Yes, Boxwood is toxic to dogs!
Dogs accidentally consuming the Boxwood 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).
Boxwood consumption is also toxic to Cats and Horses.
Toxic Principles Of Boxwood: Alkaloids.
More Info About The Boxwood Plant
Scientific Name: Buxus spp.
Family: Buxaceae.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.