New Evidence Linking Hormone Therapy to Breast Cancer


Results of the latest multi-center study on the link of hormones and breast cancer were published today in the New England Journal of Medicine.

The study is a follow-up to the landmark Women's Health Initiative (WHI) report of 2002. That study found that postmenopausal women taking estrogen plus progestin were at far greater risk of developing breast cancer than women on placebo.

The Stanford researchers found that postmenopausal women who take combined estrogen plus progestin menopausal hormone therapy for at least five years double their risk of breast cancer every year. Just as important, the multi-center study also found that women on hormones can quickly reduce their risks of cancer simply by stopping the therapy.

Use of hormone therapy plummeted in the United States after the WHI data was published. There was a drop from 60 million prescriptions in 2001 to 20 million in 2005. It was noted that breast cancer rates declined significantly within the year, suggesting a strong link between hormone use and cancer risk. Some scientists questioned the connection, saying the dip in breast cancer rates could not have occurred so rapidly and may have been related to patterns of mammogram use.

This latest study hopes to put those questions to rest. In the study, the researchers analyzed data from two groups of women. In one instance, they continued to follow more than 15,000 women in the original landmark trial (WHI) which was halted three years early. The women in this trial had been randomly assigned to take either combined hormone therapy or a placebo. When the negative results emerged in 2002, the women were all advised to stop taking their pills. The researchers continued to monitor them, taking note of the timing and number of any new breast cancer cases and how often the women received mammograms.

The second group included 41,449 women who were enrolled in a separate WHI observational study begun in 1994. These were "real world women" who could make their own choices as to whether they would take hormones or not. About 40 percent of the women in this group were on hormone therapy when the study began and had been taking the hormones an average of 6.9 years. The women weren't given any special instructions about hormone use after the 2002 study came out, but they did receive a letter outlining the results. They were also encouraged to continue to have regular mammograms.


The researchers found the results from the two groups of women were quite similar. In the clinical trial, the incidence of breast cancer was much higher in the hormone group in the five years leading up to 2002. After the women stopped taking the hormones, breast cancer rates dropped very rapidly. The number of breast cancer diagnoses fell 28 percent within the year. Use of mammography remained the same for all the women during all phases of the trial.

In the observational study women received no specific advice to stop hormone therapy. Many voluntarily chose to do so, reflecting what women were doing across the United States. In this group of women there was a 50 percent decrease in hormone use between 2000 and 2003. There was a 43 percent reduction in their breast cancer rates between 2002 and 2003.

Among the women who opted to continue to take hormones, the researchers found a far greater risk of getting cancer than previously thought. A woman who stayed on the therapy for at least five years was found to double her breast cancer risk every year.

"This is very strong evidence that estrogen plus progestin causes breast cancer," said Stefanick, chair of the WHI executive committee. "You start women on hormones and within five years, their risk for breast cancer is clearly elevated. You stop the hormones and within one year, their risk is essentially back to normal. It's reasonably convincing cause-and-effect data."

The results, she cautioned, do not apply to women taking estrogen alone. The large WHI trial of estrogen-only did not find an increase in breast cancer for the majority of women assigned to estrogen-only therapy.

The study was sponsored by the National Institutes of Health.

Stanford University Medical Center Office of Communication & Public Affairs at
New England Journal of Medicine
Women's Health Initiative



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