Christmas foliage is beautiful, but it can be deadly. Mistletoe, poinsettia, and holly berries, among others, can be detrimental to your furry friend's health. Mistletoe can drop your dog's blood pressure to a fatal level, while holly berries cause convulsions. Poinsettia is not safe for dogs or cats and causes nausea and vomiting. If you want festive plants in your home, opt for the plastic variety.