Hormonal birth control (like the Pill), especially modern lower dose medicines, are generally safe for most women. Like all medicines, though, there is the potential for adverse effects for some users. The potential for cardiovascular effects from using hormonal contraceptives has been known for some time; women who smoke cigarettes or who have severe high blood pressure are advised not to take the Pill, and hypertension, heart attack, and blood clots are rare but possible serious side effects.
As a new study in the New England Journal of Medicine points out, though, most research on adverse effects of hormonal contraceptives has focused on blood clots (deep vein thrombosis and pulmonary embolism), with little research focusing on heart attack or clot-related stroke.
The authors of the new study set out to figure out just how common stroke or heart attack are among a group of more than a million Danish women who used oral or other hormonal contraceptives. They looked at the medical records for contraceptive prescriptions and cardiovascular events, and excluded women who had cancers or during pregnancy – both factors that would increase women’s risk of stroke on their own.
The good news is that the study found that blood clots related to hormonal contraception are rare. They did, however, find that women who used combined (estrogen plus progestin) methods showed an increased risk of heart attack and stroke. The amount of increase varied by estrogen dose and other factors. As expected, smoking raised women’s risk of heart attack and stroke. Women who had diabetes, high blood pressure, or high cholesterol were also at an increased risk.
Risk of stroke was elevated among users of the patch and vaginal ring, but was only statistically significant for the ring. Women who used progestin-only methods did not have increased risks. The researchers also found that among women who had stopped using hormonal contraceptives, their risks were similar to women who had never used them.
The authors summarize the risks:
…the risk was increased by a factor of 0.9 to 1.7 with oral contraceptives that included ethinyl estradiol at a doseof 20 μg and by a factor of 1.3 to 2.3 with those that included ethinyl estradiol at a dose of 30 to 40 μg, with relatively small differences in risk according to progestin type.
They also point out that “the absolute risks of thrombotic stroke and myocardial infarction associated with the use of hormonal contraception were low,” meaning that while the increase seems high, very few women will actually experience these outcomes compared to how many women use these methods of birth control. As an accompanying editorial explains:
Considering the absolute risks of cerebral thrombosis and myocardial infarction among nonusers of hormonal contraceptives and the relative risks among users, the number of “extra” arterial thrombotic events attributable to hormonal contraceptives is about 1 to 2 per 10,000 women per year or, equivalently, 10 to 20 per 100,000 women per year for the combined estrogen–progestin formulations that might cause arterial events. These are small numbers. For an individual woman, the probability of an event is quite small.
They also say that “Although hormonal contraception is not risk-free, the evidence is convincing that the low and very low doses of ethinyl estradiol <50 μg) in the combined estrogen–progestin contraceptives studied by Lidegaard and colleagues — whatever the progestin and whether delivered orally or by means of the patch or the ring — are safe enough.”
Although estimates vary, rates of pregnancy-related stroke (during or immediately after pregnancy) are thought to generally be higher than those associated with hormonal contraceptives.