Femara (letrozole) is an aromatase inhibitor that works by suppressing the production of estrogen. In postmenopausal women estrogen is still produced in the body by the adrenal gland. Decreasing this production is a way of decreasing the risk of recurrence.

Tamoxifen on the other hand works by blocking the estrogen receptors on the cancer cell itself resulting in slower growth of the cell or cell death. Tamoxifen can be given to post or premenopausal women but Femara would only be prescribed to women that are postmenopausal. The reason being, is that a premenopausal women will still have estrogen produced by the ovaries. Femara would not be effective in this instance.

A Phase III clinical trial that is ongoing shows preliminary results that Femara is superior than Tamoxifen in women with early stage disease that are postmenopausal. The results of the study were published in the Annals of Oncology.