Mannitol or 1,2,3,4,5,6-hexanehexol (C₆H₈(OH)₆) is a vasodilator which is used mainly to reduce pressure in the cranium, and to treat patients with oliguric renal failure . It can be administered intravenously. Chemically, mannitol is an alcohol and a sugar, or a polyol; it is similar to xylitol or sorbitol. However, mannitol has a tendency to lose a hydrogen ion in aqueous solutions, which causes the solution to become acidic. For this reason, it is not uncommon to add a substance to adjust its pH, such as sodium bicarbonate.

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 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. Mannitol also has a negative heat of solution. For this reason, mannitol is a sweetener in "breath-freshening" candies, the cooling effect adding to the fresh feel. In doses larger than 20g it acts as a laxative, and is sometimes sold as a laxative for children.

It is sometimes used as a dilutant for heroin or other illicit drugs.