The University of Oxford’s Million Women Study is a fantastic resource. Started in 1996, it was originally created to provide robust information on the impact of menopausal hormone therapy (MHT) on breast cancer in women, but it now examines the impact of other lifestyle behaviours (like obesity and diet) on a much larger range of health conditions in middle and in old age.
Looking through 2019 publications I found a meta-analysis of worldwide epidemilogical evidence around the type and timing of menopausal hormonal therapy and breast cancer risk published by the Lancet in 2019. It quantifies the impact of MHT on breast cancer…. something women really want to know when considering the use of MHT.
This chart summarises the findings: that for women of average weight in developed countries, after 5 years of MHT starting at the age of 50, about one in 50 users of oestrogen plus daily progestagen preparations (2%) will experience an increase breast cancer incidence. For oestrogen only preparations, this falls to 0.5%.
To put this in context, Cancer Research UK states that 8% of breast cancer cases are caused by obesity and a further 8% are caused by alcohol drinking. Older age is the primary risk factor for cancer.