The paper, published today in the on-line version of the British Medical Journal, is a compilation and analysis of 14 previously published studies on the subject.
It found that migraine sufferers -- of whom there are about 3.3 million in Canada -- are twice as likely to suffer stroke as people who don't suffer migraines. The risk is even higher among sufferers who see aura -- meaning their vision is clouded by bursts of light.
But the data revealed that the risk was highest, by far, for young women taking the Pill, whose risk of stroke jumped eight-fold.
"This study really strengthens the hypothesis that migraine is an independent risk factor for stroke," said Mahyar Etminan, a postdoctoral fellow in clinical epidemiology at Royal Victoria Hospital in Montreal, and the lead researcher.
But he stressed that "not everyone who has a migraine is going to have a stroke -- not at all."
Dr. Etminan said that while the risk of stroke doubled, the "risk is still quite low for people who don't smoke or take oral contraceptives."
The study does not explain why migraines -- the most common form of headache among young adults, particularly women -- would increase stroke risk. A stroke occurs when a blood clot interrupts the flow of blood to the brain.
But Dr. Etminan speculated that migraines may reduce blood flow to the brain, and that could facilitate a clot blocking an artery to the brain.
But Stephen Phillips, a neurologist and spokesman for the Heart and Stroke Foundation of Canada, said that explanation really doesn't hold water.
"Stroke obviously has to do with blood flow to the brain but we don't really know very much about the mechanism of migraine," he said.
Dr. Phillips, who suffers from migraines himself, said the research is nonetheless important because it really firms up the evidence on the long-suspected link between migraine and stroke.
"If you have a propensity to have a migraine, you also have a predisposition to stroke. That association needs a lot more study," he said.
Still, Dr. Phillips said, the new research offers up no evidence on how to avoid stroke or improve treatment of migraines, which is what sufferers would really like to know.
"Unfortunately, I have no idea what I should be doing personally to reduce my risk of stroke, or what I should be telling my patients," he said.
But Dr. Phillips, who is director of the acute stroke program at Queen Elizabeth II Health Sciences Centre in Halifax, said he definitely tells young women with migraines to not take oral contraceptives. About 43 per cent of sexually active young women take the Pill, making it by far the most popular form of contraception.
A migraine is a really bad headache that lasts for more than four hours and is accompanied by certain telltale symptoms -- such as sensitivity to light, sound or colours. Migraine can also be a chronic, debilitating condition.
Though the causes are largely a mystery, there is some sort of hormonal link. Fewer than 4 per cent of people suffer migraines before puberty, but the incidence rises as high as 25 per cent among women in their late 30s.
More than 50,000 Canadians annually die of stroke.
The new research looked principally at studies that were conducted among younger people who suffered stroke, but the condition is far more common among the elderly. According to the study, the risk of stroke is about 3.5 per 100,000 person-years at the age of 15 and it rises to 350 per 100,000 person-years by the age of 85.