A popular blood-thinning drug may increase the risk of stroke in patients with a common heart rhythm disorder.

The drug, warfarin, is an anti-coagulant that is often prescribed to prevent blood clots in the heart. But a recent study shows that for those who take the drug and have atrial fibrillation, the risk for stroke is almost doubled in the first 30 days of treatment, the Canadian Press reports.

The findings, published in the European Heart Journal, revealed that patients were at a high risk of stroke in the first week after taking warfarin, peaking at about three days. The results seemed contradictory to the drug’s intended purpose since doctors commonly prescribe warfarin to patients with atrial fibrillation to prevent the chances of stroke.

"Warfarin is a highly effective drug for the prevention of stroke, there is no question there," lead author Laurent Azoulay, an epidemiologist at Montreal’s Jewish General Hospital who specializes in pharmacology, said in a press release. "But it is documented that for some paradoxical reason, warfarin can increase the risk of stroke in the first weeks of treatment."

Data was taken from 70,000 adults diagnosed with atrial fibrillation between 1993 and 2008. Researchers followed patients for 16 years and found that 5,519 patients, or 2 percent per year, suffered a stroke. But after the first 30 days, patients on warfarin saw their risk of stroke drop to less than half that of people who didn’t take the drug.

Azoulay says the results do not diminish the drug’s effectiveness. "For the vast majority, this is not an issue," Azoulay said. "However, the results of our study suggest that physicians should be vigilant when initiating warfarin, particularly in the first week of use."

Not all doctors agree with the study’s conclusions.

"The problem we see with these sorts of studies is that they tend to be biased in that patients who get started on warfarin get started on it for a reason," Dr. Muhammad Mamdani, director of the Applied Health Research Centre at St. Michael's Hospital in Toronto. "They tend to be much sicker than those who don't get started on warfarin.”

Mamdani adds that further research needs to be done to see if the results vary.

"Patients with atrial fibrillation should be on warfarin," Mamdani said. "There may possibly be a short-term risk, but more definitive data are really needed, and I don't think this will really change practice."