Xylitol is a sugar alcohol used in candy and chewing gum. It is also found in some pharmaceuticals and oral health products such as chewable vitamins and throat lozenges.
While Xylitol is safe for humans, it can be harmful to dogs. The compound doesn't affect glucose levels in people, but when ingested by dogs it can cause a dangerous surge of insulin. (In as little as 15 minutes,the blood sugar of a dog that has eaten gum containing Xylitol may register a marked drop in blood sugar.) At higher doses, Xylitol is believed toxic to the canine liver.
A dog that has eaten an item containing Xylitol can be rapidly hit by a dangerous drop in blood sugar that causes weakness, lethargy, loss of coordination, collapse, and seizures. Those symptoms can develop within 30 minutes, and a dog so afflicted will need immediate veterinary treatment to survive. Without help, irreversible brain trauma occurs and the patient dies.