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

Different: Tamoxifen is a selective estrogen agonist/antagonist. It binds to estrogen receptors and depends on which organ it's on, it either exerts anti-estrogen or estrogen effects. Femara (letrozole) is an aromatase inhibitor. It inhibits enzymatic conversion of androgens to estrogen thereby lowering estrogen level.

Mechanism of action: The two drugs are similar. However, the way they work (mechanism of action) is different. They are used to treat hormone sensitive cancers and (tamoxifen) high risk patients. If you are premenopausal, because of the different mechanisms of action, you should not take femara (letrozole).