When tamoxifen enters the body, it competes with estrogen to interact with breast cancer cells, creating a sort of protection against the cells' reproduction. Because tamoxifen acts like estrogen in other systems of the body, it may be beneficial to health in other ways. It may act like estrogen replacement therapy in menopausal women, reducing cholesterol in the blood and slowing bone loss due to osteoporosis. But tamoxifen isn't for everyone. Although national trials have shown it to be effective in preventing recurrence of cancer in women who have already had a tumor and preventing a new tumor in the opposite breast, as well as reducing the risk of breast cancer among women at risk for the disease, no studies have shown that tamoxifen helps women who are at ordinary risk for the disease.
Tamoxifen slightly increases the chance of blood clots in the legs or lungs of women who take it.
The risk is similar to that faced by women who take estrogen replacement therapy. It also has been shown to increase the chance of getting endometrial cancer. Women taking tamoxifen face a 50 percent greater risk of getting endometrial cancer than those not taking it, with risk increased further still among those who also have taken estrogen replacement therapy and are overweight, Bernstein said. Women taking tamoxifen also may have some vision problems. Other possible complications are under study, including whether women who take tamoxifen have higher stroke risk.