December 6, 2017

expert reaction to study investigating contemporary hormonal contraception and risk of breast cancer

In a new study, scientists publishing in the NEJM examine the association between breast cancer and women who currently or recently used contemporary hormonal contraceptives and women who have never used hormonal contraceptives.

Dr Michael Jones, Epidemiologist at The Institute of Cancer Research, London, said:

“The study found that hormonal contraception is associated with a small increase in risk of being diagnosed with breast cancer in women who have not yet gone through the menopause. The results are in line with a large analysis of more than 50 studies carried out 20 years ago.

“While the findings looked at the most up-to-date hormonal contraceptives used in Denmark at the time the study began, that was still a decade or more ago. In that time, new contraceptives, using for example a lower hormone dose, have been introduced. It would be interesting to see what effect these have on the association between hormone-based contraception and pre-menopausal breast cancer – but we will have to wait many years for that data.”

Prof. Ashley Grossman, Emeritus Professor of Endocrinology, University of Oxford, said:

“This study adds to previous information that OC use does, slightly, predispose to breast cancer risk. It was thought that possibly this risk would not be present with the newer OC including progesterone-only ones, but the risk persists according to this Danish study. The risk increases with use, and persists for some years after discontinuation, so the authors feel that even though they did not control for a number of other factors (factors not controlled for included alcohol, physical activity, breast feeding, and only partly for BMI), the increased risk was indeed due to the OC use.

“However, the increased overall relative risk of around 20% does not translate into a great absolute risk in these  women (under age 50y), in whom breast cancer is not common. So, they estimate that OC use for, say, 5 years, translates into one extra case of breast cancer for every approximately 1500 women. No-one should take the OC without careful thought, but the advantages in avoiding an unwanted pregnancy will usually more than outweigh the very slightly increased risk of breast cancer. And there is also the reassuring thought that OC use may decrease the risk of ovarian and endometrial cancer.”

Prof. Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“Much of the existing evidence on breast cancer risk and hormonal contraceptives comes from studies carried out some time ago, and the hormonal contraceptives that are available have changed considerably over time. This is a major new piece of research, based on data on almost 1.8 million Danish women over a period of 18 years. The research is competent statistically, and, so far as I as a statistician can judge, in other respects as well. It provides information not only on different types of currently available contraceptive pills, but also on hormonal patches, vaginal rings and intra-uterine devices that release a hormone, contraceptive hormone implants and long-lasting hormone injections.

“In very broad terms, the results are in line with previous findings. That is, using a hormonal contraceptive does appear to be associated with an increase in breast cancer risk, but the increase is not large. The researchers estimated that, if 100,000 women used hormonal contraception for a year, there would be about 13 extra breast cancer diagnoses, compared to the position if they had not used hormonal contraception at all. That doesn’t mean 13 extra cases in that year – it’s essentially 13 more cases in these 100,000 women over the next couple of decades. That’s not a large increase in risk, and of course it has to be set against any risks to health from not using hormonal contraception, for example from extra pregnancies or abortions.

“The new study found, again broadly in line with previous studies, that the increase in risk was greater in women who had used hormonal contraception for a longer time. But the increase in risk disappeared gradually over a few years after discontinuing the hormonal contraception, particularly in women who had not used it for a long period of time.

“Like most other studies on hormonal contraceptives and breast cancer risk, this one is observational, so it cannot prove conclusively that the hormonal contraception is definitely the cause of the increased risk. However, the researchers did allow statistically for most of the important factors that might also be involved, and they give good reasons why the differences in risk that they found are likely to be causally related to the contraceptives.

“I’m not a medical doctor, but my assessment is that this new evidence doesn’t make an important change to what was previously known about hormonal contraceptives and breast cancer risk. It just brings it further up to date and adds some detail. Certainly I’d advise anyone who is concerned about risks to talk to their doctor before making any changes in their contraceptive use.”

* ‘Contemporary Hormonal Contraception and the Risk of Breast Cancer’ by Mørch et al. published in New England Journal of Medicine on Wednesday 6 December. 

Declared interests

Dr Michael Jones: No conflicts of interest.

Prof. Ashley Grossman: No conflicts of interest.

Prof. Kevin McConway: “I am a member of the advisory committee of the Science Media Centre.”