BOSTON (AP) – Dangerous blood clots in the legs recur far more often in men than in women, a study found.

The finding is something of a paradox: Men and women are known to run similar risks for having their first clot. It is unclear why men would have a higher risk of another clot.

In any case, the findings suggest that some women may be able to take anti-clotting drugs for a shorter amount of time after their first clot. The drugs can cause excessive bleeding.

The study, led by researchers at the Medical University of Vienna, was published in Thursday’s New England Journal of Medicine. They looked at 826 patients for an average of three years after an initial episode of deep-vein thrombosis, as these blood clots are called.

Twenty percent of the men developed a second clot, compared with just 6 percent of the women. After adjusting for age and other factors, the risk of another episode was more than triple for men.

Deep-vein thrombosis occurs in up to two people in 1,000. The clots form mostly in the legs, but also occur in the arms or other areas. They cause pain and swelling and can kill if they break away and lodge in the lungs.

Deep-vein thrombosis can follow surgery, injuries, childbirth and lying or sitting still for an extended period, such as during a long airline flight.

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