Prempro (Premarin + Provera)

Development and release

Prempro, developed by Pfizer, is a drug that combines estrogen and progestin to treat women’s menopause symptoms. Menopause, which typically hits women around 51 years of age, can cause hot flashes, mood swings, vaginal dryness, and the thinning of a woman’s hair, among other symptoms.

Until 1995, menopausal women would often combine estrogen-based Premarin (developed by Pfizer subsidiary Wyeth) with progestin-based Provera (developed by Pfizer subsidiary Upjohn), according to a Bloomberg BusinessWeek report. The two were combined “officially” to create Prempro, a single drug that included both hormones. Wyeth’s sales of the medication exceeded $2 billion… before a 2002 government-funded study found potentially fatal side effects.

Potential side effects

In post-menopausal women, the combination of estrogen and progestin (the active ingredients in Prempro) was found to cause increased risk of heart disease, stroke, serious blood clots in the lungs and/or legs, dementia, and cancer of the breast and/or ovaries. A 2002 National Institutes of Health-sponsored Women’s Health Initiative study provided evidence of the cancer risk claims, according to a USA Today report.

The study randomly assigned to 16,000 women aged 59 to 70 either a placebo or Prempro; the study was halted in July 2002, however, because researchers found that long-term use of Prempro raised the risk of breast cancer, heart attack, and stroke.

More than six million women used Prempro before the Women’s Health Initiative study, according to the Bloomberg report. The drug is still on the market.

An additional study, conducted by cancer specialist Rowan Chlebowski and published in the Journal of the American Medical Association in December 2010, also found an increased risk of breast cancer in Prempro users, according to the USA Today report. The cancer was more likely to spread to a woman’s lymph nodes; and women were more likely to die of breast cancer. A 2009 Lancet study also conducted by Rowan Chlebowski and his team reported that the drug’s users’ breast cancer risk appeared to decrease noticeably within two years of quitting treatment on Prempro.

For the Journal study, Chlebowski’s team followed nearly 13,000 women for about eight years after they had stopped taking Prempro pills. At the time, research showed that about 1.3 extra breast cancer deaths occurred per 10,000 women, according to USA Today.


Pfizer has had to set aside more than $700 million for lawsuits as a result of claims that Prempro caused cancer in postmenopausal patients. At the height of litigation, there were 10,000 claims for damages.

On Jan. 9, 2012, Pfizer was ordered by a Michigan court to pay two women $45 million in damages, according to CBS-affiliate WWMT. The Illinois-based women sued Pfizer claiming their use of Prempro had caused them to develop breast cancer.

In December 2011, Pfizer agreed to settle claims that its menopause medications caused cancer in three women, according to the Bloomberg report. A Philadelphia jury had previously awarded those women $72.6 million. Pfizer has lost 10 of the 18 Prempro jury-decided cases since trials began in 2006. The company was able to get some of those verdicts thrown out post-trial, however, in addition to reducing awards. As with the three women in Philadelphia, Pfizer was able to resolve some of the verdicts through settlements. Beyond that it has sought decisions in appellate courts and has petitioned to have cases dismissed before trial.

Additional verdicts in favor of plaintiffs included a $1.5 million award in Puerto Rico in December 2010. Pfizer won two cases prior to the Philadelphia trial.

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