Femara is the brand name of a drug made by Novartis containing letrozole, which is an aromatase inhibitor predominantly used in the treatment of hormonally-responsive breast cancer, after surgery. It is also used in PCOS to stimulate ovulation, usually where Clomid has failed, although some physicians prefer it to Clomid believing it to be more effective at inducing ovulation and having a lower risk of birth defects and other side effects such as hot flashes, mood swings, nausea etc. The use of letrozole in this manner was first introduced in the early 1990s by Dr. Robert Casper from Toronto, Canada who published a paper on it in 2009.
Letrozole suppresses the activity of the aromatase enzyme and thus blocks the production of oestrogen by those cells that express this enzyme. In women with PCOS, too much oestrogen is produced in the fat cells which use the aromatase enzyme to convert androgens which are circulating in the bloodstream into oestrogen. When oestrogen levels are lowered like this, it causes the body to produce more follicle-stimulating hormone or FSH which can cause a mature follicle to develop and then rupture, thus triggering ovulation.
Letrozole is commonly prescribed on days 5 through 9 of the menstrual cycle in a dose of 2.5 mg daily, though 5 mg or 7.5 mg may eventually be used if 2.5 mg is unsuccessful.
Casper RF. (2009) Letrozole versus clomiphene citrate: which is better for ovulation induction?. Fertility and sterility, 92(3), 858-9. PMID: 17588568
Gill SK, Moretti M, & Koren G. (2008) Is the use of letrozole to induce ovulation teratogenic?. Canadian family physician Medecin de famille canadien, 54(3), 353-4. PMID: 18337526
Mitwally MF, Biljan MM, Casper RF. Pregnancy outcome after the use of an aromatase inhibitor for ovarian stimulation. Am J Obstet Gynecol. 2005;192(2):381–6. [PubMed]