According to snopes, Poinsettias are negligibly poisonous. Digesting poinsettia leaves may make your stomach sick, at best.
Every holiday season newspapers run stories about whether or not Poinsettia is toxic and to what degree. Although many species in the genus Euphorbia are highly toxic, poinsettia is not among them. Having said that, ingestion of this plant probably will make you sick (it just won’t kill you).
The Minnesota Poison Control System:
The fact is that they [poinsettias] are not poisonous. Nor are they edible and it can be expected that, when eaten in quantity, they may cause stomach upset with possible vomiting. This may happen when an overactive puppy devours an entire plant. In the case of a child who eats a single leaf, no ill effects would be expected.
Note: I am not a medical expert and do not condone nor advise eating any part of poinsettia.