In this case, because pancreatic cancer is relatively rare, the impressive-sounding 50% reduction doesn't amount to very much. According to the American Cancer Society, a 60-year-old man has a 0.41% (1 in 241) chance of developing pancreatic cancer during the next ten years. (Risk varies greatly with age and is much lower at younger ages.) So cutting that risk in half might bring it down to about 0.2% (1 in 480).
It's a 50% drop, sure, but the risk was already very small to begin with. In this case, it's more helpful to news and health care consumers to describe it as a 0.2 percentage point reduction.
And then there are the harms of regular aspirin use; the Yale news release that the tweet links to doesn't mention any. But taking aspirin regularly isn't a harmless intervention - far from it. It's well known that taking aspirin every day can cause serious bleeding in the gastrointestinal system and, less frequently, in the brain. That's why guidelines for aspirin use in cardiovascular disease prevention don't recommend it for people at low risk of a heart attack. The potential benefits may be outweighed by the risks of a serious bleed.
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