Yes, Christmas cactuses are toxic to dogs. The plant contains saponins, which are compounds that can irritate the digestive system if ingested. Ingesting even a small amount can cause vomiting, diarrhea, or other gastrointestinal problems. It's important to keep your pets away from Christmas cactuses and to supervise them closely if they're around the plant. If your dog accidentally ingests any part of the plant, seek veterinary care immediately.