Often the woman's cancer will be tested to see if it is sensitive to the amount of oestrogen in the system. If the cancer is oestrogen sensitive, tamoxifen will be given.

Because tamoxifen is such a weak estrogen, its estrogen signals don't stimulate very much cell growth. And because it has stolen the place away from more powerful estrogen, it blocks estrogen-stimulated cancer cell growth. In this way, tamoxifen acts like an "anti-estrogen."

Tamoxifen may also take the place of natural estrogen in the receptors of healthy breast cells. In that way it holds down growth activity, and possibly stops abnormal growth and the development of a totally new breast cancer. By blocking natural estrogen from getting to the receptors, tamoxifen is helpful in reducing the risk of breast cancer in women at high risk who have never had breast cancer. It also can help women who have already had breast cancer in one breast by lowering the risk of a new breast cancer forming in the other breast.

One study found that radiation plus tamoxifen was much better than tamoxifen alone at reducing the risk of breast cancer coming back after a lumpectomy in women with hormone-receptor-positive breast cancer. This was true even for women with very small cancers.

For pre-menopausal women, tamoxifen is the best hormonal therapy. But tamoxifen is no longer the first choice for post-menopausal women. If you've been on tamoxifen for two to three years and now you're in menopause, your doctor may recommend that you switch to an aromatase inhibitor to finish your five years of hormonal therapy. However, you can still get a lot of benefit if you take tamoxifen for up to five years and then switch to an aromatase inhibitor.

Tamoxifen was first used to fight breast cancer at the Christie Hospital in Manchester, England, in 1969. It has since proved its worth as means of stopping the spread or recurrence of the disease in women who have already been treated for it.

But, it was noticed back in the early 1980s that some women who were receiving the drug for cancer in one breast did not develop any tumorous growth in the other. This prompted the suggestion that Tamoxifen might have another preventative role for those women who are at risk of getting breast cancer but have yet to develop any signs of the disease.