The agency said tamoxifen should not be taken for longer than five years because there is no information about benefits after that period and because extended use raises concerns about increasing the chances of side effects. The FDA decision has proved to be controversial among medical experts and healthcare advocates. Critics of the move point to several concerns, including over-prescription and what some consider to be inadequate research. FDA approval resulted from a single study conducted by the National Cancer Institute involving 13,000 women identified as being at high risk of developing breast cancer because of their age, personal and family medical histories, and other factors. The study showed that tamoxifen treatment could cut a woman's chances of developing the disease by as much as 44 percent.
Tamoxifen works in breast tissue by interfering with estrogen, a hormone that promotes cell growth.
Zeneca Pharmaceuticals in Wilmington, Del., manufactures the drug under the trade name Nolvadex.