Taking vitamin E could slightly increase the risk of a particular type of stroke, a study says.
The British Medical Journal study found that for every 1,250 people there is the chance of one extra haemorrhagic stroke - bleeding in the brain.
Researchers from France, Germany and the US studied nine previous trials and nearly 119,000 people.
But the level at which vitamin E becomes harmful is still unknown, experts say.
The study was carried out at Harvard Medical School, Boston, and INSERM in Paris.
Haemorrhagic strokes are the least common type and occur when a weakened blood vessel supplying the brain ruptures and causes brain damage.
Researchers found that vitamin E increased the risk of this kind of stroke by 22%.
The study also found that vitamin E could actually cut the risk of ischaemic strokes - the most common type of stroke - by 10%.
Ischaemic strokes account for 70% of all cases and happen when a blood clot prevents blood reaching the brain.
Experts found vitamin E could cut the risk, equivalent to one ischaemic stroke prevented per 476 people taking the vitamin.
However, they warned that keeping to a healthy lifestyle and maintaining low blood pressure and low cholesterol have a far bigger effect on cutting the risk of ischaemic stroke than taking vitamin E.
More than 111,000 people have a stroke every year and they are the third biggest cause of death in the UK.
Those who survive are frequently left with disability.
While none of the trials suggested that taking vitamin E increased the risk for total stroke, the differences were notable for the two individual types of strokes.
The authors concluded: "Given the relatively small risk reduction of ischaemic stroke and the generally more severe outcome of haemorrhagic stroke, indiscriminate widespread use of vitamin E should be cautioned against."
Previous studies have suggested that taking vitamin E can protect the heart from coronary heart disease, but some have also found that the vitamin could increase the risk of death if taken in high doses.
Dr Peter Coleman, deputy director of research at The Stroke Association, said: "This is a very interesting study that shows that the risk of haemorrhagic stroke can be slightly increased by high levels of orally taken Vitamin E, although what is a high level has not clearly been ascertained.
"More research is required to discover the mechanism of action and the level at which Vitamin E can become harmful.
"We urge people to maintain a lifestyle of a balanced diet, regular exercise and monitoring their blood pressure to reduce their risk of a stroke but would be very interested in seeing further research into this study," he said.