Oral contraceptives, or birth control pills, are statistically associated with a substantial and permanent reduction in ovarian and endometrial cancer risk, researchers at Uppsala University in Sweden reported last month.

The research confirms previous findings linking oral contraceptives to an increased risk for breast cancer, the second most frequently diagnosed cancer in women. But the new study, published in Cancer Research, provides evidence that when it comes to cancer, the benefits of birth control pills may outweigh the risks for a large proportion of the female population.

Investigators analyzed medical data on 256,661 women born in the United Kingdom between 1939 and 1970. Those who were currently taking or had previously taken birth control pills were classified as “ever users” (82%), while those who had never taken them were classified as “never users” (18%). Over the decades, 17,739 were diagnosed with breast cancer, 1,966 with ovarian cancer and 2,462 with endometrial cancer.

Using several statistical analysis techniques, the researchers determined that although ever users had a slightly increased risk for breast cancer (10%), they had a significantly decreased risk for ovarian cancer (28%) and endometrial cancer (32%). Moreover, the increase in breast cancer risk was small and disappeared after two years or less. By contrast, the decrease in ovarian and endometrial cancer risk was large and lifelong; it directly correlated with the duration of oral contraceptive use and lasted more than 30 years.

“Ovarian cancer is deadly and hard to treat,” senior study author Åsa Johansson told The New York Times. “The mortality rate for breast cancer is lower. If you have a close relative who died from ovarian cancer, you might make one decision about oral contraceptives. If you have one who died from breast cancer, you might make another.”

The researchers hypothesized that the observed differences in cancer risk between ever users and never users could be ascribed to variations in hormonal exposure. Hormones such as estrogen and progesterone—which can over many years promote breast cancer and reproductive cancers by encouraging cell growth and development—proliferate during ovulation. Birth control prevents ovulation.

For related coverage, read “Use of Oral Contraceptives Not Linked to Long-Term Cancer Risk.” And to learn more about endometrial cancer, see “Endometrial Cancer Incidence Rising Worldwide.”