Any type of hormonal contraceptive may increase the risk of breast cancer, new research suggests.
The study indicates that use of progestogen-only hormonal contraceptives is associated with a higher risk of breast cancer.
While previous studies found that use of the combined contraceptive pill, which combines oestrogen and progestogen, is associated with a small increase in the risk of developing breast cancer that declines after stopping use.
The researchers looked at the absolute excess risk over a 15-year period in women with five years’ use of oral contraceptives.
In women from the age of 16, some 84 per 100,000 non-users were likely to get breast cancer, while for five years of oral contraceptive use from age 16-20, the 15-year risk is 92 per 100,000 – resulting in an excess risk of eight per 100,000.
The 15-year risk for non-users from age 25 was 505 per 100,000, but with five years of oral contraceptive use from age 25-29, the 15-year risk was 566 per 100,000 – an excess risk of 61 per 100,000 (from 0.50% to 0.57%)
The 15-year risk for non-users from age 35 was 1953 per 100,000, but for five years of oral contraceptive use from age 35-39, this increased to 2218 per 100,000 – an excess risk of 265 per 100,000 (from 2.0% to 2.2%).
Absolute excess risk means the additional number of women who would be expected to develop breast cancer in those who used oral contraceptives compared with those who did not.
She added: “Yes, there is an increase here, and yes, nobody wants to hear that something that they’re taking is going to increase their risk for breast cancer by 25%.
“The main purpose of doing this research was really to fill a gap in our knowledge.
“We’ve known for many years that combined oral contraceptives, which women have been using for decades, also have an effect on breast cancer risk, a small increase in risk which is transient.
“We weren’t absolutely sure what the corresponding effect of these progestogen-only contraceptives would be.
“What we’ve shown is that they’re just the same in terms of breast cancer risk, they seem to have a very similar effect to the other contraceptives, and the effect that we’ve known about for many years.
Kirstin Pirie, statistical programmer at Oxford Population Health, and one of the lead authors, said: “Given that a person’s underlying risk of developing breast cancer increases with advancing age, the absolute excess risk of breast cancer associated with either type of oral contraceptive will be smaller in women who use it at younger ages.
“These excess risks must, however, be viewed in the context of the well-established benefits of contraceptive use in women’s reproductive years.”
The research suggests use of progestogen-only contraceptives has increased substantially in recent years, but information on their association with breast cancer risk was limited.
In 2020, there were almost as many prescriptions issued in England for progestogen-only oral contraceptives as there were for the combined pill.
Researchers analysed data from 9,498 women who developed invasive breast cancer between ages 20 to 49 and 18,171 closely matched women without breast cancer.
The data was collected by the Clinical Practice Research Datalink (CPRD).
Some 44% of women with breast cancer and 39% of women without breast cancer included in the study had a prescription for a hormonal contraceptive an average of three years before diagnosis.
Around half of these women were last prescribed a progestogen-only contraceptive.
Researchers combined the CPRD results on oral contraceptive use with those from other previously published studies to estimate absolute excess risks.
The scientists note that their study, published in Plos Medicine, did not provide information regarding longer-term associations between hormonal contraceptives and breast cancer risk.
The study was funded through core funding provided to the Cancer Epidemiology Unit by Cancer Research UK.
Dr Kotryna Temcinaite, head of research communications at Breast Cancer Now, said: “For both types of contraceptives, if you stop using them, this added risk of breast cancer reduces over time.
“The study didn’t look at what hormonal contraceptives the women may have used in the past or consider how long they may have been on the progestogen-only contraception.
“It also didn’t factor in whether a family history of the disease contributed to their level of risk. So further work is needed to help us fully understand the impact of using this type of contraception.
“Breast cancer is rare in young women. A slight increase in risk during the time a woman uses hormonal contraceptive means only a small number of extra cases of the disease are diagnosed.”