Fueling the ongoing debate over the safety of hormone replacement therapy (HRT), the National Institute of Environmental Health Sciences (NIEHS) added the entire class of steroidal estrogens Wednesday to its registry of "known human carcinogens." Linked to ovarian cancer, estrogen therapy has been a hot topic in the medical community in recent months because of several new studies that have been released on the safety of HRT.

According to the results of a July 2002 National Cancer Institute study that followed 44,241 women over a period of 20 years, women using estrogen alone faced nearly twice the risk of developing ovarian cancer compared to similar women not taking the hormone. Women who used estrogen alone for 20 years or more were three times as likely to develop ovarian cancer. A spokesperson for the NIEHS' Environmental Toxicology Program said the new listing is based on a two-year review of medical literature on the safety of estrogen therapy. There are currently 228 substances listed by the NIEHS as "known human carcinogens."