Honey locust trees do not contain toxins, but black locust leaves, bark, and thorns do contain poison. Black locust thorns grow from one-fourth of an inch to one inch in length. Black locust tree thorns are toxic to people and it is recommended that they not be ingested by animals.
Horses or livestock that consume black locust leaves or flowers become life-threateningly ill and must have immediate veterinary attention. Although black locust is toxic, it is used in traditional Indian medicine as a diuretic and laxative. In France and Italy, the flowers of the black locust are battered and fried, which reportedly kills the toxins in the plant.