My 95 Year Old Mother Was Taking Strontium For Her

My 95 year old mother was taking strontium for her severe osteoporosis without any improvement. She swears by this drug and doesn't take any calcium so her bones are thinner that before. I refused to buy her strontium for the last 6 months and she has been taking Ca+ bid. Does strontium actually replace calcium in her bones? What other side effects are worrisome?