Warfarin “may have broad anti-cancer potential”, according to a Norwegian study, which “may have important implications for choosing medications for patients who need anticoagulation”.

In animal models, warfarin enhances immune responses against cancer and inhibits a signalling pathway that drives tumorigenesis, so researchers examined the records of 1,256,725 people born between 1924 and 1954.

They defined ‘use’ as taking warfarin for at least six months based on prescriptions. The study allowed a gap of at least two years from first warfarin prescription to any cancer diagnosis during the seven-year observation period (2006 to 2012). About one in 10 (10.6 per cent) developed cancer and 7.4 per cent used warfarin for a median of 4.7 years.

Compared with non-users, warfarin users were 16 per cent less likely to develop any cancer after adjusting for age and sex. The risk of lung, prostate and breast cancer all declined (reductions of 20, 31 and 10 per cent respectively). No significant difference emerged in the risk of colorectal cancer.

In the 33,313 patients with atrial fibrillation or atrial flutter, warfarin was associated with a 38 per cent reduction in any cancer. The risk of lung, prostate, breast and colorectal cancer all declined (61, 40, 28 and 29 per cent).

Thromboembolic disease is associated with an increased risk of cancer, particularly in the first year after diagnosis, but patients with atrial fibrillation or atrial flutter lack the occult pre-existing cancer common in patients who developed thromboembolic events. The stronger association between warfarin and cancer incidence in people with atrial fibrillation or atrial flutter “supports the hypothesis that warfarin may exert considerable cancer protection against major carcinoma types”.

The authors of the study say it “reveals a remarkable association between warfarin use and lower cancer incidence across a broad range of malignant neoplasms”. The difficulties of monitoring warfarin encouraged a move to new oral anticoagulants but the authors remark that, “an unintended consequence of this switch to new oral anticoagulants may be an increased incidence of cancer, which is an important consideration for public health”.

^{JAMA Intern Med doi:10.1001/jamainternmed.2017.5512}