That question has been bandied about by toxicologists and horticulturalists for decades. The latest that I heard is that they are only mildly toxic--much less than most species of Euphorbia. A hefty mouthful is likely to cause mild to medium stomach distress, though more serious consequences are possible in small children or people who are allergic. Dogs, however find Poinsettias more poisonous, so you would want to take your pooch to a vet quickly, if it ate some.

Most of the writing about poinsettias fails to distinguish between 'poisonous' and 'harmful'.

Poinsettias are poisonous but, as Tugbrethil says, the amount you'd have to consume is so large that there are no reliable records of it ever being harmful.

The hysteria about is based on an alleged poisoning of a young child nearly one hundred years ago. It has been shown that the case was not poinsettia poisoning but the old belief persists.

A lot of people are adamant that poinsettia is not poisonous and quote research to that effect. Research I've seen, however, was financed by the horticultural industry in the USA that didn't want people to be put off buying the plants.

For me, the distinction between poisonous and harmful is important. We are surrounded by poisonous plants but it is very unusual for any of them to be harmful.

Yes, a huge number of the ornamental plants that we grow in our homes and gardens are toxic, but they are only harmful if you are foolish enough to eat them, smoke them, or inject them! So, be watchful where kids (of all ages!) or goofy pets are concerned, and don't worry so much, otherwise.