Warfarin, a blood-thinning drug, most commonly prescribed as an oral anti-clotting agent, is found to double the risk of stroke with patients who have irregular heartbeats, according to a recent study.

Individuals with atrial fibrillation (A-Fib or AF), usually experience a rapid and irregular heartbeat due to how the upper two chambers beat out of sync with the lower two chambers. The Mayo Clinic says when the chambers have an irregular or abnormal rhythm, this causes a delay in blood flow and increases the possibility of clots. If this is not regulated, the clots can escape to the brain and result in a stroke.

Blood-thinning drugs, such as warfarin, are frequently prescribed for these patients to prevent the likelihood of strokes occurring. Warfarin stops the formation of blood clots in the body because of its potent blood-thinning properties. AF patients who take the anti-clotting agent are advised to keep their blood pressure in check because the drug decreases the body’s mechanisms, which normally stop bleeding. Warfarin helps increase people’s International Normalized Ratio (INR), which attributes to a value of 1.0 to people with a normal ability to form blood clots, to a value between two to three for stroke prevention, so they are less likely to form blood clots.

On the contrary, recent clinical trials have suggested there might be an increased risk of a stroke when patients are moved from the trial drugs to warfarin. It is also well known that the blood thinner could temporarily lead to a hypercoagulable state since the drug causes a severe depletion of protein C and S, which can lead to clot formation and/or expansion, says an article published in American Family Physician.

Published in the European Heart Journal, a team of researchers from McGill University in Canada, sought to determine whether the start of warfarin is associated with an increased risk of stroke in AF patients. Data from the UK Clinical Practice Research Datalink, a nested case-control analysis conducted within a cohort of 70,766 patients with AF between 1993 and 2008, was obtained to use as the sample size in the study. The large cohort was followed for up to 16 years until there was either a stroke, death, end of registration with their primary care practice, or the end of the study period, whichever came first.

Over 5,500 patients were found to experience a stroke during the long-term experiment. The researchers found the highest risk of developing a stroke was in the first week of taking the drug. Stroke risk peaked on the third day with patients more than twice as likely to have a stroke compared to those who were not treated with warfarin. After 30 days, the risk of stroke among AF patients taking warfarin doubled. Patients who had a previous history of stroke were found to be 2.5 times more at risk of another stroke during the first month of taking the drug.  

The researchers were surprised to find that patients with a history of stroke prior to their AF diagnosis were at a higher risk of having a stroke during the first 30 days of warfarin. According to the press release, they believe this result is consistent with the belief that the risk “may be the highest in patients with hypercoagulable states.” This hints at the possibility that biological mechanisms could play a role in this association.

Despite the findings of this study, the study authors believe there is no question that warfarin is highly effective in preventing strokes in AF patients, and it should not deter physicians and patients from using this drug.

Lead researcher of the study, Dr. Laurent Azoulay, assistant professor in the Department of Oncology at McGill University and project leader at the Lady Davis Institute for Medical Research, Jewish General Hospital in Montreal, Canada, believes the increased stroke risk applies to a small cohort of patients but suggests “physicians should be vigilant when initiating warfarin, particularly in the first week of use.”

These findings warrant further studies to confirm the researchers’ results to help identify the small group of patients who may be at risk by analyzing the biological mechanisms that may come into play.

Until further evidence is provided, co-author Professor Samy Suissa, also from McGill University, said AF patients could opt for heparin, an injected anti-clotting drug, to offset the increased risk of stroke.

To learn more about warfarin and its possible side effects, click here.

Source: Azoulay L, Dell’Aniello S, Renoux C et al. Initiation of warfarin in patients with atrial fibrillation: early effects on ischaemic strokes. European Heart Journal. 2013.