If in doubt, always call your vet or the Animal Poison Line immediately.
Xylitol triggers a massive release of insulin in dogs (unlike in humans), causing blood sugar to crash dangerously low. It can also cause acute liver failure, even in small amounts. Always check ingredient labels on peanut butter, baked goods, and sugar-free products.
This is a veterinary emergency. Call your vet immediately : even if your dog seems fine. Liver damage can develop hours after ingestion with no initial symptoms.