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The question

Warfarin, a drug that makes blood less likely to clot, has long been prescribed to people who have had a heart attack, are at high risk for a stroke, have an artificial heart valve or an irregular heartbeat (atrial fibrillation) or have had surgery that makes them more susceptible to blood clots in their leg veins. Might this drug provide other protection as well?

This study

The researchers analyzed data on more than 1.2 million adults older than 50 (most in their mid-60s to early 70s) and including nearly 93,000 who took warfarin, most for at least five years. In a seven-year span, about 133,000 people were diagnosed with cancer. Overall, those taking warfarin were 16 percent less likely than those not taking the drug to have developed cancer.

Their risk was 31 percent lower for prostate cancer, 20 percent lower for lung cancer and 10 percent lower for breast cancer. For colorectal cancer and skin cancer, no difference in the likelihood of developing cancer was found. Risk reduction was stronger among people taking warfarin for atrial fibrillation. They had a 38 percent lower risk for any type of cancer as well as greater reduced risks for prostate cancer (40 percent), lung cancer (61 percent) and breast cancer (28 percent).

Who may be affected?

People taking warfarin (brand name Coumadin). Though anticoagulants are commonly called blood thinners, they do not actually thin the blood but rather interfere with the body’s clotmaking process, thus preventing clots from forming and keeping existing clots from getting bigger. In recent years, warfarin prescriptions in the United States totaled more than 20 million a year, but those numbers have declined some as newer anticoagulants have come on the market.

Caveats

The study did not determine why warfarin might affect cancer development, nor did it compare cancer diagnoses in people taking warfarin with diagnoses in those taking other anticoagulants. Data on lifestyle factors that might affect cancer risk, such as diet, weight and smoking status, were not available for analysis. The researchers noted that people taking warfarin may be in poorer health than others, which may have affected their cancer risk. It is also possible that people taking warfarin, which can be complicated to manage, may also be more vigilant about taking care of their health.

Find this study

Online Nov. 6 in JAMA Internal Medicine (jamainternal­medicine.­­com; click on “New Online”)

Learn more

Information on warfarin can be found at heart.org (search for “patient’s guide”) and at ­medlineplus.gov/druginfo (click on “W”).

The research described in Quick Study comes from credible, peer-reviewed journals.