Estrogen and Your Breast Cancer Risk

Taking estrogen may increase a woman's risk of getting breast cancer.Estrogen is probably the hardest-working hormone in a woman’s body, but it also has a dark side: Research has determined that estrogen often plays a key role in the development of breast cancer, especially after a woman reaches menopause. How? The estrogen in a woman’s body seems to raise breast cancer risk by encouraging the growth of breast tissue, which can speed up an existing tumor’s growth. Here's what you need to know.

• Combination hormone therapy (HT): If you’re considering taking estrogen and progestin (a synthetic form of the hormone progesterone) to give you relief from annoying menopause symptoms like hot flashes and night sweats, be sure to limit the time you’re on the drugs. That’s because over time your breast cancer risk climbs, says JoAnn Manson, MD, professor of medicine at Harvard Medical School and author of Hot Flashes, Hormones & Your Health.

"In the Women’s Health Initiative (WHI) trial, when women got seven years of estrogen alone, there was no increased risk of breast cancer, but after four to five years on combined hormone therapy, the risk emerges," she says. In fact, over time, estrogen plus progestin can raise a woman’s risk for breast cancer by 24%; even if you take estrogen on its own for more than 10 to 15 years, your risk may still go up.

Dr. Manson was a coauthor on a March 2008 study in the Journal of the American Medical Association that followed up on the WHI trial. It showed that even after the women stopped taking combination HT, their breast cancer risk remained elevated. "The risk of breast cancer does decline after stopping hormone therapy," she stresses, "but if a tumor has formed while a woman is on hormones it’s very likely to come to light even after she stops hormones, so there is some residual risk. Stopping drug therapy doesn’t mean a tumor evaporates, but the risk gradually declines."

• HT and early diagnosis of breast cancer: If you have no family history of the disease, you’ve probably been encouraged to get your first mammogram by age 40 and every one to two years after that to help ensure that any tumor is caught early, when it’s most treatable. That’s good advice, but taking HT may complicate things. "Estrogen-and-progestin HT can lead to increased mammographic density"—denser breast tissue—"that can obscure breast tumors and delay diagnosis," says Dr. Manson. Denser breasts are believed to be an independent risk factor for breast cancer, but they also make it harder to accurately read a mammogram and that "can lead to abnormal mammograms that may require extensive follow-up and anxiety about repeat testing and even unnecessary biopsies," Dr. Manson adds.

• HT and risk for benign breast disease: In April 2008, the Journal of the National Cancer Institute found that postmenopausal women who’d taken estrogen on its own doubled their risk of a noncancerous type of breast disease, but one that’s associated with a higher risk of breast cancer. A September 2008 study led by the same author, Thomas E. Rohan, MD, PhD, an epidemiologist at the Albert Einstein College of Medicine, in New York City, found similarly disturbing evidence in a study of women taking estrogen and progestin: Combined HT raised a woman’s risk of benign breast disease by 74%.

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