Yes, Lenten Rose is toxic to dogs!
Dogs accidentally consuming the Lenten Rose 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).
Lenten Rose consumption is also toxic to Cats and Horses.
Toxic Principles Of Lenten Rose: Bufadienolides, glycosides, veratrin and prtoanemonin.
More Info About The Lenten Rose Plant
Scientific Name: Helleborus niger
Family: Ranunculaceae.
Also known as: Hellebore, Christmas Rose, Easter Rose.
The answer is based on information found on The American Society for the Prevention of Cruelty to Animals (ASPCA) website.