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Women who are using hormone replacement therapy (HRT) have an increased risk of breast cancer - and the risk is much higher for some types of HRT than others. Women who have worked night shifts, on the other hand, do not have any increase in risk of breast cancer. How do we know this? - because we use the NHS to support high-quality medical research. 

These are two major findings from long-term studies carried out or analysed in the Cancer Epidemiology Unit in the last 25 years. The Million Women Study needed very large numbers to look at HRT and breast cancer risk and so we recruited over 1.3 million women in the late 1990s to the study, by including a questionnaire with the invitation to routine NHS breast screening. Women reported their use of HRT and agreed that we could follow their health using routinely-collected NHS data on deaths and cancer registrations. We were therefore able to compare the risk of breast cancer in women using HRT with the risk of breast cancer in women who were not using HRT, in the largest such study in the world. Our findings on HRT helped to change prescribing guidelines, thus avoiding unnecessary use. HRT use in the UK and around the world fell markedly, followed by a fall in breast cancer rates.  It is thought that tens of thousands of breast cancers have been prevented worldwide. Without the NHS, the study would have been both prohibitively expensive and scientifically less reliable. Without routine NHS follow-up, knowing what had happened to the health of all women in the study over many years would not have been possible. 

The question of night shift work and breast cancer risk has been a long-standing concern. We were able to combine information from the Million Women Study with information from another study run from the Cancer Epidemiology Unit, EPIC-Oxford, and with information from the nationwide UKBiobank, all of which use NHS data for follow-up. We could then give a more reliable answer than previous studies, and were able to demonstrate clearly that night shift work does not increase the risk of breast cancer.  

Research that uses information about the nation’s health to learn about disease and to improve health-care has been part of the NHS since 1948. In 2018, with electronic health records and vast computing power, the opportunity for NHS research is greater than ever. With a nationwide and universal health system, the UK is one of the best countries in the world to conduct reliable, cost-effective research.