The study, published online in The New England Journal of Medicine, included more than 1.6 million Danish women. That makes it the largest study ever to examine the impact of different formulations of birth control pills and other forms of hormonal contraception on blood clots that can lead to heart attack and stroke.
Low-Dose Estrogen Pill Safer
"Risk today is significantly lower than it was decades ago in the era of high-dose pills," says University of Copenhagen professor Ojvind Lidegaard, MD, who led the research.
Hormonal contraceptives that did not contain estrogen, such as the progestin-only IUD sold in the U.S. as Mirena, were not found to raise the risk for blood clot-related heart attack and strokes at all.
But birth control delivered via skin patches and vaginal rings was associated with a significantly elevated blood clot risk, similar to that seen in older high-dose estrogen pills.
Two-Fold Increase in Risk
In related research published last fall, Lidegaard and colleagues linked use of new-generation birth control pills containing the progestin hormone drospirenone with a higher risk of blood clots in the legs that can dislodge to blood vessels in the lungs (also known as a pulmonary embolism).
The oral contraceptives Yaz, Yasmin, and Ocella contain drospirenone. In April, the FDA concluded that birth control pills containing drospirenone "may be associated with a higher risk for blood clots than other progestin-containing pills." The FDA will put that on the pills' labels.
But in the new study, these products were no more likely to cause blood clots that lead to heart attacks and stroke than any other combination contraceptive.
Age was the single-most important risk factor for developing blood clots related to hormonal contraceptive use.
The researchers followed 1.6 million Danish aged 15-49 from 1995 until 2009. During this time, there were about 3,300 clot-related strokes and 1,700 clot-related heart attacks among the women studied.
Use of combined oral contraceptives with ultra-low doses of estrogen (20 mcg) was associated with a 50% increase in risk, while use of traditional low-dose pills (30 to 40 mcg) was associated with an 80% increase in risk.
Overall, the study shows that heart attacks and strokes caused by blood clots were twice as likely in women using combination oral contraceptives. But the risk is still very low for most women, especially those younger than 35 with few risk factors for these events, Lidegaard tells WebMD.
"A 20-year-old with a very low baseline risk for heart attack and stroke should not be all that concerned that her risk may be double, but a woman who is 35 or older with 10 times the baseline risk may want to choose another form of birth control."
Progestin-Only IUD Appears Safest
He adds that progestin-only contraceptives, which appear to convey no increased risk at all, may be a good choice for these women.
In an editorial published with the study, Arizona State University researcher Diana B. Petitti, MD, MPH, writes that the Danish study should reassure women and their doctors about the safety of oral contraceptives.
She notes that the small increase in risk could perhaps be eliminated if women with risk factors for heart attack and stroke, such as smokers and those with high blood pressure, avoided combination hormonal contraceptives.
The FDA recommends against the use of combination oral contraceptives in women over age 35 who smoke.
Petitti tells WebMD that these women and older women who are obese or have high blood pressure should be cautious, but she adds that most women who take oral contraceptives have little to worry about.
"We know a lot about who is at risk for these events," she says. "Women should have their blood pressure checked before starting a combined contraceptive, and if they have hypertension they should probably use another form of birth control."
The finding that progestin-only hormonal birth control does not raise arterial blood clot risk should be particularly reassuring to older women, notes Kathy Hoeger, MD, chief of the reproductive endocrinology division at the University of Rochester and director of the Strong Fertility Center.
"Just like younger women, women in their late 30s and 40s need reliable and safe contraception," she tells WebMD.