The artificial sweetener Xylitol, which is found in many sugar-free products including chewing gum, is toxic to dogs at small doses. It can cause their blood sugar to drop very low which is dangerous. This can happen anytime between 30 minutes and 12 hours after ingestion. Higher doses can cause serious problems with the liver which may not show up for up to 72 hours. If your dog eats anything containing xylitol, contact your veterinarian or the Animal Poison Control center for instructions immediately.