WASHINGTON (AP) — Millions of people who take aspirin to prevent a heart attack may need to rethink the pill-popping, Harvard researchers reported Monday.

A daily low-dose aspirin is recommended for people who have already had a heart attack or stroke and for those diagnosed with heart disease.

But for the otherwise healthy, that advice has been overturned. Guidelines released this year ruled out routine aspirin use for many older adults who don't already have heart disease — and said it's only for certain younger people under doctor's orders.

How many people need to get that message?

Some 29...