Promethazine DESCRIPTION NewsLinks Promethazine hydrochloride is a racemic compound; the empirical formula is C17H20N2S HCl and its molecular weight is 320.88. Promethazine hydrochloride, a phenothiazine derivative, is designated chemically as N, N, a-trimethyl-10H-phenothiazine-10-ethanamine monohydrochloride. Promethazine hydrochloride occurs as a white to faint yellow, practically odorless, ...