• Arimidex reduces the risk of breast cancer by 53% after 7 years follow up and 50% after 10.9 years follow up.
• Arimidex also reduced the risk of other cancers compared with placebo, such as endometrial cancer, ovarian cancer, lung cancer, and melanomas.
• In the IBIS-II trial, too few deaths were reported in the Arimidex Vs Placebo arms (2 versus 3 respectively) to make any conclusions about overall survival differences.
• The protective effects of Arimidex continue even after you stop taking it, so long as you have been taking it for around five years.

Arimidex (anastrozole) is a medication that is used to treat breast cancer in postmenopausal women. In most cases, Arimidex is taken for five years, although some women may take it for a shorter period.

Many women worry that stopping Arimidex after five years will cause their cancer to come back; however, the International Breast Cancer Intervention Study II (IBIS-II) Prevention trial reports that:

• Arimidex reduced the likelihood of breast cancer by 50% in women considered at high risk, at 10.9 years follow-up.
• An estimated 29 women need to be treated with Arimidex for 5 years to prevent one woman from developing breast cancer during treatment and within the next 5 years. For tamoxifen, 49 women need to be treated in order to prevent one developing breast cancer.
• Invasive ER-positive breast cancer was reduced by 54% with Arimidex treatment. A nonsignificant effect was observed for invasive ER-negative breast cancer.
• Overall, Arimidex also reduced the risk of ductal carcinoma insitu (DCIS), with the biggest reduction seen in cases known to be ER-positive.
• Arimidex also reduced the risk of other cancers, other than breast cancer. 129 women who took Arimidex developed other cancers compared to 192 who were given placebo ( a pretend pill). Arimidex decreased the risk of endometrial cancer, ovarian cancer, lung cancer, and melanomas.
• Women were considered high risk if they had two or more blood relatives with breast cancer, a mother or sister who developed breast cancer before the age of 50, or a mother or sister who had breast cancer in both breasts.
• In the IBIS-11 trial, too few deaths were reported in the Arimidex Vs Placebo arms (2 versus 3 respectively) to make any conclusions about overall survival differences.
• Breast cancer incidence among post menopausal women at high risk for breast cancer continued to be significantly reduced 5.9 years after stopping five years of the Arimidex (average of 10.9 years post breast cancer treatment)

The IBIS-II Prevention trial enrolled 3,864 postmenopausal women at increased risk for developing breast cancer from 2003 to 2012. Among the participants, 1,920 were randomly assigned to anastrozole for five years and 1,944 to placebo.

The five-year adherence rate was 74.6 percent for Arimidex and 77.0 percent for placebo, which is not significantly different.