Birth control pills cause a small but significant increase in the risk of the most common type of stroke, according to a comprehensive report in the journal MedLink Neurology.

For healthy young women without any stroke risk factors, the risk of stroke associated with oral contraceptives is small. But in women with other stroke risk factors, "the risk seems higher and, in most cases, oral contraceptive use should be discouraged report co-authors.

Oral contraceptives increase the risk of ischemic strokes, which are caused by blood clots and account for about 85 percent of all strokes. In the general population, oral contraceptives do not appear to increase the risk of hemorrhagic strokes, which are caused by bleeding in the brain.

There are about 4.4 ischemic strokes for every 100,000 women of childbearing age. Birth control pills increase the risk 1.9 times, to 8.5 strokes per 100,000 women, according to a well-performed "meta-analysis" cited in the report. (A meta-analysis combines the results of multiple studies.) This is still a small risk; 24,000 women would have to take birth control pills to cause one additional stroke, according to the report.

But for women who take birth control pills and also smoke, have high blood pressure or have a history of migraine headaches, the stroke risk is significantly higher. Such women should be discouraged from using oral contraceptives, the report said.

Hormone replacement therapy with estrogen alone or combined with progesterone increases the risk of ischemic stroke by 40 percent; the higher the dose, the higher the risk, the report said.