If in doubt, always call your vet or the Animal Poison Line immediately.
Raisins contain the same unknown toxin as grapes but in a much more concentrated form. A single raisin or a small piece of fruit cake containing raisins has caused kidney failure in dogs. Particularly dangerous in Christmas and Easter foods.
Emergency vet visit required. Even one raisin warrants a call to your vet or your national animal poison helpline.