Modern birth control still carry one huge side effect: breast cancer.

Breast cancer has long been linked to hormonal contraception — but the prevailing wisdom among young women was that modern birth control, which uses lower doses of estrogen, is now much safer than the birth control past generations of women used. This study proves belief wrong.

The study, published in the New England Journal of Medicine, tracked 1.8 million Danish women between the ages of 15 and 49 for over a decade. For every 100,000 women, it estimated, 13 are annually diagnosed with breast cancer caused by their use of hormonal birth control.

Overall, the study found, current and recent use of hormonal birth control was linked to a 20 percent increased risk of breast cancer. The longer women used birth control, the higher their risk — users who’d been on hormone birth control for under one year saw about a 9 percent increase in relative risk, while people who’d been on it for more than a decade saw their relative risk rise by about 38 percent.

And it seems no form of hormonal birth control is safe. The study found that any form of conception that releases hormones, from the pill to the hormonal IUD, carries similar drawbacks. (Not all IUDs release hormones.)

“No type of hormone contraceptive is risk-free unfortunately,” Lina Mørch, a researcher at the University of Copenhagen and the study’s lead author, told USA Today

The study, however, didn’t track outside factors that could also increase a woman’s breast cancer risk, like physical exercise and alcohol use. It also didn’t include women who’d received infertility treatments.

Plus, young women — who represent the bulk of birth control users — still have relatively low chances of developing breast cancer.

“The absolute increase in risk is 13 per 100,000 women overall, but only 2 per 100,000 women younger than 35 years of age,” David Hunter, an epidemiology and medical professor at Oxford University, wrote in an editorial that accompanied the study. “Most of the cases that occurred in this analysis occurred among women who were using oral contraceptives in their 40s.”

Using hormonal birth control can also help women stave off other forms of cancer. American Cancer Society researcher Mia Gaudet told ABC News that even after this study, hormonal birth control still has a “net cancer benefit,” since it can decrease women’s risk for ovarian and endometrial cancer.

Ultimately, Hunter said, this study simply reestablishes the need to continue researching and manufacturing new forms of birth control.