Avocado pits aren't poisonous to humans in small quantities, but they contain a fungicidal toxin called persin that can harm domesticated animals. Persin is an oil-soluble compound found in the leaves, bark and pits of avocados.
Avocado leaves and seeds are harmful when ingested in large quantities, but as of 2014, few cases of toxicity have been reported. Avocado pits are especially toxic to horses, cattle, goats and birds. Cats and dogs show slight signs of illness after eating large amounts of avocado flesh and peels, and eating the pits can cause gastric obstruction. Although some persin leaches into the flesh of avocados from the pits and skins, the concentration tends to be low. People with latex, banana, melon and peach allergies may be allergic to avocados.