Landmark research by Oxford University has shown that modern forms of hormonal contraceptives increase the risk of breast cancer by around one quarter.
The analysis of data by researchers at Oxford Population Health Cancer Epidemiology Unit, published on Wednesday in PLOS Medicine, showed that use of progestogen-only hormonal contraceptives is associated with a 20-30% higher risk of breast cancer.
Almost 30,000 women under the age of 50 took part in the study, which tracked cases of breast cancer among those taking a range of contraception, including progesterone-only drugs, which have spiked in popularity in recent decades. In the last 20 years, the number of women taking such drugs has doubled, with approximately three million now on hormonal contraceptives in the UK.
In 2020, there were almost as many prescriptions issued in England for progestogen-only oral contraceptives as there were for combined oral contraceptives.
It comes as the numbers taking traditional combined pills have halved, meaning numbers are now equal for each. The increased popularity of hormonal contraceptives has been rooted in a belief that they carry less health risks, with evidence having suggested they are less likely to cause blood clots and strokes,
However, the new research found that progestogen-only drugs and coils present at least equal risks as the combined contraceptive pill when it comes to breast cancer in women under the age of 50.
Research conducted in the past has shown that using 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. Despite a substantial increase in the use of progestogen-only contraceptives, information on their association with breast cancer risk has been limited up until now.
Researchers from Oxford University analysed data from 9,498 women who developed invasive breast cancer between the ages of 20 to 49, and 18,171 closely-matched women without breast cancer who acted as controls in the study. The research found that around two thirds of women in their 20s had a prescription for hormonal contraception, which fell to around one quarter of those in their 40s.
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 being diagnosed – around half of whom had last been prescribed a progestogen-only contraceptive.
Key findings
Researchers found that there was a significant increase in the risk of breast cancer associated with hormonal contraceptive use. The researchers estimated that the absolute excess risk of developing breast cancer over a 15-year period in women with five years use of oral contraceptives ranged from eight in 100,000 women for use from age 16 to 20, to 265 in 100,000 for use from age 35 to 39.
When the findings for progestogen-only contraceptives were combined with previously published studies, there was an increased risk of breast cancer in current and recent users of all four types of progestogen-only preparations.
The increased cancer risk for women taking progestogen-only drugs was 26 per cent, compared with 23 per cent for those on combined pills. The risk rose to 32 per cent among women with progestogen-releasing intra-uterine devices, while those on hormonal implants experienced an increased risk of 25 per cent.
Scientists emphasised that the overall risks for breast cancer among young women remain low, with the vast majority of women who get breast cancer being over 50. They described the increased risk as “transient” – adding that it largely disappears after coming off the contraceptives.
According to Breast Cancer Ireland, one in nine women will be diagnosed with breast cancer in their lifetime. 3,700 new cases of breast cancer are diagnosed annually in Ireland; 23% of all women diagnosed with the disease are between 20-50 years of age. Breast cancer is the third most common type of cancer in Ireland.
Researchers said that those taking hormonal contraceptives, such as the pill and the coil, should take into account the increased risk of cancer. However, they said that the increased risk should be balanced against benefits of the pills – including protection against other forms of the disease, as well as their contraceptive function.
Overall, women who took such pills for 15 years had a 2.2% risk of breast cancer by their late 30s. This compared with a 2% risk among those who had not taken hormonal contraception. For those in their late 20s, the risk increased from 0.5% to 0.57%.
Kirstin Pirie, statistical programmer at Oxford Population Health, one of the lead authors, said:
“The findings suggest that current or recent use of all types of progestogen-only contraceptives is associated with a slight increase in breast cancer risk, similar to that associated with use of combined oral contraceptives.
“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 researchers also stated that although the findings provide evidence about the short-term associations between hormonal contraceptives and breast cancer risk, they do not provide information regarding longer-term associations, or the impact of total duration of contraceptive use on breast cancer risk.