Lactose

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A lactose molecule.

Commonly known as "milk sugar", lactose is a disaccharide sugar made up of the two monosaccharides, glucose and galactose. It has many uses, among them being a common solid vehicle for binding small amounts of drugs into a tablet form that can be handled.

A significant number of people have lactose intolerance, or the usually genetic but sometimes acquired lack of the enzyme lactase. The unabsorbed lactose then is fermented by intestinal bacteria, which produce acid and gas. These byproducts may cause anything from mild discomfort to severe diarrhea.