Letrozole (Generic name Femera) is a specific and potent aromatase inhibitor
Aromatase is the enzyme that is responsible on conversion of estrogen precursors (such as androgens=male steroids) to estrogen in peripheral tissues like fat tissue, breast and the brain.
Letrozole interact directly and specifically with Aromatase and neutralize it, thus lowers and prevents estrogen synthesis from its root. This mechanism is different then drugs like Tamoxifen and Clomifen which don't have any effect on estrogen synthesis, but interact with estrogen receptors to lower and prevent estrogen effects.
So Tamoxifen and Clomifen treat estrogen effects, and Letrozole lowers estrogen levels in first place.
Letrozol was developed for the treatment of advanced breast cancer in postmenopausal women.
Like other anti estrogen it's used successfully for male infertility and treatment of gynocomastia (abnormal breast tissue growth in males)
Letrozole was proven to some extent to be more effective then Clomifen to treat infertility in males. For this purpose a dosage of 1-2.5mg per day is required for 3 weeks, with one week break, and repeat on this cycle according to results
Letrozole is exceptionally effective in lowering estrogen serum levels, high estrogen levels are the direct cause of gynocomastia, thus Letrozole was proven effective as a first measure to prevent gynocomastia. Athletes who are using exogenous Testosterone, and other steroids that can be converted to estrogen such as Dianabol, are prone to estrogenic side effects. A dosage of 0.5-2.5mg per day is necessary in such cases along the steroid cycle. The dosage is dependent on the individual sensitivity to estrogen.
Letrozole may lower as well gynocomastia symptoms after this phenomena already occur though it can't treat it completely, and is advanced cases of gynocomastia a surgery intervention is required to remove the tissue.
Overall, and compared all the anti estrogens in the market, Letrozole has the strongest impact on lowering estrogen serum levels, and prevention of estrogenic side effects, hence in individual cases with high sensitivity to estrogen Letrozole is the drug of choice. How ever in other individuals the high estrogenic suppressing effect of Letrozole is unnecessary and other anti estrogens such as Arimidex and Aromsin are overally more effective. This is because moderate estrogen serum levels are required for optimal anabolic environment in tissues such as muscle, bones and ligaments