Are Lilacs poisonous to dogs?

My 6 month old Black Lab pup has a love/ hate relationship with flowers of all kinds. He loves to sniff them and experience new smells, but then he will get very playful and demolish every one in his path. The past couple weeks the lilacs in our area have come into bloom and he has done a number on the neighbors bushes in the backyard {they are overgrown into our dogs area from the neighbors yard}. I am not at all concerned about the bushes since the neighbor doesn't care for them anyway and I am allergic to the flowers, but I am concerned that they could make my puppy sick. Just wondering if lilacs are poisonous to dogs?