Complex sentence: Evidence shows that they reduce the risk of osteoporotic fracture in those who have had previous fractures however do not reduce fracture risk in those with osteoporosis who have not previously had a fracture.
Split into simpler sentences: Evidence shows that they reduce the risk of osteoporotic fracture in those who have had previous fractures. However, they do not reduce fracture risk in those with osteoporosis who have not previously had a fracture.