Okay, the title might be a tad hyperbolic. Mistletoe isn't trying to harm your pet. But both of these holiday plants are indeed toxic.
Poinsettias. They're a staple houseplant of the holiday season, despite being a large tropical shrub that can grow to be up to 15 feet, sometimes more, when planted outdoors in tropical regions. These gorgeous plants are pleasant to look at, but one who decides to bring them into the home should be aware that they are a mildly toxic. Proceed with caution if you plan on adding them to your home when you have dogs and/or cats. When our furry family members do decide to munch on them, it rarely leads to serious complications and fatalities are even rarer. Still, it's worth keeping in mind.
The milky white sap found in poinsettias contains chemicals similar to those in detergents and when large quantities are ingested, mild signs of vomiting, drooling, or sometimes diarrhea could manifest. Skin irritation can occur when in contact with this sap as well. Since the sap runs through the entire plant, all parts of the plant (leaves, stems, flowers, and roots) are toxic. Interestingly, people with latex allergies can also be impacted by the sap due to the Poinsettia being a close relative of the plants that are used to produce natural rubber latex and the reactions will be the same.
Due to the low level of toxicity seen with poinsettia ingestion, you are safe to use them in your house with caution. All reasonable precautions should be made such as keeping plants and decorations out of your dog’s or cat's reach and never leaving your dog unattended when the plants, christmas decorations (which may pose an entirely different danger), and potentially hazardous holiday foods are present.
While medical treatment is rarely necessary when a dog ingests a leaf from a poinsettia plant, you should contact your veterinarian if clinical signs appear in your dog (vomiting, lethargy, skin irritation, etc.).
Mistletoe is less common these days, at least in the living variety, but it still shows up and is sometimes used in both fresh and dry arrangements and wreaths around the holidays. There are 1500 species of mistletoe, varying widely in toxicity to humans; the European mistletoe (Viscum album) is more toxic than the American mistletoe (Phoradendron serotinum), though concerns regarding toxicity are more prevalent in the US. The effects are not usually fatal, but even more caution should be taken than with Poinsettias. Symptoms of mistletoe toxicity include gastrointestinal upset (vomiting and diarrhea in dogs and cats; but colic in horses), difficulty breathing, weakness (due to low blood pressure and/or decreased heart rate), and generally odd behavior. If any symptoms present themselves and you fear your pet has ingested mistletoe, contact your veterinarian immediately. Never allow horses near American Mistletoe.
Mistletoe toxicity is a bit different than that of the Poinsettia. The danger lies in a toxic protein called Phoratoxin. This protein is present in the leaves and stems, but has the highest concentration in the berries.
Other common plants which tend to be utilized for holiday decor that can be hazardous to pets are as follows; Peace Lily (Spathiphyllum), Calla Lily (Zantedeschia aethiopica), Amaryllis (Christmas Lily), Lily of the Valley (Convallaria majalis), Autumn Crocus (Colchicum autumnale), Giant Dracaena (Cordyline australis), and all cultivars of the extremely popular Holly (Aquifoliaceae).