American chestnuts are not poisonous to dogs, according to the ASPCA. The Daily Puppy warns an unrelated plant called the horse chestnut is considered toxic to dogs and other animals when consumed in large quantities.
According to The Daily Puppy, the bark, leaves and nuts of the horse chestnut tree are poisonous to dogs when ingested. The nuts from the tree not only contain toxins, such as saponin, aescin and glycoside, but can also cause choking in dogs. When dogs consume horse chestnuts, or "conkers," vomiting, diarrhea, seizures, depression and coma can occur.
The Daily Puppy notes that a dog who ingests horse chestnuts or any part of the tree requires veterinary care. The vet may induce vomiting in the dog to expel the toxic elements, or she may administer activated charcoal to prevent the dog's intestines from absorbing the toxins.