10 Foods Your Dog Should Never Eat Chocolate. Chocolate contains a very toxic substance called methylxanthines, which are stimulants that can stop a dog's metabolic process. ... Avocados. ... Onions and Garlic. ... Grapes and Raisins. ... Milk and other Dairy Products. ... Macadamia Nuts. ... Sugary foods and drinks. ... Caffeine. More items...