Calcitriol is a man-made active form of vitamin D. Most people get enough vitamin D from exposure to the sun and from fortified food products (e.g., dairy products, vitamins). Vitamin D helps control parathyroid hormone and the levels of certain minerals (e.g., calcium, phosphorus) that are needed for building and keeping strong bones. Calcium citrate is used to prevent or treat low blood calcium levels in people who do not get enough calcium from their diets. Zinc is a mineral that helps keep the immune system strong and is often used to help battle the common cold. Magnesium is required for the proper growth and maintenance of bones. Magnesium is also required for the proper function of nerves, muscles, and many other parts of the body. And other supplements for growth and development of bone mass.