Breast cancer study
A recent study conducted by Morch and colleagues at the University of Copenhagen examined the link between hormonal birth control and breast cancer. While the study cites an elevated risk of breast cancer of 20% (95% CI: 14%-26%) among current or recent users of hormonal contraception compared to never users, the effect is small in absolute numbers, equating to 13 additional cases of breast cancer per 100,000 current and recent users of hormonal contraception aged 15-49 (95% CI: 10-16) in one year. It may be the case that current or recent users of oral contraceptives interact with the medical system at higher rates than those who never used hormonal contraception and are therefore potentially more likely to get regular breast exams or mammograms. Furthermore, the authors do not account for other advantages of hormonal methods to present a full picture of the risks and benefits, including potential prevention of other types of cancer, reduction in mortality overall, or non-contraceptive benefits women gain from these methods including the life and well-being benefits of being able to plan when to have children and to complete education or pursue a career.
The study takes advantage of the Danish national health registry to follow women aged 15-49 from 1995 to 2012. The period of time covered by the study allowed the researchers to examine more recent formulations of birth control pills that use lower doses of estrogen than those that were first on the market and that were the subject of most previous research linking hormonal contraceptives to breast cancer.
The study found a small, significant increase in the risk of breast cancer among current users of hormonal contraception. The study cites an elevated risk of 20% (95% CI: 14%-26%) among current or recent users of hormonal contraception compared to never users, which out of context may seem like a large increase. However, when put in terms of absolute risk, a 20% increase in risk equates to 13 additional cases of breast cancer per 100,000 current and recent users of hormonal contraception aged 15-49 (95% CI: 10-16) in one year. The majority of these cases were observed in women over age 40. Among women younger than age 35, there were an estimated two extra breast cancer cases per 100,000 current or recent users of hormonal contraception (95% CI: 1-4) compared to never users in one year.
Given the size of the dataset used for this study, the authors were able to examine the differences between different formulations of hormonal birth control and generally found that all formulations, including hormonal intrauterine devices, increased the risk of breast cancer compared to never users. The authors also found that risk of breast cancer increased with longer duration of use and remained elevated at least up to five years after discontinuation.
However, the results from this study should not be interpreted as casual evidence that hormonal contraception causes breast cancer. This study is observational in nature, and although authors did adjust for some confounding factors in their analyses including education level, family history of breast cancer, and parity, they did not control for other known risk factors for breast cancer including age at first menstruation, history of breastfeeding, alcohol consumption (see here and here), or physical inactivity. The authors did control for tobacco use, body mass index, and age at first birth among parous women in a sensitivity analysis, but this analysis could suffer from selection bias as the rates of missingness for these variables was high and these variables were only available for parous women. A difference in screening may also potentially explain some of the increased risk picked up in this study. It may be the case that women who are current or recent users of oral contraceptives are interacting with the medical system at higher rates than those who never used hormonal contraception and are therefore potentially more likely to get regular breast exams or mammograms.
Although this study adds a piece of evidence to a body of literature associating hormonal contraception with breast cancer, the findings are not new or surprising. The results of this study align with findings from previous studies examining the link between hormonal
contraception and breast cancer. Despite the small increase in risk among users, users of hormonal contraception should weigh the benefits of hormonal contraception with the risks. Beyond the obvious contraceptive benefits that allow users to plan their reproductive lives, oral contraceptives are associated with substantial reductions in the risks of ovarian (see here and here), endometrial, and colorectal cancers later in life and multiple studies have linked use of oral contraception to a decrease in overall mortality or have observed no effect (see here, here, and here) on overall mortality. They are also used for treatment of other medical disorders such as endometriosis-associated pain or irregular or excessive bleeding. Physician and expert responses (see here, here and here) to this article highlight that women, especially younger women, should not change their choice in contraception because of this study and should consider the benefits of each method of contraception alongside the potential risks. For other responses to this article, see the NPR and Boston.com articles.