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.
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.
I believe tamoxifen has been superceded by Arimidex - after my diagnosis of invasive ductal cancer and chemotherapy I was switched after 2 months of tamoxifen ( in 2004) to Arimidex, which I believe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.