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The use of oral hormonal contraceptives and their link to breast cancer is a long-standing discussion among the scientific community.

One of the reasons why it has been difficult to reach definitive conclusions is that almost all of the research into this relationship comes from observational studies, both large prospective cohort studies and population-based case-control studies. This has made it difficult to determine causality.

However, overall, these studies have provided consistent evidence that the risks of breast and cervical cancer are increased in women who use oral contraceptives, while the risks of endometrial, ovarian and colorectal cancers are reduced, according to the US National Cancer Institute.

The same source states that a meta-analysis of more than 150,000 women who participated in 54 epidemiologic studies showed that, overall, women who had at some point used oral contraceptives had a slightly higher relative risk of breast cancer—7%—compared to women who had never used them.

One of the most recent papers, a 2023 meta-analysis conducted by researchers at the Cancer Epidemiology Unit, part of Oxford Population Health at the University of Oxford, found that all forms of hormonal contraception may increase the risk of cancer.

This research concluded that the use of progestogen-only hormonal contraceptives is associated with a 20 to 30% increased risk of breast cancer. It added that previous studies have shown that use of the combined contraceptive pill, which combines estrogen and progestogen, is associated with a small increase in the risk of developing breast cancer, a risk that decreases after suspending its use.

The Nurses’ Health Study, which has been perhaps the most extensive observational study, followed over 116,000 nurses aged 24 to 43 when they enrolled in the study in 1989. This study also found that participants who used oral contraceptives had a slight increase in breast cancer risk.

Another study, conducted with Danish women, found a slightly higher risk of breast cancer, which was similar regardless of the type of formulation of the oral contraceptive. The increased relative risk observed in this study translates into one additional case of invasive breast cancer for every 7,690 women using hormonal contraceptives.

A summary by the non-profit organization Susan G Komen states that:

• Current or recent use of birth control pills is linked to a slight increase in the risk of breast cancer.
• Studies show while women are taking birth control pills (and shortly after), their breast cancer risk is 20%-30% higher than for women who’ve never taken the pill.
• However, this extra risk has a fairly small impact because the risk of breast cancer for most young women is low. So, even those who have a slightly higher risk are unlikely to get breast cancer.
• Once women stop taking birth control pills, their risk of breast cancer begins to decrease. After about 5 years, their risk of breast cancer is similar to risk for women who’ve never taken the pill.

Breast cancer is the most common with 2.3 million diagnoses annually, according to data from the World Health Organization.

This story was produced using content from original studies or reports and from other medical research, as well as health and public health sources, highlighted in related links throughout the article.