Dogs should not eat grapes as they are highly toxic to them. Even small amounts of grapes or raisins can cause serious health issues, including vomiting, diarrhea, lethargy, loss of appetite, dehydration, and potentially life-threatening kidney failure. The exact toxic substance in grapes is unknown, but the reaction can vary between dogs, with some dogs being more sensitive than others. Because of this unpredictability and the seriousness of the potential outcomes, grapes and raisins should be completely avoided in a dog's diet. If a dog does consume grapes or raisins, immediate veterinary attention is crucial to induce vomiting, administer activated charcoal, and monitor kidney function.