Not really, Daniel, but it could have other negative effects. For example, sometimes people add preservatives to Christmas tree water that may contain fertilizers. While these preservatives are not poisonous, they can cause stomach upset. Stagnant tree water can also be a breeding ground for bacteria, which can lead to vomiting, nausea, or diarrhea. It's a good idea to make sure a skirt or a cloth covers the bottom of your tree to deter your dog and cat from drinking the water.