Previous studies have shown 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. 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 combined oral contraceptives.
The 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 who acted as controls. The data were collected by the Clinical Practice Research Datalink (CPRD). 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 whom were last prescribed a progestogen-only contraceptive.
The data were used to calculate the strength of the association between use of each type of hormonal contraceptive and breast cancer risk. These calculations were then adjusted to take into account established risk factors such as body mass index (BMI), number of recorded births, and the time since a woman’s last birth. The researchers combined the CPRD results on oral contraceptive use with those from other previously published studies to estimate absolute excess risks, meaning the additional number of women who would be expected to develop breast cancer in those who used oral contraceptives compared to those who did not.