All forms of hormonal contraception, including the pill and the coil, were found to increase the risk of breast cancer by around a quarter, in a new study from Oxford University.
The researchers analysed GP records from nearly 30,000 women using a range of contraceptives and found the risk of developing breast cancer to be the same for all forms of hormonal contraceptives, including progesterone-only drugs.
Progesterone-only contraception has seen a doubling in use in the past twenty years due to health concerns around the combined pill. This analysis, published in PLOS Medicine, showed for the first time that the risk of developing breast cancer when using progesterone-only contraception was the same as all other hormonal contraception.
Breast cancer is one of the most common forms of cancer, and according to NHS figures, In the UK, 1 in 8 women will be diagnosed in their lifetime. Previous research has found that combined oral contraceptives, which contain both oestrogen and progesterone, are associated with a slight increase in the risk of breast cancer. There is limited data about the possible effect of progesterone-only contraceptives despite the use of progestogen-only contraceptives increasing substantially in recent years. In 2020 there were almost as many prescriptions in England for oral progestogen-only contraceptives as for combined oral contraceptives.
The researchers examined the records of 9,498 women under the age of 50 who had invasive breast cancer diagnosed between 1996 and 2017. They compared the data with the records of over 18,000 women who did not develop breast cancer.
The results show a 20 to 30 percent relative increase in breast cancer risk associated with both combined and progesterone-only contraceptives, whatever the mode of delivery.
The researcher found that the risk fell with increasing time after discontinuing the oral contraceptive medication. Ten years after stopping, there was a very low excess risk observed with oral contraceptive use, with 8 cases per 100 000 hormonal contraceptive users at age 16-20 years and 265 cases per 100 000 users at age 35-39 years.
Kirstin Pirie, a statistical programmer from the Cancer Epidemiology Unit at the University of Oxford, said: ‘These 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.’
The authors highlighted that the underlying risk of breast cancer increases with advancing age, and the absolute excess risk associated with using either type of oral contraceptive will be smaller in women who use it at younger rather than at older ages. They advised that these risks should ‘be viewed in the context of the well-established benefits of contraceptive use in women’s reproductive years.’
Professor Gillian Reeves, lead author of the study, also from Oxford University, said that the new research filled a knowledge gap, showing that progesterone-only contraceptives also have a small, but transient, increase in the risk of breast cancer and said the study did not mean women needed to change the contraception they were using.