N-[2-(Phenylthio)phenyl]-1-piperazine-d8-carboxamide Hydrochloride is the labeled analog of N-[2-(Phenylthio)phenyl]-1-piperazinecarboxamide Hydrochloride, an impurity of Quetiapine (Q510000), a benzothiazepine with mixed serotonin and dopamine receptor antagonistic properties; used as an antipsychotic.