Why can't dogs eat grapes?

Grapes and raisins can cause kidney failure or toxic reactions in dogs. Some dogs are more prone to developing kidney failure from the food than others, and the reason why they cause this ailment is unknown.

Some dogs can eat grapes without developing illness but later eat them and become ill. Therefore, it is important for dog owners to keep grapes away from their pets at all times. Even a few grapes or raisins can adversely affect them. Signs of grape toxicity include vomiting, diarrhea and lethargy that occur about 12 hours after ingestion. People who suspect grape toxicity in a dog should take the animal to the veterinarian immediately.