A new study adds to the evidence on hormonal contraceptives and breast cancer. But experts stressed the overall risk remains low.
A study published Wednesday found that women using contraceptive intrauterine devices that deliver a certain hormone are at increased risk of breast cancer, though risk of the disease in these women remained low overall.
The research looked at 78,595 women in Denmark between the ages of 15 to 49 years who used levonorgestrel IUDs, which in the United States are known by the brand names Mirena, Liletta, Kyleena and Skyla. Researchers compared them to 78,595 women who did not use the devices but otherwise had similar profiles. Those who used levonorgestrel IUDs had a 40 percent higher chance of developing breast cancer. For women in their 30s in the United States, that would take the risk of breast cancer from about 1 in 204 to about 1.4 in 204. It's about the same increase in breast cancer risk that has been shown with oral contraceptive pills.
Even with that increased risk, breast cancer remains rare among women under 50, and extremely rare for women under 30.
Dr. Daniel Breitkopf, the chair of the department of obstetrics and gynecology at the Mayo Clinic in Rochester, Minn., said that when he discusses the risks of hormonal contraceptives, he makes this clear. "The way I talk to patients about that is, if I tell you I'm going to double your chances of winning the lottery, you're not going to go out and buy a lottery ticket -- because the chances of winning are still very, very low."
And he stressed that for many patients at average risk of breast cancer, the benefits of the contraceptives often outweigh the risks.
Hormonal IUDs, which have become increasingly popular in recent years, prevent pregnancy in part by delivering small amounts of hormones inside the uterus. In Denmark, where the study took place, hormonal IUDs are the preferred form of contraception among women who have given birth, said Lina Morch, the head of the research team at the Danish Cancer Institute. Dr. Morch was the lead author of the new paper, which was published in JAMA.
There is a long-established link between other types of hormonal contraception and elevated breast cancer risk, with previous studies showing that women who take birth control pills have a 20 to 60 percent increased risk of breast cancer compared to women who don't take them.
One 2010 study of more than 100,000 female nurses found that pills that used levonorgestrel, a type of progestin, appeared to drive most of the excess risk associated with oral contraception. That study, along with others, have suggested that the increase in risk declines when women stop taking the pills, or shortly thereafter.
A large study in the United Kingdom last year also found that contraceptives with progestin -- whether taken as pills, injected or delivered by IUD -- were associated with a roughly 20 to 30 percent increased risk of breast cancer.
The latest paper builds on previous research by looking specifically at women who had not used any other hormonal contraception for five years up to the study's start date, which made it possible to more closely connect their outcomes to the type of contraception being studied. Researchers matched IUD users to nonusers who had a similar history of hormonal contraceptive use before that time and who shared other characteristics, including age, number of pregnancies and education levels. The mean age of women in the study was 38.3 years.
In the IUD user group, there were an additional 14 breast cancer cases per 10,000 women over five years. The number of extra cases increased among women who used the devices for longer than five years, but there were not enough of these long-term users to draw statistically significant conclusions, Dr. Morch said.
One potential limitation of the study was that it did not account for how often women were screened for breast cancer, said Dr. Christopher Li, an epidemiologist focused on breast cancer at Fred Hutch Cancer Center in Seattle, Wash., who was not involved in the research. Those with IUDs might be more likely to access health care and be screened, leading to higher rates of cancer detection, Dr. Li said.
But Denmark has universal access to health care, Dr. Morch said, so researchers did not expect there to be differences in screening between IUD users and nonusers. (Routine screening there begins at age 50.) Dr. Morch added that the study controlled for education levels, which could account somewhat for differences in people's knowledge about health and how often they use the health care system.
The study also did not break out how the risk of breast cancer linked to IUD use varied by age, which matters because breast cancer risk generally rises with age. Dr. Morch said that while the paper did not include the result, she did analyze the increased risk of breast cancer just in patients 35 and older. That group saw an extra 17 breast cancer cases per 10,000 women over five years.
Dr. Breitkopf said that while IUDs do come with risks, which also include blood clots and an increased likelihood of depression, they can safely and effectively prevent pregnancy, and significantly reduce heavy menstrual bleeding. "For a lot of people, that's a life-altering improvement," he said.
Dr. Breitkopf said he would not recommend progestin birth control for patients who have a strong family history or genetic risk for breast cancer, but "for most people who are at average risk for breast cancer, it's still a good option."