The Women’s Health Initiative Study was a large American study which looked at health outcomes such as the incidence of cardiovascular disease, breast cancer, colorectal cancer and hip fracture in over 160,000 postmenopausal women aged between 50-79. One part of the study included a trial of HRT where women were randomised to either a synthetic form of combined HRT or placebo. 27,347 women were randomised to the HRT group. The trial was stopped early in 2002 following increased incidence of breast cancer, blood clots, cardiovascular disease and stroke in the HRT group.

However, one of the criteria for entry in the trial was that women could not be perimenopausal or have menopausal symptoms. As a result, most women included in the trial were older than the average woman commencing HRT for symptom control who is usually in her early to mid-50s. The trial assessed disease prevention and wasn’t utilised in women for whom HRT is indicated, that of symptom control.

Women were also given a synthetic HRT which is less commonly prescribed in Europe. In addition, a proportion of women in the placebo arm of the trial were taking HRT (started outside of the trial protocol) or had a previous history of HRT use. Finally, an 18- year follow-up analysis published in 2017 showed that HRT was not associated with an increase in all-cause mortality, cancer mortality or cardiovascular disease.