The Link Between Birth-Control Pills and Breast Cancer

A new study finds that pills with a high level of estrogen increase breast-cancer risk significantly. But that still might not mean you should change your prescription.

The birth-control pill is regularly hailed as one of the greatest medical advancements for women. Breast cancer, meanwhile, is considered one of the biggest health threats.

The only problem? There’s some evidence that the first increases your risk of the second. A new study out today suggests that having recently taken birth-control pills increases breast cancer risk by about 50 percent. Women who were on a formulation of the pill with an especially elevated level of estrogen nearly tripled their risk of getting breast cancer, and a pill with even a moderate amount of the hormone increased the risk by about 60 percent.

For the study, published in the journal Cancer Research, scientists at the Fred Hutchinson Cancer Research Center and the University of Washington in Seattle followed 1,102 women diagnosed with breast cancer, most of whom were in their 40s. Rather than rely on self-report data, the researchers dug into the women’s pharmaceutical records to determine the exact oral contraceptives they were taking.

They divided the types of pills into three categories: Those formulated with a low level of synthetic estrogen, or about 20 micrograms of ethinyl estradiol; those with a moderate dose, or 30 to 35 micrograms of ethinyl estradiol or 50 micrograms of mestranol; and those with a high dose, or 50 micrograms of ethinyl estradiol or 80 micrograms of mestranol. Estradiol is one of the two main chemicals in the pill, along with progestin.

Overall, women who recently took high-estrogen-dose pills were 2.7 times more likely to have breast cancer, while those who took moderate-dosed pills were about 1.6 times more likely. There was no increased risk for the low-dose pills.

Two specific types of pill formulations increased cancer risk the most: Those containing ethynodiol diacetate (which are sold as Continuin or Femulen) increased the risk 2.6 fold, and triphasic combination pills containing 0.75 milligrams of norethindrone (sold as Ortho 75) increased the risk 3.1 fold.

Estrogen acts as a signal to the breasts to stimulate the growth of epithelial cells. When there’s too much estrogen, “you're giving a higher signal than what's normally there,” said Rowan Chlebowski, an oncologist at the University of California Los Angeles and a spokesman for the American Association for Cancer Research. “Estrogen normally makes the breast proliferate—women get larger breasts. A few proliferations could allow the growth to continue in an unregulated way.” That unregulated growth, in turn, could become a cancerous tumor.

The results are consistent with past studies that have found that breast cancer risk increases slightly with some oral contraceptive pills, though other studies have not found such a link.

As with most things OB-GYN related, that’s frustratingly confusing. The pill is essential; not getting cancer is too. How do you choose what's more important—a lifetime of easy reproductive autonomy, or ratcheting down your risk of a deadly disease by marginal amounts?

The caveats to this study are in some ways just as important as the findings:

  • The high-dose pills are incredibly uncommon. “I haven't seen a woman in 20 years who was on 50 or 80 micrograms. It's really uncommon, and it's not something that I prescribe,” Owen Montgomery, chairman of the department of obstetrics & gynecology at Drexel University in Pennsylvania, told me. Most of these women have specific medical issues that necessitate being on a high-dose pill, or they’ve found that their side-effects on other formulations of the pill are just too debilitating to keep taking them. Women might take a higher-estrogen pill if they have extreme breakthrough bleeding, for example. Only half of one percent of women were using the high-dose pills by the last phase of the study, which took place from 2005 to 2009.
  • The cancer risk returned to normal for women who stopped using birth-control pills. The elevated risk was only found for women who had taken the pill within a year.
  • Breast cancer is already really rare—a woman’s risk of developing it at age 40 is only about 1.5 percent, and it’s only 2.38 percent at age 50. Increasing either of those numbers by even a factor of three makes a difference, but not much of one. Even the study’s author, Elisabeth Beaber, a staff scientist at the Fred Hutchinson Cancer Research Center, said the findings aren’t enough to suggest you should change your prescription if you do happen to be taking one of these high-dose pills.
  • The pill has cancer-fighting effects, too. It’s been shown to protect slightly against ovarian and endometrial cancer. So if you have a family history of endometrial cancer rather than breast cancer, a higher-dose pill might still be worth taking.
  • Finally, all medicines carry risks. “Every time I prescribe something for one of my patients, there's always a balance between risk and benefit,” Montgomery said. “If aspirin came with a package insert, you'd be scared to death to take aspirin.”

For Montgomery, all this study shows is that, “you have to make sure that with your patient, for her unique history, that you use something where the benefit outweighs the risk. For the vast majority, the usefulness of the pill outweighs the risk by a huge margin.”

And since the pill is still America’s most popular contraceptive method, despite decades of yes-it-does-wait-no-it-doesn’t cancer risk research, it seems many women agree.

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Olga Khazan is a staff writer at The Atlantic, where she covers health.

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