What is this plant, is it a weed, and is it poisonous?
It looks like a species of wild Ageratum, a genus of plants in the family Asteraceae. It might be Ageratum houstonianum or Ageratum conyzoides which are commonly called "billygoat weed". There are several species in the genus. The Wikipedia link mentions both "weed" and "toxicity" in their article about the genus Ageratum, but some species are grown for their flowers. I hope the following links are helpful.