Letrozole is the chemical name of Novartis selective third generation aromatase inhibitor (AI), a drug that works by blocking the aromatase enzyme responsible for the production of estrogen.
In clinical use, Letrozole is primarily administered to halt the progression of breast cancer in women. It is generally used as part of an aggressive treatment in post-menopausal women, to fight and reverse the spread of breast cancer after other treatments (such as Tamoxifen therapy) has failed. Its probably the most efficient product on the market for this purpose (5). Letrozole is very similar in both structure and action to its AI predecessor Arimidex.
In athletics and bodybuilding, it is used as an ancillary compound within anabolic steroid cycles for its estrogen reducing properties, and has the additional benefit of modestly increasing testosterone levels. Many anabolic steroids aromatize (convert to estrogen via the aromatase enzyme), a process that is responsible for many of the undesirable side effects which accompany anabolic steroid use such as acne, gynecomastia, water-retention, etc.
Letrozole also does quite a few things that would be of interest to both bodybuilders and athletes. Firstly, it has been shown to reduce estrogen levels by 98% or greater (1). In at least one documented incident, Letrozole reduced test subject estrogen to undetectable levels, while increasing LH, FSH and SHBG (4). For the bodybuilder, less estrogen in the body means less chance of estrogen-related side effects. This makes Letrozole an appropriate choice for even the heaviest bulking or cutting cycles, including those which incorporate harsher androgens. Also, if you are a competitive bodybuilder, Letrozole is a must have product for contest preparation as no other ancillary compound supports the coveted dry and tight appearance quite as well.
An effective dose of Letrozole is .25 to .5mg/day. I use .25mgs/day, but be forewarned, if you go over this amount it can kill your sex drive. Also worth noting is Letrozoles substantial estrogen rebound effect that occurs after discontinuation. Maximum inhibition of the aromatase enzyme has been cited at doses as low as 100 mcg. (2)
Its effects on serum lipids including cholesterol, both HDL and LDL are, in the words of one researcher: "inconsistent." However, you could certainly suffer an impaired lipid profile and immune system if estrogen levels remain too low for long periods of time.
As previously mentioned, Letrozole can be used to raise LH and FSH, these are hormones which signal your testes to produce more testosterone. (6) These properties mean that Letrozole can be used for post-cycle therapy (PCT), and I have successfully used it for this purpose. However, for various reasons, Tamoxifen is a better PCT choice.
How good is Letrozole when compared with Aromasin (Exemestane) and Arimidex (Anastrozole), its primary rivals? Letrozole is 10-20x more potent than Anastrozole, and about as potent (but with a slightly different mechanism) as Exemestane. It also long-lasting. Letrozole has a whopping 2-4 day half-life, and youll need to take Letrozole for approximately 60 days (with wild variance of 2-6 weeks) to achieve a steady blood plasma level (8).
Those are impressive numbers, but heres one of the most interesting things about Letrozole:
It may reduce/eliminate/reverse existing gynecomastia!
In a study conducted on mice (and yes, I know its not perfect), gyno-like changes in the mammary gland were totally destroyed! Heres a direct quote from that study:
Our results also indicate aromatase overexpression-induced changes in mammary glands can be abrogated [destroyed] with very low concentrations of the aromatase inhibitor, letrozole.(7)
In addition, both I and a friend successfully used Letrozole to eliminate our gyno while both using 2.5mgs/day, tapering down to .25mgs/day, and then finally off. The gyno never returned for either of us.
Based on its availability and cost (when you consider the fact that .25mgs/day is more than enough protection from estrogen-related sides on most cycles), not to mention its overall utility for a variety of functions (destroying gyno, preventing estrogenic sides, and for PCT), Id say that this stuff is pretty great.
1. Clin Cancer Res. 2005 Apr 15;11(8):2809-21.
2. J Clin Endocrinol Metab. 1995 Sep;80(9):2658-60.
3. Eur J Obstet Gynecol Reprod Biol. 2002 Nov 15;105(2):161-5
4. Epilepsy Behav. 2004 Apr;5(2):260-3
5. Semin Oncol. 2004 Dec;31(6 Suppl 12):3-8.
6. Diabetes Obes Metab. 2005 May;7(3):211-5.
7. J Steroid Biochem Mol Biol. 2001 Dec;79(1-5):27-34. Aromatase overexpression transgenic mice model: cell type specific expression
and use of letrozole to abrogate mammary hyperplasia without affecting normal physiology.
8. (Clin Cancer Res. 2003 Jan;9(1 Pt 2):468S-72S.).