Anastrozole and Letrozole are both effective in the treatment of breast cancer and there is no particular difference between the two. Anastrozole and letrozole are both antitumor drugs that are used to treat advanced breast cancer in postmenopausal women that cannot be controlled with tamoxifen and other anti-hormonal treatments. They may also be helpful in the adjuvant treatment of early breast cancer in postmenopausal women who are estrogen positive. Treatment with anastrozole may be considered for patients who are estrogen receptor negative and have a positive clinical response to tamoxifen. Adverse effects such as nausea, bone pain, arthralgia, and dyspnea may occur with the use of both medications, and patients are advised to choose to use them according to their condition under the guidance of their physician.