Tamoxifen was approved by the U.S. Food and Drug Administration in 1998 for breast cancer prevention in women who have at least a
1.67-percent chance of developing the disease over the next five years.
Such women are considered at high risk for breast cancer. Groups such as
the U.S. Preventive Services Task Force and the Canadian Task Force on
Preventive Health Care recommend that physicians counsel women above
this threshold about the benefits and risks of tamoxifen as a means of
preventing the disease.
Tamoxifen is a selective estrogen receptor-modulating drug used to treat estrogen receptor-positive breast cancers. In addition, it has
been shown to reduce the incidence of invasive breast cancer among
high-risk women by up to 49 percent.
However, tamoxifen is associated with significant adverse effects, including cataracts requiring surgery, deep vein thromboses, endometrial
cancer and stroke. Women taking tamoxifen, if they do develop breast
cancer, are also more likely to develop an estrogen receptor-negative
tumor, which has a worse prognosis.
In the new study, Melnikow and her colleagues calculate that tamoxifen can be expected to extend life expectancy only when a woman's
five-year risk of developing breast cancer reaches 3 percent or more.
This is especially true for women who have not had a hysterectomy, and
therefore face the risk of endometrial cancer related to tamoxifen use.
To arrive at their findings, Melnikow and her colleagues used a complex mathematical model based on a hypothetical group of 50-year-old