Women's Health Initiative

The Women's Health Initiative (WHI) was a series of clinical studies initiated by the U.S. National Institutes of Health (NIH) in 1991, to address major health issues causing morbidity and mortality in postmenopausal women. It consisted of three clinical trials (CT) and an observational study (OS). In particular, randomized controlled trials were designed and funded that addressed cardiovascular disease, cancer, and osteoporosis.^{[citation needed]}

In its entirety, the WHI enrolled more than 160,000 postmenopausal women aged 50–79 years (at time of study enrollment) over 15 years, making it one of the largest U.S. prevention studies of its kind, with a budget of $625 million.^[1] A 2014 analysis calculated a net economic return on investment of $37.1 billion for the estrogen-plus-progestin arm of the study's hormone trial alone, providing a strong case for the continued use of this variety of large, publicly funded population study.^[2][3] In the years following the WHI, studies have shown a decrease in breast cancer rates in postmenopausal women, attributed to the decline in use of hormone replacement therapy.^[4]

However, initial interpretation and communication about the studies' findings have been criticized for failing to clarify that the studies were weighted toward women already 60 or older (average age 63).^[5] This meant that women in their 50s, who tend to be healthier and have more menopausal symptoms, were underrepresented.^[5] Systemic hormone therapy has decreased dramatically among U.S. women since the WHI results were published.^[6]