All hormone contraceptives, including progestagen-only “minipills,” carry a small “excess risk” of breast cancer, according to a new study by researchers at the University of Oxford, but the overall risk remains low.
The study, published Tuesday in the journal PLOS Medicine, bridges a research gap by investigating the link between breast cancer and progestagen-only contraceptives.
“Use of combined oral contraceptives, containing both estrogen and progestagen, has previously been associated with a small increase in breast cancer risk but there is limited data about the effect of progestagen-only hormonal contraceptives,” the authors wrote.
The study found that progestagen-only contraceptives are associated with a 20% to 30% relative risk for breast cancer, which aligns with previous research on combined hormone contraceptives. The overall risk of breast cancer among hormone contraceptive users is low, especially for younger women, and past studies have shown that the excess risk is gone entirely about 10 years after stopping.
The study does not indicate that hormonal contraceptives cause breast cancer but rather investigated potential links to the disease.
What are progestagen-only contraceptives?
Progestagens, or progestin, are a type of synthetic hormone drug that mimic the natural hormone progesterone, which is crucial for menstruation and pregnancy.
Unlike combined hormone contraceptives, progestagen-only contraceptives do not have estrogen. Types of progestagen-only contraceptives available in the UK and US include the implant, the hormonal intrauterine device (IUD), the contraceptive injection and the minipill.
In the UK, which is ranked highest for access to contraception on the 2023 Europe Contraception Policy Atlas, prescriptions for progestagen-only contraceptives are almost as high as for combined oral contraceptives.
In the US, the combined pill is the most popular hormonal contraception, but a significant percentage of women are choosing an implant or IUD, according to the US Centers for Disease Control and Prevention.
Progestagen-only contraceptives work to prevent pregnancy by thickening mucus in the cervix to stop sperm reaching an egg and, in some cases, can stop ovulation completely.
The CDC and the British National Health Service also cite additional benefits.
For example, women who have severe period pain are often advised to opt for the implant or IUD, as progestagen-only contraceptives can weaken or stop periods for some women. This can serve as a more sustainable and affordable option for long-term pain management for conditions like endometriosis. The minipill is also cited as a safe option for women who are breastfeeding.
Like combined hormonal contraceptives, progestagen-only contraceptives are designed to prevent unwanted pregnancies but should still be used in tandem with condoms, as they do not protect women from sexually transmitted infections.
Risks vs. benefits
The new study analyzed data on almost 10,000 women under the age of 50 in the UK who were diagnosed with invasive breast cancer between 1996 and 2017, as well as more than 18,000 who did not have breast cancer.
“On average, 44% of women with breast cancer and 39% of matched controls had a hormonal contraceptive prescription, with about half the prescriptions being for progestagen-only preparations,” the study stated.
The Clinical Practice Research Datalink, which collected the data for the study, also combined the results with previous research, including women from a wider age range.
The resulting 15-year “excess risk” associated with five years of use of combined or progestagen-only contraceptives in high-income countries was estimated at 8 per 100,000 users ages 16 to 20 and 265 per 100,000 users ages 35 to 39.
Kirstin Pirie, a researcher at the University of Oxford and lead author of the study, said excess risks must be viewed in the context of the “well-established benefits” of contraceptive use during a woman’s reproductive years, such as birth control and hormone regulation. Pirie also noted that breast cancer risks are lower for younger women, given that the underlying risk increases with age.
Claire Knight, a senior health information manager at Cancer Research UK, which provided core funding for the study, said that hormonal contraception is a personal choice, as “there are lots of possible benefits to using contraception, as well as other risks not related to cancer.”
“Women who are most likely to be using contraception are under the age of 50, where the risk of breast cancer is even lower. For anyone looking to lower their cancer risk, not smoking, eating a healthy balanced diet, drinking less alcohol and keeping a healthy weight will have the most impact,” Knight said.
Limitations of the research
The researchers noted some limitations in the study, including a lack of complete information on a woman’s prescription history. However, while that limitation affected the ability to assess the long-term associations of contraceptive use with breast cancer risks, the authors say it is unlikely to have disproportionately affected the short-term findings.
The study says data was lacking on a family history of breast cancer, but the researchers said that previously published findings for combined oral contraceptives were unaltered after adjusting for family history.
The authors also said they attempted to investigate whether there are differing breast cancer risks between hormonal and nonhormonal IUDs. However, they said too few women in England had been prescribed nonhormonal IUDs to make a reliable comparison.