Dear moneu: Tamoxifen is used in women whose cancers are ER/PR positive (estrogen receptor/progesterone receptor). It is used to reduce the chance of recurrence both locally and distantly as well as reduce the risk of developing a contralateral (opposite breast) breast cancer. Tamoxifen can also be used to reduce the risk of developing breast cancer in women who are at high risk.
I believe tamoxifen has been superceded by Arimidex - after my diagnosis of invasive ductal cancer and chemotherapy I was switched after 2 months of tamoxifen ( in 2004) to Arimidex, which I believe has a much better prognosis for recurrence than tamoxifen - that is if you are ER+.
It is my understanding (and I could be wrong) that Tamoxifen is still used instead of Arimidex depending on your menstrual status. Tamoxifen for women who are ER/PR+ and have not gone through menopause and Arimidex for ER/PR+ for women who have undergone menopause.
Twocapes is correct in the use of Tamoxifen and Arimidex. Right now Arimidex and Femara are not being given to premenopausal women. I was on Tamoxifen for 5 years, diagnosed with bc on the other side from the first bc and am now on Femara.