The safety of hormone replacement therapy has come under the spotlight again following research which claims that a decline in its use is probably responsible for the recent drop in cases of women being diagnosed with breast cancer in the USA.

There has been a rapid decline in HRT use since 2002 when a Women's Health Initiative study found that such treatment was associated with an increased risk of breast cancer and recent data has linked the decrease in the latter to the drop in hormone use. However other researchers have said the decline is due to a decrease in mammography screening over the same period.

In light of this debate, Karla Kerlikowske of the University of California, San Francisco and colleagues from the Breast Cancer Surveillance Consortium collected data between 1997 and 2003 from over 600,000 mammograms performed on women aged 50 to 69. The findings, published online in the Journal of the National Cancer Institute, show that use of HRT among the study population declined 7% a year between 2000 and 2002, then by 34% between 2002 and 2003. Over the same period, breast cancer incidence rates declined annually by 5% and oestrogen-receptor positive breast cancer rates fell 13% each year from 2001 to 2003.

The authors of the study said that the results suggest that a decline in postmenopausal hormone therapy use has contributed to the decrease in breast cancer incidence “and that the small decline in screening mammography observed in the USA is unlikely to explain the national declines in breast cancer incidence". Prof Kerlikowske added that “there is clearly a causal link” and “for people who stop taking hormones, their risk decreases fairly quickly. That's a positive, exciting message”.

The study authors speculate that oestrogen and progestin work “synergistically” to promote breast tumour formation and growth and say that in the absence of those hormones, tumours may grow very slowly, stop growing, or “regress completely”. They conclude that women who require oestrogen-progestin therapy to control postmenopausal symptoms should be encouraged to use it “for the shortest time possible”.