Phenothiazines

Phenothiazines are a class of heterocyclic compounds centered on the phenothiazine nucleus, a tricyclic structure in which two benzene rings are fused to a central thiazine ring containing sulfur and nitrogen. This chemical framework has given rise to a broad group of pharmacologically active agents, including several traditional antipsychotics and a number of antihistamines and antiemetics.

Pharmacology and clinical use are dominated by phenothiazine derivatives that act as central dopamine D2 receptor antagonists, especially in the mesolimbic pathway. This Dopaminergic blockade helps reduce the positive symptoms of schizophrenia and other psychoses. Many phenothiazines also block histamine H1, α1-adrenergic, and muscarinic receptors, contributing to sedation, orthostatic hypotension, and anticholinergic effects. In addition, certain derivatives such as promethazine and prochlorperazine are used as antiemetics. Chlorpromazine is a historically important antipsychotic, illustrating the therapeutic versatility of this class.

Safety and side effects are a major consideration. Typical phenothiazines can cause extrapyramidal symptoms (parkinsonism, dystonias, akathisia) and tardive dyskinesia with long-term use. Neuroleptic malignant syndrome is a rare but serious risk. Anticholinergic effects (dry mouth, constipation, blurred vision), sedation, and hypotension are common, and some agents may prolong the QT interval. High-potency phenothiazines tend to produce more EPS, while low-potency ones cause greater sedation and autonomic effects. Thioridazine carries a risk of pigmentary retinopathy and is used less commonly today.

Historically, phenothiazines revolutionized psychiatry in the mid-20th century and remain chemically defined as a broad class. While newer antipsychotics have supplanted many uses, phenothiazines are still employed for certain indications and settings, especially where older agents are preferred or available.