Both chocolate and grapes are known to be highly toxic to dogs. Chocolate contains theobromine, which dogs cannot metabolize effectively. Even small amounts can lead to symptoms like vomiting, increased heart rate, and, in severe cases, seizures or death. Grapes, on the other hand, are associated with acute kidney failure in dogs, and the exact substance that causes this reaction is still unknown.

Given that these two foods pose such significant risks, it's crucial to avoid feeding them to dogs altogether. If a dog has ingested chocolate or grapes, it is essential to seek veterinary attention immediately, as prompt treatment can significantly improve outcomes. Always consult a veterinarian before introducing any human food to your pet's diet.