Yes, Grapefruit is toxic to dogs!
Dogs accidentally consuming the Grapefruit plants can show the following clinical symptoms: Vomiting, Prolonged Depression, Incoordination, Hypersalivaton, Sleepiness Or Excitation, Dilated Pupils, Low Body Temperature, Low Blood Pressure, Coma, Seizure And Death (In Rare Cases).
Grapefruit consumption is also toxic to Cats and Horses.
Toxic Principles Of Grapefruit: Essential oils and psoralens.
More Info About The Grapefruit Plant
Scientific Name: Citrus paradisii
Family: Rutaceae.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.