Even if you don't have a dog, I'm sure you know several people who do, who may not know of this danger to their pet.
Xylitol is a naturally occurring substance that is widely used as a sugar substitute. In nature, it is found in berries, plums, corn, oats, mushrooms, lettuce, trees, and some other hardwood trees and fruits. Products that may contain xylitol include sugar-free gum, breath mints, baked goods, cough syrup, children's chewable vitamins, mouthwash, and toothpaste, to list a few.
It's safe for humans, however, it is extremely toxic to dogs. Even small amounts of xylitol can cause hypoglycemia (low blood sugar), seizures, liver failure or even death in dogs.The most common source of xylitol poisoning is sugar-free gum.
If you personally use products containing xylitol, make sure they are stored safely, out of reach of your pets. Do not share any food that may contain xylitol with your pets. Only use pet toothpaste for pets, never human toothpaste.
If you suspect that your dog has ingested a product containing Xylitol, please contact your vet.
The 'Vets Now' organisation has some useful information on poisons.