I've just been reading about Xylitol, the sweetener used in Smint, Orbit Complete and lots of other sweets and chewing gums. It's a bit like the aspirin of the mint-world. It helps re-mineralise teeth, improves bone-density in osteoporosis sufferers and helps prevent ear infections. It has no known side-effects, other than a mild laxative effect at high doses. Unless you're a dog. Even small doses can cause hypoglycæmia (low blood sugar levels) in dogs, which can be fatal.
Charlie says, "Don't play with matches and keep your Smints off the floor."
[Updated to add: Paracetamol is equally dangerous to cats. In case you were interested.]