Do Birth Control Pills Cause Breast Cancer? The Latest Study

Last month, you might have seen headlines warning you that hormonal birth control increases risk for breast cancer. This news came from a study that examined the medical records of nearly 1.8 million Danish women — and the huge sample size lent heft to the findings, several of which stood in stark contradiction to commonly held beliefs about modern hormonal contraception.

Some types of hormonal contraception could increase breast cancer risk, while others may not. But the Pill also reduces risk for endometrial and ovarian cancers.

Birth control comes in two “flavors” — hormonal and nonhormonal. Hormonal contraception is among the most effective, and includes birth control pills, hormonal IUDs, the shot, the vaginal ring, the implant, and the patch. Nonhormonal contraception ranges from very effective, including surgical sterilization and the copper IUD, to the not-quite-as-effective, including condoms, diaphragms, and withdrawal. (With the exception of condoms, birth control does not provide protection against STDs.)

Hormonal contraception is one of the greatest achievements in the history of medicine, and offers those wishing to control their fertility an array of effective options. However, as with all effective medications, there is potential for side effects. And, because many forms of hormonal birth control contain types of estrogen, and exposure to estrogen is a risk factor for breast cancer, many people wonder if hormonal birth control might increase users’ chances of developing breast cancer later in life.

Most birth control pills contain a combination of two hormones: estrogen and progestin (synthetic progesterone). Other hormonal methods, such as the ring and the patch, also use combinations of these two hormones.

There are also pills that don’t contain estrogen, called POPs, or progestin-only pills — aka the “minipill.” Additionally, hormonal IUDs, the implant, and the shot are progestin-only methods.

What Previous Studies Have Shown

The connection between hormonal contraception and breast cancer is murky, because the association is difficult to study properly. There are so many different types of hormonal contraceptives, each with different dosages, different chemical formulations, and different ways of entering the body. We can’t tease these differences apart on the one hand, but make blanket statements about hormonal contraception as a whole on the other hand. But we can look at the available evidence and see where it points.

Before December’s study was published, most people thought older versions of the birth control pill were probably linked to small increases in breast cancer risk, but that more modern birth control pills, with lower doses of estrogen, were not linked to breast cancer at all. Most studies were conducted when the Pill had higher doses of estrogen, and before estrogen-free forms of hormonal contraception were available. People also thought that different types of birth control, such as the vaginal ring or the IUD, could deliver hormones in a more “targeted” manner, having less effect on breast tissue. So the hope was that modern contraception — with lower doses of estrogen or no estrogen at all, or that delivered hormones in a more specific way — would have less risk.

One large study, published in 2010, found that only “triphasic” pills, which contain a form of progestin called levonorgestrel, were associated with increased risk — a result that came as a surprise to the medical community, and led to speculation that maybe other forms of progestin were safer. In 2013, an analysis of the available evidence showed that using the birth control pill increased lifetime risk of breast cancer by 0.89 percent — a tiny increase, but one that statisticians did not deem likely to be due to chance.

So, the consensus was that birth control pills possibly carry with them a slight increase for breast cancer risk — but there were a lot of unanswered questions. Did more modern forms of contraception carry even less risk, if any at all? And were only certain types of progestin, like levonorgestrel, the culprit for the small overall increase in risk?

What This New Study Tells Us

Last month’s study compared users of hormonal contraception to women who had never used it, and found that current and recent users had a 20 percent higher risk of breast cancer. What’s more, those who had used hormonal contraception for more than a decade had a 38 percent increase in breast cancer risk.

Image: xkcd

That might sound huge, but risk always needs to be put in perspective. If your chances of getting cancer are already small, a 20 percent increase in that risk will not dramatically alter your absolute risk. The study’s authors kindly put those numbers in a context that might be easier to grasp: For every 7,690 women using hormonal contraception, there will be one additional case of breast cancer each year.

Nevertheless, these results challenged the widely held belief that only older forms of the Pill might increase breast cancer risk. But what about progestin-only birth control? The form of progestin called levonorgestrel continued to raise red flags: Current or recent use of progestin-only IUDs containing levonorgestrel was associated with a 21 percent increase in breast cancer risk. Additionally, progestin-only pills containing levonorgestrel were associated with a 93 percent increase in risk.

Breaking Down Risk by Contraception Type

The authors didn’t just lump all forms of hormonal contraception together to come up with that 20 percent overall increase in breast cancer risk — they also looked at different types of contraception individually. Organizing the data this way helps us understand if some types of birth control might carry less breast-cancer risk than others — but dividing the overall group into more than a dozen subgroups leads to a loss in statistical power, so only a few results were statistically significant.

Oral contraceptives (the Pill)

First, birth control pills that combined estrogen (ethinyl estradiol, 20 to 40 micrograms) with different forms of progestin were associated with varying increased risk of breast cancer:

  • cyproterone: 44 percent increase in risk
  • levonorgestrel: 33 percent increase in risk
  • norgestimate: 22 percent increase in risk
  • gestodene: 20 percent increase in risk
  • desogestrel: 12 percent increase in risk

There weren’t enough data to evaluate risk associated with other forms of progestin (norethisterone, drospirenone, and dienogest combined with estradiol valerate) used in combined birth control pills. Results for progestin-only pills using norethisterone or desogestrel weren’t statistically significant either.

Nonoral contraception: the patch, the ring, the shot, and the implant

A few birth control methods seemed to be associated with a decrease in breast cancer risk, but none of these results was statistically significant. They included the birth control patch, the vaginal ring, the contraceptive shot, and the implant. Further studies with larger sample sizes must be conducted to evaluate these methods’ association with breast cancer risk. (Given how relatively unpopular these methods are, those large sample sizes might be difficult to come by.)

The Significance of “Significant”

According to the New York Times, the study found that “Women who rely on birth control pills or contraceptive devices that release hormones face a small but significant increase in the risk for breast cancer.”

“Small but significant”? What does that even mean?

To most people, “small but significant” might sound like a contradiction in terms, like saying “tiny but huge.” But scientists and statisticians use the word “significant” in a way that is very different from how most English speakers understand the word. In this sense, “significant” doesn’t mean “big,” “major,” or “dramatic” — it actually means something more like “not likely to be coincidental.”

The “small but significant” increase in breast cancer risk simply refers to the finding that hormonal contraception slightly raises breast cancer risk, and this slight increase in risk is unlikely to be due to chance.

The Upshot

Last month’s study was a bombshell in many ways, but it had its weaknesses as well. Some doctors say that yearly visits to obtain birth control prescriptions increase opportunities to detect breast cancer, and the study did not control for health-care visits. The authors also weren’t able to adjust for other factors that might influence breast cancer risk, such as alcohol consumption, physical activity, age of first menstrual period, or breastfeeding.

It’s also important to remember the “good” side effects of the Pill. Not only is it good at preventing pregnancy — and all the health and lifestyle implications pregnancy carries with it — it can also help people with painful or heavy periods, and conditions like endometriosis. And, when it comes to cancer, the Pill seems to reduce users’ risk of ovarian cancer, endometrial cancer, and possibly colorectal cancer.

So is the Pill good or bad? That’s a question only you can answer for yourself — taking into account your family history, your other risk factors, how much risk you’re willing to accept, and the benefits the Pill can provide. If you’re worried about how the Pill could affect your risk, make an appointment at your local Planned Parenthood, where a health care provider can review your options with you and help find something appropriate for your lifestyle and preferences.