Ever since "the pill" was introduced in the United States in the 1960s, there has been concern about the potential increased risk of heart attack and stroke associated with taking oral contraceptives. According to the study's author, earlier research showed a significant -- three to five-fold -- increase in stroke risk in women taking oral contraceptives.
A new formulation of contraceptive -- one with less estrogen and less progestin -- has been prescribed increasingly since the 1980s. It's been unclear, say the researchers, whether these new formulations posed a risk for stroke. Researchers pooled results of two studies -- one in California and one in Washington -- involving 373 women between the ages of 18-44 who had a stroke and 1,191 women of similar age who had not had a stroke.
Compared to women who were not using oral contraceptives, women currently using low-dose formulations of "the pill" had only a 9 percent higher risk of an ischemic stroke, which occurs when blood flow to the brain is stopped by a blood clot or clogged vessel. In particular, there was no increased risk among women who didn't smoke or have high blood pressure.
When the researchers looked at the risk of hemorrhagic stroke -- a stroke caused by bleeding in the brain and more common in younger women than older women -- risk was increased only 11 percent. "We feel that neither of these increases is a statistically important difference," says the study's lead author, Stephen Schwartz, Ph.D., University of Washington School of Public Health, Seattle.
In some of their analyses, the researchers found that users of oral contraceptives were at lower risk of stroke than women who had not used "the pill." One likely explanation, Schwartz says, is that doctors tend to prescribe these pills to women who are already healthy.
The results are similar to an earlier study the researchers published in the September 15 issue of Circulation: Journal of the American Heart Association on heart attack risk associated with oral contraceptive use. That study showed there was not a link found between heart attack risk and low-dose oral contraceptives. The scientists say that strokes and heart attacks are both rare in young women, occurring in fewer than 2 out of every 10,000 women each year in the populations involved in this study.
"For both heart attack and stroke, there is pretty convincing evidence that women who do not have other cardiovascular disease risk factors -- smoking, high blood pressure, obesity -- are not at increased risk if they use low-dose contraceptives," Schwartz says. "There still remains the question of whether certain subgroups at risk for stroke -- smokers, those with high blood pressure -- are at increased risk when low-dose oral contraceptive use is added."
"It's relatively rare for a woman who smokes or has high blood pressure to be prescribed oral contraceptive pills in the United States," he adds. "For this reason, the ability to assess risks in these groups in this study was very limited. It does point to the need to do more thorough research on certain subgroups of women who might be more susceptible to strokes when using oral contraceptives."
Co-authors include Diana B. Petitti, M.D.; David S. Siscovick, M.D.; W.T. Longstreth, Jr., M.D.; Stephen Sidney, M.D.; Trivellore E. Raghunathan, Ph.D.; Charles P. Quesenberry, Jr., Ph.D.; and Joseph Keleghan, M.D.