Warfarin is a widely used anticoagulant prescribed to as many as 10 percent of adults in Western countries.

Studies disagree on whether warfarin is associated with cancer.

Any association between warfarin and cancer would be important to identify given the availability of newer non-warfarin anticoagulants.

In an observational study using Norwegian national registry data, published in JAMA Internal Medicine, warfarin use was associated with lower risk of any cancer and of three of the most common cancers (prostate, lung, female breast) compared to warfarin non-use.

The finding could have implications for choosing medications for patients who need anticoagulation but further studies to understand the mechanisms underlying any protective association are warranted.

About 1.25 million people born in Norway between 1924 and 1954 divided into those taking (92,942) and not taking warfarin (more than 1.1 million).

Individuals taking warfarin for atrial fibrillation or atrial flutter were studied as a subgroup.

In the subgroup of people using warfarin for atrial fibrillation or atrial flutter, cancer risk was lower at any site and in all four common sites (lung, prostate, breast, and colon).

In observational studies, researchers observe exposures and outcomes for patients as they occur naturally in clinical care or real life.

Because researchers are not intervening for purposes of the study they cannot control natural differences that could explain study findings so they cannot prove a cause-and-effect relationship.

Researchers did not collect information on other medications or risk factors that could influence cancer development.

New cancers may actually have been cancer recurrences.

Prescription of warfarin may be a marker for other health care factors that lead to cancer prevention.

Source: JAMA Internal Medicine