The Pill comes with a small risk of breast cancer: How worried should women be?

For decades, a slight link between breast cancer and birth control pills has been recognized, but results of studies were inconsistent, and few involved pills with newer formulas or lower hormone doses.

Now, the largest study ever to examine the link concludes that taking any contemporary hormonal contraceptive for at least five years raises breast cancer risk by about 20 percent for current and recent users. That increased risk grows with longer use, and it persists for at least five years after quitting the pill or methods such as the hormone-releasing IUD.

The Danish study, published this week in the New England Journal of Medicine, was accompanied by an editorial that said the pill's "small increased risk" should be considered in context. Breast cancer is unusual among women in their 20s and 30s, so a 20 percent increase still puts very few women at risk. Plus, oral contraceptives have benefits beyond pregnancy prevention. That includes relieving acne and heavy periods, as well as reducing the chances of ovarian, endometrial, and possibly colon cancer.

"Indeed, some calculations have suggested that the net effect of the use of oral contraceptives for five years or longer is a slight reduction in the total risk of cancer," David J. Hunter, a population health researcher at the University of Oxford in England, wrote in his editorial.

Lankenau Medical Center radiation oncologist Marisa Weiss, founder of the online breast-health resource breastcancer.org, offered a different perspective: Doctors should inform patients that the pill can boost breast cancer risk and discuss switching to non-hormonal contraception such as a diaphragm at five years.

"It's almost a default, putting young women on the pill, without too much thought," said Weiss, who is a breast cancer survivor. "While the increased breast cancer risk is not that huge, it has a significant public health impact" because so many young women use hormonal birth control. "I think [the new study] is a wake-up call for the ob-gyn world."

In 2012, about 10 million U.S. women were on the pill, the most popular form of birth control, federal data show. Surveys suggest access to birth control has been improved by the Affordable Care Act, which requires health plans to cover all forms of contraception without out-of-pocket costs.

The new study, led by University of Copenhagen epidemiologist Lina S. Morch and gynecologist Ojvind Lidegaard, analyzed data for nearly 1.8 million women who joined a Denmark hormone research registry between 1995 and 2012. Women who used hormonal contraceptives had more breast cancers than those who didn't, translating to a 20 percent higher risk.

The researchers calculated one extra case of breast cancer would occur for every 7,690 women using hormonal contraception for a year. (However, few cases of breast cancer occurred among users of three hormonal methods — the skin patch, the implant, and the Depo-Provera shot.)

Morch said her team hoped that contemporary pill formulations, with far lower hormone doses than products made in the 1960s, would not increase breast cancer risk, "but we did not find that."

Contraceptives use estrogen and progestin, or progestin alone, to prevent pregnancy by interfering with ovulation or by changing the uterine lining. But those hormones also work in the breast, where they can promote the growth of abnormal cells, fueling tiny tumors that may be present.

Even before the new study, the World Health Organization and medical groups recommended against hormonal contraception for breast cancer survivors and women at high risk of the disease because of family history or breast abnormalities.

"Making a higher-than-average risk a little higher isn't a good idea," Weiss said.

But weighing the tradeoffs can be highly personal and complex.

Three years ago, at age 30, Amanda Morris of Herndon, Va., bought an ancestry.com DNA test that led to  an unexpected, life-changing discovery: she inherited a BRCA1 mutation, a huge risk factor for both breast and ovarian cancer. She underwent a double mastectomy, then opted to go on the birth control pill to reduce her ovarian cancer risk while she and her husband considered surgery to remove her ovaries.

Last year, the adoptive mother of two had that surgery, which triggers premature menopause. Confident that she had drastically reduced her chances of cancer, she decided to stay on the low-dose pill as hormone replacement therapy.

"I said, 'I don't want to feel like I'm 70.' My oncologist totally understood. My breast surgeon was a little iffy," recalled Herndon, who has become a volunteer with FORCE, a nonprofit devoted to people with BRCA defects.  "I've had great results — virtually no menopausal symptoms."