Phenothiazine ring-system compounds — tricyclic structures containing both sulfur and nitrogen heteroatoms — are the chemical backbone of a historically important class of antipsychotic drugs, including chlorpromazine and thioridazine, as well as veterinary antiparasitic agents and industrial antioxidants. The 'not further fused' qualifier limits this subheading to the core phenothiazine tricycle; compounds where additional rings are appended to that system fall outside the category. Despite the decline of first-generation phenothiazine antipsychotics in clinical practice, the scaffold remains relevant in agrochemical and materials chemistry.

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