Poinsettias are considered mildly toxic to cats. However, their potential for harm is much less than that of other common ornamental plants associated with Christmas. Poinsettias, if ingested, can cause irritation of the mouth and digestive system, but there are no recorded reports of fatalities to cats from consuming them.
Poinsettias gained their poisonous reputation from an erroneous news story in 1919 about a little boy who died after eating a single leaf. While poinsettias do contain a milky sap that causes irritation on contact, it is very rare for ingestion to cause much more than mild diarrhea in either humans or cats. Other popular plants used to decorate for Christmas are actually much more dangerous to cats. For example, breathing in lily pollen can cause liver toxicity in cats and ingesting small amounts can be swiftly fatal. Holly and mistletoe are also very toxic to cats.