According to The American Society for the Prevention of Cruelty to Animals (ASPCA), citrus fruits like Lemon (Citrus limonia), Orange (Citrus sinensis) and Grapefruit (Citrus paradisii) are toxic to dogs, cat and horses.
The Toxic Principles are essential oils and psoralens.
Signs and Symptoms: Vomiting, diarrhea, depression; potential dermatitis. The fruit is edible, skins and plant material can cause problems.
Source: link.
