Christmas trees can be toxic to dogs if they ingest the needles or drink the water from the stand. The sap of the tree can also cause an allergic reaction in some dogs, leading to skin irritation and vomiting. Additionally, if the tree is not properly secured, it can fall over and injure a dog. To prevent these issues, it is important to keep the tree out of reach of pets, secure it properly, and keep the water in the stand fresh and clean.