I can understand that giving candies or sweets to a cat or dog can rot their teeth.
Products containing Xylitol are potentially lethal if fed to pets.
Xylitol, an artificial sweetener, is present in many products, such as candy, sugar-free chewing gums, toothpaste and baked goods. Ingestion of these foods by dogs results in a significant, and often sustained, insulin-mediated hypoglycemic crisis (Cope, 2004). 1
Xylitol is metabolized differently by dogs and cats than humans.
In dogs, Xylitol cause the dog's pancreas to release much larger amounts of insulin; this overproduction of insulin can lead to hypoglycaemia, and possibly resulting in coma and death.
The effects of ingesting candies or products with Xylitol are seen fairly quickly (within 30 minutes) and are the signs consistent with hypoglycemia. Trembling and shaking, weakness, lethargy, vomiting, and possible loss of consciousness.
If your pet has ingested such food products, induced vomiting is recommended and urgent veterinary help should be sought.
An important note for cats: The same effect can be seen in cats, however less cases of such poisoning are seen due to cats being more fussy about what they eat than dogs.