Symphytum is a dangerous herb to dogs because it contains comfry which is toxic to the liver. I have a rescued Dogue de Bordeaux who has an old bone fracture that was causing him extreme pain. I give him Arnica pellets which are very effective to relieve the pain and completely safe.
Symphytum 30C contains a homeopathic potency of comfrey, making the amount ingested negligible. It is recommended by vets trained in Homeopathy and is considered safe for short term use.