TL;DR
Aspirin, one of the cheapest pills in any medicine cabinet, may protect some people's brains from dementia, and their genes may be what marks them out. Researchers at Monash University went back to an old trial of more than 13,500 older adults and found that among people whose genes push them toward a high platelet count, 1 in 100 aspirin takers developed dementia, against about 3.3 in 100 on a dummy pill. The same people also had about twice as many major bleeds, roughly 4.4 in 100 against 2.1 in 100. The study's own lead author says people should not start taking aspirin to prevent dementia because of it, and nobody should: the finding is a first hint that needs its own trial, and the bleeding cost is real. What it might mean for the future is a world where a simple gene score tells doctors which people a cheap preventive drug could help, and which it could hurt.
Below the line, for paid subscribers: The full story: where it stands, how we got here and what's dated next; our Odds board on 4 milestones, starting with "A trial that enrolls only people with high platelet-gene scores, to test aspirin or a narrower platelet drug for dementia prevention, is announced by 2028"; our labeled speculation on what it adds up to; and every source.