Dogs should not consume milk. Doing so can cause digestive issues such as stomachaches and diarrhea in dogs, because dogs do not have high amounts of the lactase enzyme that breaks down lactose from dairy products. Although milk can cause digestion issues in dogs, it is not considered toxic.
Not all dairy products are bad for dogs. Typically, the amount of lactose in a product determines how well a dog can digest it. For example, while milk contains a substantial amount of lactose, other dairy products, such as cheddar cheese, contain very small amounts of lactose, which makes them easier for dogs to digest.