copy and pasted from the Harvard website:

Strontium is a chemical element found in water and food. When taken orally, it's absorbed in the body in small amounts, mainly in areas where bone is being remodeled — that is, undergoing the natural process by which it is broken down and formed. In the 1950s, it was used to treat osteoporosis, but it fell out of favor because of adverse effects on bone mineralization. The dose may have been too high, or the patients may have had dietary deficiencies. Interest in the use of strontium has recently revived, this time in a new form called strontium ranelate. In humans, it increases bone mineral density, improves bone microarchitecture (an indicator of bone strength), and decreases the risk of fractures in postmenopausal women with osteoporosis. In 2006, the Cochrane Collaboration, an international organization that evaluates medical research, pooled the data from four randomized clinical trials of strontium ranelate for the prevention or treatment of osteoporosis. The reviewers found that postmenopausal women with osteoporosis who took two grams of strontium ranelate daily for three years had 37% fewer spine fractures and 14% fewer nonspinal fractures, compared with women taking a "placebo". (Both groups took supplemental calcium and vitamin D.) Strontium ranelate also increased bone mineral density at both the spine and hip. It appears to have few adverse effects, although the reviewers noticed a small increase in the risk of blood clots in the legs or lungs and recommended further investigation of this potentially serious side effect. No studies have directly compared strontium ranelate with other commonly prescribed drug treatments for osteoporosis, such as raloxifene (Evista), estrogen, or bisphosphonates, which include alendronate (Fosamax), risedronate (Actonel), and ibandronate (Boniva). The Cochrane review suggests that strontium ranelate is about as effective as Fosamax and Actonel in reducing the risk of spinal fractures, but less effective for preventing nonspinal fractures (such as hip fractures). Strontium ranelate was approved for use in Europe in 2004 (where it is marketed under the trade name Protelos), but it is neither approved nor available in the United States. Over-the-counter supplements containing the mineral strontium are available in health food stores. These products are not the same as strontium ranelate. ***There is no evidence that they have similar beneficial effects or that they have any role in treating or preventing osteoporosis. ***
— Celeste Robb-Nicholson, M.D.Editor in Chief, Harvard Women's Health Watch