M.D. Anderson scientists confirm that the decline in HRT probably caused the drop in breast cancer rates.
The strongest evidence of the risks of hormone replacement therapy came in a study by the University of Texas M.D. Anderson Cancer Center. Researchers found that new breast cancer cases fell by 7% nationwide in 2003, the year after many women had stopped using the drugs.
After analyzing the potential reasons for this decline, the researchers concluded that the change could only be linked to the decline in women receiving hormone replacement therapy treatment.
12–15% drop in breast cancer for older women
While the 7% drop was “astounding” in the words of one scientist, the change was even more noticeable in the type of cancer associated with hormone replacement therapy and in the age group that usually receives this treatment.
Among women 50–69 who receive HRT drugs during menopause, cancer rates fell by 12%. And for estrogen-positive tumors—which are fueled by the hormones in HRT—cancer rates fell by 15%.
Scientists say these numbers suggest that the decline in hormone replacement therapy use is the only probable cause of the falling cancer rates. About one-third of women over age 50 were using Prempro and other HRT drugs before 2002; about two-thirds of these women stopped using hormone replacement therapy in 2003 alone.
The risks of hormone replacement therapy have been known by scientists since 2002. That year, the National Institutes of Health conducted a study in the Women’s Health Initiative for Wyeth’s hormone replacement therapy drug Prempro. 
