Prempro (conjugated estrogens/medroxyprogesterone acetate), Premarin (conjugated estrogens), and Premphase (conjugated estrogens/medroxyprogesterone acetate) are hormone replacement therapy (HRT) drugs used to treat symptoms of menopause such as hot flashes, night sweats, and vaginal dryness. Such drugs have been linked to lobular breast cancer, ductal breast cancer, coronary heart disease, stroke, blood clots, deep vein thrombosis, pulmonary embolism, ovarian cancer, scleroderma, and lupus.

Between 20 percent and 50 percent of women in the Western world who are between the ages of 45 and 70 have taken or are now taking HRT drugs. In the past, they were prescribed to every woman who experienced unpleasant symptoms of menopause. They were even expected to decrease certain health risks. Thousands of users assumed that drug companies like Wyeth had tested the safety of the medications.

On July 9, 2002, the National Institutes of Health (NIH) revealed the abrupt halt in use of Wyeth's Prempro in the Women's Health Initiative (WHI) study because of unacceptable risks associated with the drug. Letters were sent to study participants urging them to stop taking the drugs because they led to increased risk of breast cancer, heart disease, and stroke.

The WHI study was a $700-million, eight-year, publicly funded study of HRT. It was intended to evaluate the benefits of hormone replacement and hoped to find that drugs like Prempro and Premarin helped in the prevention of heart disease, breast and colon cancer, and osteoporosis. Instead, the risks associated with these drugs outweighed any benefits found. The findings were so shocking that the study was stopped with almost three years left to go.