The effects of histamine H1-receptor antagonists, promethazine and << homochlorcyclizine >>, both of which are inhibitors of [[ CYP2D6 ]], on the steady-state plasma concentrations (Css) of haloperidol and reduced haloperidol were studied in 23 schizophrenic inpatients receiving haloperidol, 12 to 36 mg/d, for 2 to 29 weeks.