Coumadin® (warfarin) may help prevent cancer

That headline sounds like clickbait, doesn't it? After all, if it sounds too good to be true, it usually isn't.

Taking warfarin (brand name: Coumadin®) isn't easy: there's what we physicians call a "therapeutic window" of effectiveness in warfarin dosage that prevents your blood from clotting too easily (being "too thick") and at the same time doesn't contribute to excessive bleeding (being "too thin"). Because everyone's bodies are a little different, this means that there's no "one size fits all" dosage - those who are taking warfarin need to have their blood monitored carefully in order for their dosage to stay within that therapeutic window.

This also means that those who are taking warfarin need to monitor their dietary intake of Vitamin K-containing foods very carefully so that their daily intake of Vitamin K remains stable. To put it very simply, wide variations in Vitamin K intake can mean wide variation in warfarin effectiveness - and this can lead to life-threatening conditions like blood clots or excessive bleeding.

In an article published in JAMA Internal Medicine (1), researchers in Norway noted that warfarin, because of its effects on Vitamin K activity, indirectly blocks cancer cells from multiplying and enhances the activity of tumor-killing cells. (This is a bit of an oversimplification.) Would those who were prescribed warfarin develop cancers at lower rates than those who were never prescribed warfarin?

The authors were fortunate to live in a country with universal health care and a centralized health registry, a national database that contains information on every Norwegian resident since the inception of universal health care in that country. A unique identification number is assigned to every citizen or resident of Norway, allowing researchers to correlate health and prescription information with the Cancer Registry of Norway. The data from these registries are anonymized for researchers.

Their analysis included all persons born between 1924 and 1954 who were living in Norway between 2006 and 2012 (over 1.2 million people). The authors accessed the prescription database to determine who had been prescribed warfarin between 2004 and 2012 and took the medication for at least 6 months. Those warfarin users who developed cancer of any kind within two years of taking warfarin were excluded from analysis; the authors then compared the overall incidence of cancer in those remaining who took warfarin with those who never took warfarin.

After taking into account age and sex, the authors found that 9.4% of those who took warfarin were diagnosed with cancer over the course of the study period. On the other hand, 10.6% of those who did not take warfarin were diagnosed with cancer. Warfarin users were about 20% less likely to develop lung cancer, 31% less likely to develop prostate cancer, and 10% less likely to develop breast cancer. This despite the fact that over 60% of those who took warfarin were male and tended to be older than non-warfarin-users. Even more remarkable is that those who were prescribed warfarin for atrial fibrillation had an even lower rate of cancers than those who took warfarin for other reasons.

What this means for you

Once again, this type of study only shows that taking warfarin is associated with lower risk of cancer: it does not show definitively that taking warfarin is the reason cancer rates were lower. That said, the proposed mechanism (the reason warfarin might have that effect) is known and plausible. More research is needed, but in the mean time, if you are facing the challenge of warfarin treatment, know that there might be a positive unintended consequence: a lower risk of cancer.

References:

1. Haaland GS, Falk RS, Straume O, Lorens JB. Association of Warfarin Use With Lower Overall Cancer Incidence Among Patients Older Than 50 Years. JAMA Intern Med. Published online November 06, 2017. doi:10.1001/jamainternmed.2017.5512

First posted: November 8, 2017