Saccharin

a white, crystalline powder with about 500 times the sweetening power of sucrose. It is not very soluble in water but is used extensively as a sweetening agent, in the form of the sodium salt.

Taken from Dictionary of Science

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Saccharin is an artificial sweetener. The basic substance, benzoic sulfimide, has effectively no food energy and is much sweeter than sucrose, but has an unpleasant bitter or metallic aftertaste, especially at high concentrations. In countries where saccharin is allowed as a food additive, it is used to sweeten products such as drinks, candies, medicines, and toothpaste.

Taken from Wikipedia