The first sign of osteosarcoma in dogs is usually lameness of the affected area. Dogs will start to limp and become painful to the touch. The tumors start deep within the bone, so the cancer can be fairly advanced before obvious symptoms appear. Other common signs of bone cancer in dogs may include: joint pain, swelling, loss of appetite and lethargy. In many cases, the pain seems to improve with the introduction of pain medications and anti-inflammatories, but often this relief is short-term. Using natural pain management supplements in conjunction with veterinary prescriptions can help provide relief without the risk of serious side effects.