All hormonal contraceptives carry a slightly increased risk of breast cancer, including the increasingly popular progestogen-only pills, according to a study published on Tuesday.
The researchers who carried out the study stressed that the increased risk of breast cancer needs to be weighed against the benefits of hormonal contraceptives, including the protection they provide against other forms of female cancer.
Previous studies have established an increased risk of breast cancer from two-hormone, or combined, contraceptives that use both estrogen and progestogen.
While the use of progestogen-only contraceptives has been on the rise for well over a decade, little research had been performed previously on their links to breast cancer.
The study, published in the journal PLOS Medicine, found that the risk of a woman developing breast cancer was about the same for hormonal contraceptives using both estrogen and progestogen as for those using just progestogen.
According to the study, women taking hormonal contraceptives have a 20% to 30% higher risk of developing breast cancer than those who do not use them.
The findings are similar to those published previously, including in a vast 1996 study.
The risk remains about the same regardless of the delivery method — oral pill, IUD, implant or injection — or whether it is a combined pill or progestogen alone.
Taking into account that the likelihood of breast cancer increases with age, the authors of the study calculated how much absolute excess risk is associated with hormonal contraceptives.
For women taking hormonal contraceptives for a period of five years between the ages of 16 to 20, it represented eight cases of breast cancer per 100,000, they said.
Between 35 and 39 years old, it was 265 cases per 100,000.
'Very small increase in absolute risk'
"Nobody wants to hear that something that they're taking is going to increase their risk of breast cancer by 25 percent," said Gillian Reeves, a professor of statistical epidemiology at the University of Oxford and a co-author of the study.
"What we're talking about here is very small increase in absolute risk," Reeves said.
"These increases in risk for breast cancer have to, of course, be viewed in the context of what we know about the many benefits of taking hormonal contraceptives," she added.
"Not just in terms of birth control, but also because we know that oral contraceptives actually provide quite substantial and long term protection from other female cancers, such as ovarian cancer and endometrial cancer."
The study also confirmed, like others, that the risk of breast cancer declines in the years after a woman stops using hormonal contraceptives.
Stephen Duffy, a professor at Queen Mary University of London who did not take part in the study, described the findings as "reassuring in that the effect is modest."
The study involved data from nearly 10,000 women under the age of 50 who developed breast cancer between 1996 and 2017 in the United Kingdom, where the use of progestogen-only contraceptives is now as widespread as the combined method.
Reeves said there were several explanations for the growing use of progestogen-only contraceptives.
They are recommended for women who are breast-feeding, who may be at risk of cardiovascular problems or smokers above the age of 35.
"It might just be because women are taking hormonal contraceptives possibly into later years now," Reeves said.
"So they are naturally at higher risk of those other conditions for which risk is increased with combined contraceptives."