For healthy young women without any stroke risk factors, the risk of stroke associated with oral contraceptives is small, researchers said.
But in women with other stroke risk factors, "the risk seems higher and, in most cases, oral contraceptive use should be discouraged," said report co-authors Marisa McGinley, Sarkis Morales-Vidal, and Jose Biller, of Loyola University Medical Center and Loyola University Chicago Stritch School of Medicine.
Worldwide, more than 100 million women currently use oral contraceptives or have used them in the past. Strokes associated with oral contraceptives were first reported in 1962.
Early versions of the pill contained doses of synthetic estrogen as high as 150 microgrammes. Most birth control pills now contain as little as 20 to 35 microgrammes. None contain more than 50 microgrammes of synthetic estrogen.
Oral contraceptives increase the risk of ischemic strokes, which are caused by blood clots and account for about 85 per cent of all strokes, researchers said. 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.
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 researchers said. Hormone replacement therapy with estrogen alone or combined with progesterone increases the risk of ischemic stroke by 40 per cent; the higher the dose, the higher the risk, the report said.
The report was published in the journal MedLink Neurology.