How is toxicity of sodium nitroprusside?

Nitroprusside combines with hemoglobin and is subsequently metabolized to 5 cyanide ions and cyanmethemoglobin. Cyanide then converts to thiocyanate via rhodanese-mediated donation of a sulfur group (liver) (from thiosulfate or cysteine); this final step can quickly become overwhelmed with large quantities of the drug. Antidotal therapy given, preferably using a combination of hydroxocobalamin (to bind intracellular cyanide) and sodium thiosulfate (to act as a sulfur donor).