Thiocyanic acid, also known as hydrogen thiocyanate, is a hydracid that is cyanic acid in which the oxygen is replaced by a sulfur atom. It has a role as an Escherichia coli metabolite. It is a hydracid, a one-carbon compound and an organosulfur compound. It is a conjugate acid of a thiocyanate. It is a tautomer of an isothiocyanic acid. In the past, thiocyanate was used therapeutically to treat severe hypertension, but the results of this therapy were inconsistent from clinic to clinic, and even within a single study. Thiocyanate is believed to play a role in an endogenous antibacterial system (lactoperoxidase/thiocyanate/hydrogen peroxide system) present in milk. It has been added commercially to some milk preparations as an antibacterial agent.