Harvard Heart Letter

Preventing blood clots: Is warfarin still right for you?

Using warfarin effectively is challenging—perhaps even more so than doctors have realized.

blood test warfarin preventing blood clots
Image: arun011 /Thinkstock

For more than 60 years, warfarin has been a mainstay for people with atrial fibrillation. Used properly, warfarin may prevent as many as 60% of strokes related to this common heart rhythm disorder each year. But warfarin is notoriously tricky to manage because blood levels have to be kept within a narrow range to avoid side effects such as unwanted bleeding.

As a result, warfarin users must get frequent blood tests (see "What is an INR?") when they start the drug to fine-tune their dose. Once their INR values are consistently in the right range, most people scale back to testing every month or so. But a recent study suggests that people who take warfarin need to stay extra vigilant over the long term.

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