Aspirin not effective in some people
Failure to take the drug may be the most common reason.
Despite aspirin's low cost and availability without a prescription, it is a powerful drug: its antiplatelet activity helps prevent clots from forming inside stents or arteries. That's why people with coronary artery disease who take aspirin have fewer heart attacks and strokes than people who don't take aspirin.
But aspirin is not fully effective in some people, leaving them vulnerable to a potentially fatal clot. Just how many people are aspirin resistant is unknown: most experts think about 2% of people are resistant, though some estimates are higher.