According to the Federal Drug Administration (FDA), mistletoe contains lectins and phoratoxins, which are poisonous to cats. "These chemicals affect the heart, causing low blood pressure and slowed heart rate," reads the FDA's website. "Fortunately for Fido, Tigger, and Trigger, severe mistletoe toxicity is uncommon and usually only occurs if your pet eats a large amount." Cats who eat big quantites of this holiday green may suffer from stomach pain, difficulty breathing, vomiting, diarrhoea, and low blood pressure.