Breast cancer rates among postmenopausal women in Canada dropped nearly 10 percent after news of a big study in 2002 that found taking hormone replacement therapy could increase breast cancer risk, researchers said on Thursday.

The decline coincided with a sharp decline in use of the drugs after publication of the Women’s Health Initiative study, which found an increased risk of ovarian cancer, breast cancer, strokes and other problems from hormone therapy.

Before 2002, doctors widely prescribed hormone replacement therapy, or HRT, to lower the risk of heart disease or osteoporosis, both of which go up sharply after menopause.

Sales of U.S. market leader Wyeth’s combined estrogen-progestin therapy Prempro have fallen by about 50 percent since 2001 to around $1 billion a year. Wyeth is now owned by Pfizer.

To see if the drop in use affected breast cancer rates, Prithwish De of the Canadian Cancer Society and colleagues analyzed data from various Canadian registries and from a national health survey of women aged 50-69.

They specifically scanned for prescriptions on hormone replacement therapy, breast cancer incidence, mammography rates and women’s self-reported use of hormone replacement therapy.

They found in 2002 that 12.7 percent of Canadian women aged 50-69 used HRT but this fell to 4.9 percent of these women by 2004. During the same period, breast cancer rates fell 9.6 percent, but mammogram rates remained stable.

They said the drop was likely linked to media coverage about the Women’s Health Initiative study and a British study released around the same time known as the Million Women Study - both of which showed increases in breast cancer risk in women who used hormone replacement therapy.

In 2005, breast cancer rates in Canada began to creep back up among women aged 50 to 69. That rebound may mean that hormone replacement therapy speeds the growth of hidden breast cancers, but does not cause them, the team reported in the Journal of the National Cancer Institute.

About 75 percent of breast cancers are estrogen-receptor positive, meaning they are fed by estrogen.

The study was limited because it was based on data women reported themselves, and it did not include data on how often or how long they were treated, the team said. They said more long-term studies might help clarify the link.

Doctors now recommend hormone replacement therapy for women suffering severe menopause symptoms, but caution that they should use the lowest dose possible for the shortest period of time.

More than 400,000 women die from breast cancer globally each year.

SOURCE:  Journal of the National Cancer Institute, published online September 23, 2010.