Nepicastat is a 1,3-dihydroimidazole-2-thione containing a 5,7-difluoro-1,2,3,4-tetrahydronaphthalen-2-yl group substituted on one nitrogen and an aminomethyl group on the adjacent carbon (the S enantiomer). It is a potent and selective inhibitor of bovine and human dopamine beta-hydroxylase, an enzyme that converts dopamine to norepinephrine. It functions as an EC 1.14.17.1 (dopamine beta-monooxygenase) inhibitor. It belongs to the classes of tetralins, organofluorine compounds, primary amino compounds, and 1,3-dihydroimidazole-2-thiones.