Answer:
When set loose I fly away. Never so cursed as when I go astray.
What is given but kept by the giver?
I can be written, I can be spoken, I can be exposed, I can be broken.
Who is he that runs without a leg. And his house on his back?
It is the great nemesis of the lactose intolerant.
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?