Does birth control prevent—or cause—cancer?

During a routine check-up while I was engaged, a gynecologist asked whether I wanted to start taking birth control for pregnancy prevention. I declined since my soon-to-be husband and I planned to use an alternative but equally effective way to delay pregnancy called the Sympto-Thermal Method (STM). STM is a type of Fertility Awareness Method (FAM) with a pregnancy prevention rate rivaling that of birth control without any of the side effects or risks–including the cancer risks that accompany hormonal birth control use.  

Still, my OB/GYN continued to push birth control, claiming it could help “regulate” my menstrual cycle so that I wouldn’t menstruate on my wedding day.  I declined again, and never went back.  Had I not known about an effective alternative, I fear that like many other young women, I too would have started using birth control and been left to endure the risks and side effects that blindside so many women.

Perhaps the least addressed concern for women, yet among the most pressing, is how birth control affects their risk of developing cancer. [1]

After all, the International Agency for Research on Cancer, which is part of the World Health Organization, classified the most commonly used hormonal birth control in the United States as a group 1 carcinogen in 2007. [2],[3] Being labeled a group 1 carcinogen means that there is “sufficient evidence” that an agent—in this case, hormonal birth control that contains synthetic progesterone (called progestin) and estrogen (called estrogen estradiol or EE2)—has the potential to cause cancer in humans. 

Explaining the link between birth control and cancer 

All hormonal birth control has synthetic progestin as a main active ingredient. But while hormonal IUDs, certain birth control pills, Depo Provera shots, and implants are “progestin-only,” combined oral contraceptives, euphemistically known as “the Pill,” have two primary active ingredients: progestin and estrogen estradiol. It is important to distinguish these synthetic versions from the natural hormones they’re trying to imitate because they act differently within the body.

Your body reacts in various ways to these man-made hormones, and studies have shown that this exposure increases the risk of breast and cervical cancer and decreases the risk of uterine, ovarian, and colorectal cancer.  

Although more research is needed to understand exactly how hormonal birth control affects cancer risk, here are a few working theories: 

• Birth control increases the risk for breast cancer by overstimulating breast cells, which leads to the multiplication of the type of cells that form tumors  
• Birth control increases the risk for cervical cancer by changing the appearance of cervical cells [4]
• Birth control reduces the risk of uterine cancer, also called endometrial cancer, by slowing the growth of cells in the uterus 
• Birth control reduces the risk of ovarian cancer by decreasing the number of times a woman ovulates during her lifetime  

Because high levels of bile acid in the bloodstream are linked to colorectal cancer, and because exposure to estrogen lowers the amount of bile acid in the bloodstream, it is a plausible theory that hormonal birth control use may also decrease one’s risk of colorectal cancer. [5] However, more research is needed.  

To understand your cancer risk with birth control, start with your baseline risk 

Cancer risk generally increases with age and depends on a spectrum of factors, such as genetics and lifestyle.  Oftentimes, you hear that hormonal birth control raises or lowers your risk for a particular cancer by a certain percentage. For example, birth control increases your risk for breast cancer by 20 percent, an amount often referred to as a slight increase. But what does this mean?

To understand your risk of developing breast, cervical, ovarian, uterine, and colorectal cancer, let’s start with your baseline risk for each. For the average woman, the risk of developing a specific cancer can be approximated using the Know Your Chances online tool from the National Cancer Institute.  

To best illustrate this, let’s start with an example: A 25-year-old woman has been on hormonal birth control for years and plans to continue until she is 35. She wants to know her estimated risk of developing any of the cancers influenced by hormonal birth control. 

First, we have to determine the baseline risk of developing each cancer for a woman who has an average risk for developing cancer—that is, not having any known risk factors such as hormonal birth control use. Then we’ll see what the increase (or not) in risk would be for her specifically since she has the added risk factor of hormonal birth control use. 

Average baseline cancer risk for women without known risk factors (like hormonal birth control use)

Because her chance of developing cancer between age 25 and 35 is so incredibly low—no greater than 0.07% of women who aren’t on hormonal birth control are diagnosed with cervical, colorectal, uterine, or ovarian cancer during this time span and only 0.2% with breast—let’s instead focus on a time period later in life when her risk of cancer is greater and still affected by hormonal birth control: her risk of cancer between age 35 and 55. 

The rationale for calculating cancer risk several years after this woman has been off birth control is that the effects linger, and may actually become more pronounced for breast cancer 15 to 20 years after stopping, according to comments made by Dr. Joel Brind in the No Adverse Outcomes roundtable.

According to the online tool, in a population of 10,000 women, the following are estimated to develop each cancer between the age of 35 and 55:

• 298 with breast cancer
• 26 with cervical cancer
• 56 with colorectal cancer
• 52 with uterine cancer
• 22 with ovarian cancer

As you can see, the risk of developing cervical, colorectal, uterine, or ovarian cancer is small—no greater than 0.6% of these women were diagnosed with each of those cancers, but notice the stark difference between breast cancer and the rest. Nearly 3% of these women, or one in 33, were diagnosed with breast cancer between age 35 and 55. Also, the number of women diagnosed with breast cancer is more than double that of the three cancer types potentially prevented.

With the baseline risk established, we can now estimate this woman’s cancer risk if she continues taking hormonal birth control as planned.

Explaining “relative risk:” When your doctor says your cancer risk is increased or decreased by a certain amount, this is what it means 

Each study examining the effects of hormonal birth control on cancer risk tends to arrive at slightly, and sometimes drastically, different percentages by which your cancer risk changes with hormonal birth control use, because the groups of women included vary from one study to the next. But the commonly cited risks are as follows: Birth control use increases your risk of breast cancer by 20% and cervical cancer by 100%, while decreasing your risk of ovarian cancer by 50%, colorectal cancer by 20%, and uterine cancer by 30%.  

But those percentages are known as relative risks and can be difficult to comprehend—what do they mean for you? Let’s do the math again to figure out what those risks mean, exactly.  

Cancer risks for women taking hormonal birth control

Using the online tool, in a population of 10,000 women who are taking hormonal birth control, the following are estimated to develop each cancer between 35 and 55:

• 358 with breast cancer
• 52 with cervical cancer
• 45 with colorectal cancer
• 36 with uterine cancer
• 11 with ovarian cancer

Translating further, these numbers mean that an estimated 60 additional women will develop breast cancer and 26 cervical cancer, or a total of 86 additional cancers for every 10,000 women in this age bracket taking hormonal birth control. Meanwhile, 11 fewer women will develop colorectal cancer, 16 uterine cancer, and 11 ovarian cancer, or a total of 38 avoided cancers.

Put yet another way, this 25-year-old woman’s risk of developing breast cancer once she has stopped birth control at 35 rose from 30 to 36 in 1,000 and cervical cancer from 3 to 5 in 1,000. Her risk of developing colorectal cancer fell from 6 to 5 in 1,000, uterine cancer from 5 to 4 in 1,000, and ovarian cancer from 2 to 1 in 1,000.

Not all birth control cancer risks are created equal–and women deserve that information 

Now, while these numbers seem to show that birth control is causing many more cancers than it’s preventing—and that very well may be true—remember that these numbers aren’t perfect estimates of cancer risk in women taking hormonal birth control for two reasons. First, the online tool is limited. Second, more research is needed to better understand how these risks vary by type of hormonal birth control, duration of use, starting age of use, and many other factors.

Still, the online tool provides a concept of risk and how even a so-called “slight” increase in breast cancer risk often dwarfs a drastic drop in uterine or ovarian cancer risk because a woman’s starting risk of breast cancer is so much higher than the rest. After all, breast cancer is the second most common cancer after skin cancer, affecting 1 in 8 women in the US, and leading to over 43,000 deaths in 2021 alone. Worldwide, 2.3 million new cases of breast cancer were diagnosed in 2020, and it it is also the most prevalent cancer worldwide.

Understanding these rates is important, as two doctors in the United States explain in this paper, because “many studies promoting the benefits of OCPs [oral contraceptives] in the literature tout lowered risks of uterine and ovarian cancers, yet minimize the increase in risk of breast and cervical cancers caused by these same OCPs.” [6]

An effective alternative to birth control  

The good news is that fertility awareness methods (FAMs), which allow women to track their fertility using a variety of evidence-based techniques, can effectively prevent pregnancy without the side effects or the increased cancer risks associated with hormonal birth control use. As a biomedical writer and editor who specializes in reporting cancer research, choosing a FAM made the most sense to me for planning my family without raising my cancer risk.

Note from the editor: We encourage readers to review the risks of breast cancer as reported in the study that supports our FDA petition, as it shows a summary of multiple different studies that specifically show the increased risks based on length of birth control use. The 20% risk rate mentioned in the article above is a conservative estimate; our petition study shows that it ranges between 19% and 37% (see Table 17 on page 85 of the petition study).

References: 

[1] Goldfield N, Neinstein L. Patient understanding of oral contraceptive side effects. West J Med. 1985 Mar;142(3):417-8. PMID: 3993024; PMCID: PMC1306063.

[2] International Agency for Research on Cancer., World Health Organization. (2007). IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. Lyon: World Health Organization. Available from: https://www.ncbi.nlm.nih.gov/books/NBK321672/pdf/Bookshelf_NBK321672.pdf

[3] Cooper DB, Patel P, Mahdy H. Oral Contraceptive Pills. [Updated 2022 Jan 21]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan. PMID:  28613632

[4] Mahgoub, Wala & Ali, Ibrahim. (2018). Histological Changes in the Cervical Cells Associated with Hormonal Contraception. Journal of Gynecology and Womens Health. 12. doi: 10.19080/JGWH.2018.12.555833. 

[5] Nguyen TT, Ung TT, Kim NH, Jung YD. Role of bile acids in colon carcinogenesis. World J Clin Cases. 2018 Nov 6;6(13):577-588. doi: 10.12998/wjcc.v6.i13.577. PMID: 30430113; PMCID: PMC6232560.

[6] Peck R, Norris CW. Significant Risks of Oral Contraceptives (OCPs): Why This Drug Class Should Not Be Included in a Preventive Care Mandate. Linacre Q. 2012;79(1):41-56. doi:10.1179/002436312803571447

Additional Reading:

How birth control, pregnancy, and abortion each impact a woman’s breast cancer risk

Reasons Women Need Periods: Breast Health

As Breast Cancer Rates Soar, the WHO Doubles Down on Promoting Birth Control

Hormonal Contraceptives Increase Breast Cancer Risk: A Doctor’s Report on Why

Fight Breast Cancer with Fertility Awareness

Natural Womanhood Citizens Petition to the FDA Requesting Accurate Side Effects Information on Hormonal Birth Control