Dogs should not eat grapes because it can lead to kidney failure. Some dogs have eaten grapes without adverse effects, but the risk of kidney failure is well-documented so grapes are not considered safe for dogs.
Grape toxicosis can be caused by raisins and other products derived from grapes. The exact reason grapes are toxic to dogs is unknown as of 2014, but it has been observed in dogs who eat even a small amount of grapes. If a dog accidentally eats grapes or raisins, the owner should induce vomiting as soon as possible and call his veterinarian for advice. Grape toxicosis can cause long-term illness or death.