Hormonal Contraceptives and the Risk of Breast Cancer

A Danish study reported that hormonal contraceptive use can increase a woman's risk for breast cancer, especially if it is used for a prolonged period of time.

Key Points

Highlights:

• This study from Denmark seeks to see whether there is a relation between hormonal contraceptive use and increased risk of breast cancer.

Importance:

• Approximately 140 million women worldwide use hormonal contraceptives, thus it is imperative to know if this medication will adversely impact the user in any way.
• Individuals with endometriosis are often prescribed hormonal contraceptives. Endometriosis patients and their health care providers ought to know the risks associated with the medication to make an informed decision.  

What’s done here?

• This nationwide prospective cohort study included 1,797,932 women participants.
  • These participants were between the ages of 15-49; did not have cancer, venous thromboembolism; and did not receive any infertility treatment.
  • Any female living in Denmark who was 15 years of age or older before December 31st, 2012 included in the study;  except those migrated to Denmark after 1995.
• Danish Civil Registration System, The National Register of Medicinal Product Statistics, The Danish Cancer Registry and other registries were used to gather the data and evaluated statistically.

Key results:

• In the entire study population, there were 11,517 cases of breast cancer.
• Breast cancer risk in recent and current hormonal birth control pill users was 1.20, or 20 % more, relative to the women in the study who had never taken any type of hormonal contraceptive.
• A woman who has never taken a hormonal contraceptive is at decreased risk for breast cancer compared to a woman who no longer uses hormonal contraceptives but has a history of hormonal contraceptive use, defined here as 5 years or more.
• The longer a woman uses a hormonal contraceptive, the greater their risk for breast cancer: an individual who has used hormonal contraceptives for less than a year has 9% increased risk, whereas an individual that has used it for more than 10 years has a 38% increased risk.
• Current and prior use of oral hormonal combination contraceptives, consisting of estrogen and progestin, were found to have an associated risk of 1.0 – 1.6.
• The progestin-only intrauterine system increased breast cancer risk to 1.21.
• For every 100,000 women, there were 13 additional cases of breast cancer that could be attributed to hormonal contraceptive use.
  • In women under 35, there were 2 additional cases of breast cancer per 100,000 women.
• The authors of this paper state that absolute increase in breast cancer risk was small with regards to this study.

Limitations of the study:

• This study was conducted using women from one geographical area of the world. Thus, the results of this study may not be pertinent to individuals from other parts of the globe.
• The authors of the study state that they did not adjust for the age of the first occurrence of menstruation, breastfeeding, alcohol consumption, and physical activity. Additionally, the researchers only had information about the body-mass index for the patients that were parous.
• Due to the age of the registries, some of the confounding factors could not be found for the older participants.
• Some of the participants characterized as “never have taken a hormonal contraceptive” could have taken a hormonal contraceptive before the study period because the study only looked at women who had purchased a hormonal contraceptive during the study period.
• The number of additional breast cancer cases does not include the cases diagnosed after the discontinuation of long-term hormonal contraceptive use.

Lay Summary

A study titled “Contemporary Hormonal Contraception and the Risk of Breast Cancer” was recently published in The New England Journal of Medicine and was immediately picked up and summarized by many well-known newspapers and broadcasting journals, including but not limited to NPR, CNN, and NBC. Which then begs the question: What makes this Danish study so interesting and pertinent to a large reader base that a number of popular websites would rush to publish the results? This question can be answered by two impactful words found in the study’s title: hormonal contraception and breast cancer. In short, the study of interest, conducted by Mørch et al., focuses exclusively on the implications, namely breast cancer risk, of utilizing hormonal birth control, which is used by approximately 140 million women worldwide. Over the course of this particular article, the Danish study will be discussed in greater detail and the implications of this study on endometriosis shall also be deliberated.

The Danish National Patient Registry is one of the world’s oldest nationwide hospital registries and is a valuable tool for epidemiological research, providing longitudinal registration of diagnoses, treatments, and examinations, with complete nationwide coverage. The current nationwide prospective cohort study had 1,797,932 women participants between the ages of 15 and 49. These women did not have cancer, venous thromboembolism, and did not previously receive any type of treatment for infertility. The researchers consulted a variety of nationwide registries, including the National Register of Medicinal Product Statistics and the Danish Cancer Registry, in order to ascertain individual hormonal contraceptive use and incidence of breast cancer. The information from the various registries was also used to find potential co-founders. Statistical analysis was conducted on the data derived from this study.

In total, there were 11,517 cases of breast cancer. The risk of breast cancer in recent and current users of hormonal birth control was 1.20, namely 20% more, compared to those who had never taken any hormonal contraceptive. A woman who has taken birth control for more than 5 years but discontinued use, is still at increased risk for breast cancer relative to an individual who has never taken any form of hormonal birth control. Additionally, it was found that the risk of breast cancer increased with prolonged use of hormonal birth control. An individual who used hormonal contraception for less than a year had an associated risk of 1.09, whereas an individual who has taken hormonal contraceptive for more than 10 years had an associated risk of 1.38. The risk of breast cancer associated with the current or past use of oral hormonal combination contraceptives, consisting of estrogen and progestin, ranges from 1.0 to 1.6. The progestin-only intrauterine system also increases an individual’s risk of breast cancer to 1.21. The study noticed that for every 100,000 women there are 13 additional breast cancer cases that are thought to be tied to oral contraceptive use; however, there were only 2 additional cases per 100,000 women for women under the age of 35. Thus, the study proves that breast cancer is a rarer occurrence for younger women. In short, the results of the study show that the risk of breast cancer increases with the use of hormonal contraceptives and the increase is a function of the duration of use. That being said, the authors of this paper state that the absolute increases in breast cancer risk were small. 

This study does not specifically concentrate on endometriosis; however, as far as hormonal contraceptives are often provided to those people suffering from endometriosis, an endometriosis victim who is taking or has taken hormonal contraceptive should pay attention to the results of this study.