Yes, Bittersweet is toxic to dogs!
Dogs accidentally consuming the Bittersweet 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).
Bittersweet consumption is also toxic to Cats and Horses.
Toxic Principles Of Bittersweet: Sesquiterpene alkaloids.
More Info About The Bittersweet Plant
Scientific Name: Celastrus scandens
Family: Celastraceae.
Also known as: American Bittersweet.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.