In the long human struggle against the slow unraveling of the mind, an unexpected ally may have emerged from an unlikely quarter. Researchers analyzing the health records of 120,000 adults have observed that those taking common osteoporosis medications develop Alzheimer's disease at notably lower rates — a finding that invites medicine to look again at tools already in hand. The discovery does not yet constitute proof, but it opens a door that the field of dementia prevention, long marked by disappointment, has rarely seen ajar.