Dieffenbachia is indeed poisonous. One common name for it is dumb cane, because of the effect it has on those who accidentally ingest it. The plant sap contains calcium oxylate, a chemical compound that causes swelling of the tongue and throat. Swelling can be so severe that airflow through the throat is blocked, preventing normal speech and breathing. Obviously, this can have serious consequences. Fortunately, few accidental poisonings occur. However, to be safe, keep your dieffenbachia out of the reach of small children and pets, who may not understand that the plant needs to be left alone.