chlorpromazine, a tranquillizer; promethazine hydrochloride (Phenergan), a long-acting antihistaminic; and diethazine hydrochloride (Diparcol), used in treatment of parkinsonism. A large group of dyes has the phenothiazine structure, including methylene blue.

Compounds in which the sulfur and nitrogen atoms are adjacent (1,2-thiazine and its derivatives) are extremely rare. 1,3-Thiazine itself is not known; an important compound containing its structure is cephalosporin C, a useful antibiotic.