The whole plant is extremely poisonous. From the roots to the flowers, leaves and fruit. It's been the key element in many murder mysteries, lately. I think based on a true-life crime. I've also heard that an extract would be an effective terrorist weapon. That could be a big rumor. I did read at 2 different horticulture web site, that you hands should be washed well after handling the plant and your clothes should be washed.
I researched this carefully, after I grew 5 of them last year, and have more starting this year. Mine has beautiful red leaves, and I wouldn't be a true Oaklander, if I were planning to kill anything and didn't use a gun.