Sodium thiosulfate (Na2S2O3), also spelled sodium thiosulphate, 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”.[2]
It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system.[3]