McMaster University researchers have found consistent evidence that use of hormone replacement therapy (HRT) is associated with breast cancer globally. This study comes at a time when more women are again asking for this medication to control hot flashes and other symptoms of menopause.

The rising trend is at odds with a U.S. Women's Health Initiative (WHI) study of 2002 which found a higher incidence of breast cancer, heart attack and stroke among women using HRT. Those findings led to a rapid decline in HRT use and a subsequent reduction in the incidence of breast cancer in many countries. However, HRT is now being offered to women in smaller doses and for a shorter period of time.

In their study, the McMaster researchers found "convincing evidence" for a direct association between decreased HRT use after the WHI study and the declining incidence of breast cancer. Their research appears in the January 2012 issue of the Journal of Epidemiology and Community Health.

"The evidence is compelling that HRT use increases the risk of breast cancer, and its cessation reduces this risk," the researchers said.

Dr. Kevin Zbuk, assistant professor of oncology at the Michael G. DeGroote School of Medicine at McMaster and lead author on the study says: "In our study we examined all studies that have reported breast cancer and rates of HRT use after the WHI study. There is very clear evidence that the countries with the highest HRT rates had the largest decrease in breast cancer incidence when HRT use started to decline.

"Given the potential harms associated with HRT use, physicians and patients alike should be reminded of the lessons learned from the WHI trial. If HRT is needed, it should be used for the shortest time and at the lowest dose necessary to relieve symptoms."