What is harmful? Lactose.
Why is this harmful? All dogs are Lactose intolerant to a degree and cannot properly digest dairy. Giving your dog milk can cause a significant imbalance to their system and trigger other health issues as well. Not all dogs react when having milk or dairy products, however it is advised not to take the risk.
What symptoms should I look out for? Smelly farts, diarrhea, vomiting.