What is the difference between femara (letrozole) and arimidex?

Both interfere with . Estrogen production and are used in treatment of breast cancer where the primary was er+. Side effects are similar, cost is much higher fro Letrozole (femara). It is newer. These are each used only in postmenopausal patients. Tamoxifen around for decades is used in premenopausal.
Both AI. Femara (letrozole) is letrazole, an aromatase inhibitor that is slightly more potent than Arimidex though belong to the same class of drugs.