Taking low-dose aspirin every day may be as routine as brushing your teeth, but new research has found that the one-dose-fits-all amount commonly prescribed may be doing a lot of people no good at all.

Doctors have advised millions of people, mostly over the age of 50, to take a low-dose aspirin once a day to help prevent heart attack or stroke. “Low-dose” is typically 81 milligrams (mg)—that’s one-quarter the amount of the standard 325 mg aspirin tablet that’s been used for headaches for generations. And now pharmacy shelves are filled with bottle after bottle of these very unintimidating little pills, frequently marketed with heart shapes on the package.

To see just how well this dose works, researchers from Oxford University in the UK and other institutions analyzed 10 trials of low-dose aspirin involving more than 117,000 people. They looked at aspirin’s effects based on people’s body weight in 10-kilogram (22-pound) increments from less than 50 kg and up, and found that the benefits of daily low-dose aspirin are quite good, possibly even better than previously thought…but only if you weigh between 110 and 154 pounds, said lead researcher Peter M. Rothwell, MD, PhD. People in this weight range were able to reduce their risk of a heart attack, stroke or other major heart-related event by an average of 23%.

What’s the problem? There was no heart-health benefit seen from taking low-dose aspirin among people who weighed more than 154 pounds—and that accounted for about 80% of the men and nearly 50% of the women in the trials. These people had a higher risk for a fatal heart attack than those weighing less than 154 pounds, underlining the lack of benefits they were getting from low-dose aspirin.

As to why low-dose aspirin is ineffective when body weight reaches a certain point, the thinking is that people with more body mass have a greater volume of enzymes that clear aspirin from the bloodstream, reducing the amount of the drug getting where you need it—your circulatory system. This could also explain why aspirin is less effective at preventing stroke, on average, in men than in women.

A positive finding was that heavier people can get the benefits of aspirin by taking higher daily doses, typically 325 mg (the commonly available pill size that you might take for headache relief, for example), but most doctors aren’t in the habit of prescribing aspirin doses based on their patients’ weight. Also, taking a higher daily dose of aspirin increases the bleeding risk that already exists with even a low-dose aspirin regimen.

The researchers also looked at low-dose aspirin’s effect on reducing the risk for colon cancer, another well-known benefit. They again found that it was significant only for people weighing up to 154 pounds. Larger daily aspirin doses helped prevent colon cancer in people weighing up to 176 pounds, but this benefit wasn’t seen in people weighing more than that.

So, what’s the answer for you? If you’re on aspirin as a preventive measure, there’s a good chance that your daily dose should be personalized to your weight along with other considerations, such as your particular bleeding risk, says Dr. Rothwell. But do not increase (or decrease) your dose on your own. It’s important to speak with your doctor to carefully weigh the potential risks and benefits of aspirin doses.

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