Poinsettia:

Contrary to popular belief poinsettias ARE NOT considered dangerous. In the 1970s, scientists determined that concerns over the plant were unfounded.
Holly:

The berries of these pretty plants, more often found outside than in, can be toxic if ingested. Eating 20 berries can be fatal to a child.
Mistletoe:

This romantic plant has a dark side. The entire plant is poisonous if eaten. Kind of gives a whole new meaning to 'Love Bites'.
Bittersweet:

Looking for a non-poisonous alternative?
Christmas Cactus is completely harmless.
