Norfluoxetine-D5 hydrochloride is intended for use as an internal standard for the quantification of norfluoxetine by GC- or LC-MS. Norfluoxetine is an active metabolite of the antidepressant fluoxetine. It is formed from fluoxetine by the cytochrome P450 (CYP) isoforms CYP2C9,CYP2C19,and CYP3A. Norfluoxetine inhibits serotonin (5-HT)uptake in rat brain synaptosomal membrane preparations (Ki = 44.7 nM)and isolated human platelets (IC50 = ~15 nM). It has been found in the tissues of fish exposed to wastewater effluent.