Extended use of blood-thinning medications may help people who suffer a blood clot in the lungs (pulmonary embolism) prevent a new clot, according to a new study published in the July 7 issue of JAMA.

However, the length of time those blood thinners should be given is unclear, since their benefits wear off soon after use is discontinued, according to a team of French researchers.

The team tracked outcomes for 371 adults who had experienced an “unprovoked” blood clot in the lung. All of the patients received six months of treatment with a type of anti-clotting drug known as a vitamin K antagonist, which includes the standard blood thinner warfarin.

At the six-month point, patients then received either warfarin for another 18 months, or a “dummy” placebo pill.

The extended use of warfarin did seem to help patients: Further blood clots or major bleeding occurred in only 3% of those taking the drug, compared to 13.5% of those taking the placebo. That means that taking the warfarin reduced the risk by 78%, researchers said.

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