A new study has reported a decline in use of hormone replacement therapy (HRT) since 2002 in Canada that was followed by a concurrent decrease in breast cancer. The U.S. Women’s Health Initiative clinical trial reported in 2002 that the risks of combined estrogen and progestin HRT in postmenopausal women outweighed the health benefits. The subsequent decline in use of HRT was followed by declines in breast cancer incidence in several countries, including the U.S. The current study was designed to determine whether similar reductions in breast cancer rates occurred in Canada. The authors first confirmed that there was a decline in HRT use among approximately 1,200 Canadian women aged 50 to 69 who participated in the National Population Health Survey between 1996 and 2006. This data was then extrapolated to the national female population. Data on prescriptions for HRT was obtained from a national registry of pharmacy-filled prescriptions. Breast cancer rates were obtained from the Canadian Cancer Registry and mammography rates were obtained from the Canadian Community Health Survey.

The authors found that the use of HRT declined after 2002, as expected. Between January 1, 2002, and December 31, 2004, the use of combined HRT (typically estrogen plus progestin) dropped from 12.7% to 4.9% of women aged 50 to 69. At the same time, there was a 9.6% reduction in the rate of breast cancer (from 296.3 per 100,000 women in 2002 to 268.0 per 100,000 women in 2004). Mammography rates were stable at 72% during the period, indicating that the decline in breast cancer diagnoses was not a result of fewer women having mammograms. On the other hand, there was little change in breast cancer rates in women under age 50 and over 70, who would not be expected to be on HRT. The authors conclude that there was a relationship between the declines in the use of HRT and breast cancer among Canadian women during the period 2002 to 2004.