Chestnuts themselves are not poisonous to dogs if they are the edible types like American chestnut or sweet chestnut. These types of chestnuts can actually be safe for dogs when cooked and given in moderation and can provide benefits such as antioxidants, fatty acids, fiber, and vitamins. However, horse chestnuts, also called conkers, are highly toxic to dogs because they contain a neurotoxin called aesculin. Ingesting horse chestnuts can cause severe symptoms such as vomiting, diarrhea, abdominal pain, muscle twitching, seizures, respiratory problems, and even death. Additionally, the hard shells of chestnuts can pose a choking hazard or cause internal damage if swallowed whole. It is important to always avoid giving dogs raw chestnuts or any parts of the horse chestnut tree and to consult a veterinarian before feeding dogs any new human foods. Thus, while some chestnuts are safe in small amounts when prepared properly, horse chestnuts are dangerous and should be strictly avoided by dogs.