The essential oils and psoralens in grapefruit can be toxic to dogs. The peel, seeds and pith are the most toxic parts of the fruit. Symptoms of grapefruit toxicity are digestive upset, depression and photosensitivity.
Citrus fruits, such as grapefruit, are healthy choices for humans but offer no nutritional benefits for dogs. Citrus fruits are rich in vitamin C, which is good for humans, but dogs have no dietary need for it. The acidity and bitter flavor of grapefruit may be unpleasant for a dog and can cause digestive upset. Any citrus fruit, including grapefruit, lemons and oranges, can cause vomiting and diarrhea in dogs.