What is Letrozole?
Letrozole (brand name Femara) is the most potent of the non-steroidal aromatase inhibitors. It lowers estradiol even more strongly than anastrozole - by over 98% in clinical studies. That strength is both advantage and risk: letrozole can reverse active gynecomastia, but also crashes estradiol too low quickly if you are not careful.
This information is for research and educational purposes only. No medical advice.
How it works
Like anastrozole, letrozole binds aromatase reversibly, just much more strongly. The near-complete estradiol suppression makes it the choice when other AIs are not enough - for example with already visible gynecomastia you want to push back.
Dosing
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What the research shows
The BIG 1-98 study (NEJM, 2005) compared letrozole with tamoxifen in postmenopausal breast cancer and showed significant superiority of letrozole in disease-free survival. That confirms the very strong estrogen lowering that makes letrozole so potent in research contexts.
Side effects
- Estradiol crash: the main risk - joint pain, libido loss, lethargy, poor sleep.
- Lipids: very low estradiol worsens cholesterol markedly.
- Bone density: relevant with long-term use.
Storage
Store dry and protected from light. Out of reach of children.