http://en.wikipedia.org/wiki/Mannitol

Just including a wiki link about Mannitol...

{snip}Mannitol or hexan-1,2,3,4,5,6-hexol (C6H8(OH)6) is a polyol that is used as an osmotic diuretic agent and a weak renal vasodilator. It is a sorbitol stereoisomer.

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Mannitol is used clinically to reduce acutely raised intracranial pressure, until more definitive treatment can be given, e.g. after head trauma (although significant controversy exists over this use), and to treat patients with oliguric renal failure. It is administered intravenously, and is filtered by the glomerulus of the kidney, but is incapable of being resorbed from the renal tubule, resulting in decreased water and Na+ reabsorption via its osmotic effect. Consequently, mannitol increases water and Na+ excretion, thereby decreasing extracellular fluid volume.

Mannitol can also be used to open the blood-brain barrier by temporarily shrinking the tightly coupled endothelial cells that make up the barrier. This makes mannitol indispensable for delivering various drugs directly to the brain (e.g. in the treatment of Alzheimer's disease).

Mannitol is also used as a sweetener for people with diabetes. Since mannitol has a negative heat of solution, it is used as a sweetener in "breath-freshening" candies, the cooling effect adding to the fresh feel. In oral doses larger than 20g, mannitol acts as an osmotic laxative, and is sometimes sold as a laxative for children.{end snip}