The most common formulation of birth control pills has been linked to a doubling of relative risk of heart attack and stroke, a new study finds.
Contraceptive vaginal rings and skin patches formulated in the same way were also linked to higher heart risks.
Danish researchers stressed that the absolute risk to any one woman using these pills, patches or rings remains very low.
However, they said the findings are something for doctors to think about as they prescribe contraceptives.
The pill formulation in question is a combination of the hormones estrogen plus progestin, the team reported Feb. 12 in The BMJ.
It's the most-prescribed type of hormonal contraception.
“Although absolute risks were low, clinicians should include the potential risk of arterial thrombosis [blood clots] in their assessment of the benefits and risks when prescribing hormonal contraceptive method," wrote researchers led by Dr. Harman Yonis. He's with the Department of Cardiology at Nordsjaellands Hospital in Hilleroed, Denmark.
According to the research team, an estimated 250 million women worldwide are thought to use some form of hormonal contraception. Prior research has linked this form of birth control to heightened cardiovascular risk, but findings have varied study-to-study.
The new study involved a deep dive into national prescription records for more than 2 million Danish women, who were tracked from 1996 through 2021. They ranged in age from 15 to 49.
Contraceptives included estrogen-progestin pills, vaginal rings, patches, progestin-only pills, intrauterine devices (IUDs), under-the-skin implants and intramuscular injections.
Yonis' team compared the use of each woman's contraceptive of choice (or no use) against their history (if any) of first-time heart attack and ischemic stroke. (Ischemic strokes, the most common form of stroke, are caused when a clot blocks a vessel supplying blood to the brain).
The study did not include women who'd already experienced a blood clot, or those with cancer, liver disease, kidney disease, polycystic ovary syndrome, endometriosis or infertility treatment; as well as those who used psychiatric medications or hormone therapy, or who had undergone a hysterectomy.
After accounting for factors such as age, education level and existing conditions such as high blood pressure and diabetes, the team found that women taking the combo estrogen/progestin pill had double the relative risk of an ischemic stroke or heart attack.
However, each woman's absolute risk remained low: Use of the estrogen/progestin pill translated to one extra stroke for every 4,760 women using the pill for one year, and one extra heart attack for every 10,000 women per year of use, the researchers noted.
Pills and implants that contained progestin only also raised a woman's risk for stroke and heart attack, but only slightly and less than the combination pill.
The risk was highest for vaginal rings and skin patches containing the estrogen/progestin combination. These devices raised a woman's risk for ischemic stroke by 2.4-fold and heart attack by 3.8-fold, the team found.
Dual-hormone skin patch contraceptives increased ischemic stroke risk by 3.4-fold, according to the study.
Only one form of hormonal contraception showed no rise in cardiovascular risk: The progestin-only IUD, which seemed safe even over an extended period of use.
Yonis' team stressed that the study was observational in nature, meaning it was not designed to prove that any of the contraceptives caused a rise in heart risks. It could only point to associations.
In a linked journal editorial, Therese Johansson, of the Swedish Institute of Technology, stressed that the risk to any one user of hormonal contraceptives remains very low. However, since hundreds of millions of women worldwide use these pills and devices, the study findings remain important.
Any effort to improve the safety of contraception is therefore welcome, Johannson said.