Mannitol is a sugar alcohol.
http://en.wikipedia.org/wiki/Mannitol
It's sometimes used to lower intracranial pressure for people with head injuries. (my dad told me about it when I first got the box of Copaxone. He's used it on patients with head injuries)
It's also a sweetener in breath mints and used as a sweetener for diabetic foods.
Oh, and it's also used as a laxative...(higher dosages of it than what's in the C)