Because the bulk of production animals are raised on pastures where they come into touch with various types of vegetation, exposure to plant toxins may be the most common cause of cutaneous poisoning in animals. Some plants have substances that have an immediate effect on the skin, while others have spines that can inflict mechanical harm to the skin. Many chemicals can cause allergic dermatitis, which is analogous to poison ivy in humans. The scope of this chapter does not allow for a consideration of contact sensitization. A book on immunology or allergy medicine should be read. Many plants contain colours or chemicals that, once absorbed through the gastrointestinal tract, are toxic. Other plants, which contain poisonous alkaloids that cause irreversible liver illness, can produce secondary photosensitization.