Dosage schedule and plasma levels of doxepin and desmethyldoxepin.

Plasma levels of doxepin and its metabolite desmethyldoxepin were determined in 7 depressed patients treated with doxepin hydrochloride in 3 divided doses at 1000, 1600, and 2200 hours (t.i.d.), and repeated after changing the dosage schedule to a single daily bedtime (h.s.) dose at 2200 hours. Doxepin and its metabolite were measured at 9000, 1200, 1500, and 1800 hours. None of the individual patients showed clinically significant changes in their plasma concentration of tricyclic antidepressant on the 2 dosage schedules. No difference in the clinical condition of the patients was detected on the 2 dosage schedules using the Zung Self Rating Depression Scale, however patients experienced more morning sedation while on the single h.s. dosage. This study provides pharmacological support for the prescription of doxepin on a once daily basis.