Apples are considered a safe food for dogs, with some caveats. The seed coverings contain minute amounts of a cyanide compound called amygdalin; so it is not recommended to feed a dog whole apples, apple cores, or apple slices that still contain seeds.
Large slice or whole apples can represent a choking hazard to dogs; any apples fed to a dog must be bite-sized to prevent a potentially fatal problem. Finally, dogs are mainly carnivores, and a diet very high in fruit, including apples, can cause stomach upset. Dehydrated apples can also cause issues as they are lacking in water needed for digestion.
It is also not recommended to give dogs that suffer from kidney disease or arthritis too many apples, because apples contain vitamins, minerals and fatty acids that can aggravate those illnesses. Healthy dogs do not need to have calcium, phosphorus or omega 6 levels monitored as closely and thus can consume a greater amount of apples.