8-Chloro-10-[3-[4-(2-hydroxyethyl)-1-piperazinyl-d8]propyl]-10H-phenothiazin-3-ol Dihydrochloride is the labeled analogue of 8-Chloro-10-[3-[4-(2-hydroxyethyl)-1-piperazinyl]propyl]-10H-phenothiazin-3-ol Dihydrochloride (C366795), a hydroxylated metabolite of 2-chlorophenothiazine derivatives.