Is anticoagulant warfarin associated with lower risk of cancer incidence?

The study included 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.

Prescriptions for warfarin between 2004 and 2012 (exposure); any new cancer and most common cancers (prostate, lung, breast, colon) between 2006 and 2012 (outcome).

This is an observational study using Norwegian national registry data. 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.

Results show that 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. 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).

Study limitations were outlined by the investigators:

• 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.

In conclusion, warfarin appeared to be associated with reduced cancer risk in a national European population. 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.