Shelcal HD tablet contains calcium and vitamin D3 and Shelcal 500 tablet also contains calcium and vitamin D3. The only difference between these two is the amount of vitamin D3. Shelcal HD tablet has a higher amount of vitamin D3 compared to Shelcal 500. These medicines are given in case of calcium and vitamin D3 deficiencies, for treatment and prevention of bone related disorders such as osteoporosis. You should take medicine as advised by your doctor and the same shall be based on your diagnosis and your medical history.