Sodium thiosulfate (Na2S2O3), also spelled sodium thiosulphate, is a chemical and medication. As a medication it is used to treat cyanide poisoning and pityriasis versicolor. It is an inorganic compound that is typically available as the pentahydrate, Na2S2O3·5H2O. The solid is an efflorescent (loses water readily) crystalline substance that dissolves well in water. It is also called sodium hyposulfite or “hypo”.