Poinsettia's get a bad rap at the holidays for being an extremely poisonous plant, especially to cats. In reality they are low on the list of poisons your pet may be exposed to around the holidays.
Poinsettia's, if eaten, may cause vomiting, drooling, skin irritation around the mouth or rarely diarrhea. These signs are usually very mild if seen at all and usually do not require medical treatment.
The bigger threat with holiday decorations are lilies, holly, mistletoe and tinsel. A small amount of lily being ingested can cause severe kidney issues in cats. The ingestion of holly berries or leaves can result in severe gastrointestinal upset and drooling. Luckily, mistletoe is usually hanging high enough that your pets can't reach it, but if eaten, the most common symptom is gastrointestinal irritation. If large amounts are ingested, however, it can cause hypertension, ataxia (lack of coordination when trying to walk), seizures and even death. Tinsel can be a huge risk for cats even though the product itself is not toxic. Cats can ingest the plastic tinsel and it can get stuck on the back of their tongue or in their intestines and cause an obstruction which would require immediate surgery.
If you're ever in doubt if something that your pet ate is toxic, you can always call Animal Poison Control at (888) 426-4435.