Hawthorn berries are generally safe for dogs to eat and are not considered poisonous. If dogs consume a large quantity of them, they might experience some upset stomach, but overall, hawthorn berries pose very low risk to dogs. They are even used as a natural supplement to support heart health and digestion in dogs. However, care should be taken to avoid confusing hawthorn berries with look-alike berries such as Bryony, which are toxic. Additionally, it is advisable to avoid berries that may have been sprayed with chemicals. Always consult a veterinarian before adding hawthorn as a supplement, especially if the dog has existing health conditions or is on medication.