Mannitol

Mannitol is a form of sugar. Although it is often used as a sweetener in foods for diabetics, its main medical use is as a medication to reduce pressure in the eyes (typical of glaucoma) and to reduce intracranial pressure. Because it's main benefit when used as a sweetener for diabetics is that it is poorly absorbed by the intestines, to have any therapeutic benefit, it must be injected.
Typical side effects are dehydration and problems with electrolytes. However, in severe cases, it can affect heart and kidney function.
Mannitol works by creating a difference in osmotic pressure between the brain and the bloodstream, requiring water to pass through the blood-brain barrier to restore the balance. 
Although mannitol exists in nature, most of it is produced by processing other sugars..
Mannitol at WikipediaCategory:Pharmaceuticals