Migraine sufferers are twice as likely to have a stroke as people who do not have these debilitating headaches, Canadian and American scientists reported on Monday.

A review of 14 studies into the link between the headaches and stroke showed patients who have auras, or light effects, with their migraine are even more at risk.

“The results of our study strongly suggest that migraine may be an independent risk factor for stroke,” Ali Samii, a neurologist at the University of Washington, said in a report published online by the British Medical Journal.

Three studies in the review indicated that women who have migraines and who take oral contraceptives were up to eight times more likely to have a stroke than those not taking the pill.

Samii and Mayhar Etminan, of the Royal Victoria Hospital in Montreal and their colleagues who collaborated on the review, believe the raised risk is due to the reduced blood flow to the brain that occurs during a migraine.

Ischemic stroke, which usually caused by a blood clot that blocks a blood vessel in the brain, is the most common type of stroke.

Millions of people around the globe suffer from migraine, a severe throbbing headache that can cause nausea, vomiting, or a sensitivity to light, sound and odors. The cause is unknown.

The scientists called for more research into the link between migraine and stroke risk.

“The risk of migraine among users of oral contraceptives must be further investigated,” they added.