People who have migraines are twice as likely to have a stroke as others, researchers estimate.
Women on the pill who have migraines could be at very high risk
Experts in Canada and the US looked at 14 studies that had shown a link to quantify the exact risk involved.
Migraines roughly doubled the stroke risk, while migraines with 'auras' more so than those without interruptions to the sufferers' vision.
Women with migraines who were on the oral contraceptive pill appeared to be at particular risk, reserach found.
Higher risk groups
The increased risk of stroke is probably down to the reduced blood flow to the brain which usually occurs in a migraine, the researchers believe.
The studies looked at by Dr Ali Samii, neurologist at the University of Washington, and his colleagues, suggest the risk of stroke for migraine sufferers is 2.16 times that for non-sufferers.
Those who have migraines with auras are 2.27 times as likely to suffer a stroke and in those with migraines without auras the risk is increased 1.86 times.
Three of the studies showed that women migraine sufferers who were also taking oral contraceptives were up to eight times more likely to suffer a stroke than those not taking the pill.
The researchers said the latter results were slightly at odds with other studies, which suggest a smaller degree of increased risk for such women - about double.
Therefore, they say much more research should be carried out to establish what the risk is.
"Given that the use of oral contraceptives is prevalent among young women, the potential risk of stroke among women with migraine who are also users of oral contraceptives must be further investigated," they said in their paper which is due to be published in the British Medical Journal.
'No cause for alarm'
They said as many as a quarter of women in their mid to late 30s experience migraines.
Dr Anne MacGregor, director of clinical research at the City of London Migraine Clinic and acting general secretary of the International Headache Society, said: "There is no doubt that there is a relationship between migraine and stroke, and that the risk is greater with aura migraines and women on the pill. That is indisputable.
"What is not known is the exact risk and I am not convinced that a meta-analysis of studies, such as this, will come up with the answer."
She said there were too many other factors that could skew the results, such as how the different studies were carried out.
Dr MacGregor said there were already guidelines recommending against contraceptive pill use by women with aura migraines.
Women with non-aura migraines could try using the pill but should come off it if they have any problems.
In some women the pill can even help with migraines, she said.
"We do not want people with migraine to think they are at high risk of having a stroke.
"The absolute numbers are very small. You are far more likely to get a stroke from smoking. That's the big risk factor."
The Stroke Association agreed with the call for more research.
A spokesman added: "Other risk factors such as high blood pressure and smoking may also have a role.
"Migraine sufferers who may have such risk factors and are intending to take oral contraception should be advised accordingly.
"Anyone who has concerns about this issue should discuss them with their GP."