What are some common poisonous plants?

Some common household plants that are considered "poisonous" include philodendrons, calla lilies, and peace lilies. The leaves contain oxalic acid, which combined with water, creates oxalate crystals that can imbed in tissue and cause swelling. For example, if you took a bite from a peace lily leaf, you might feel some sharp pain in your tongue and some tongue swelling. For that reason, it's considered poisonous because if the swelling were severe, you would choke.

Obvious poisonous plants include poison ivy and poison oak for the awful dermatological reactions they cause. 

Despite the reputation, poinsettias are not toxic, but like any plant, they can cause a rash if you're sensitive to them.