What Letrozole is

Letrozole is a non-steroidal triazole aromatase inhibitor, chemically close to anastrozole and working by the same reversible competitive mechanism. What separates them is potency: letrozole suppresses circulating oestrogen more completely and at a much lower milligram dose, which is why its tablets are typically 2.5 mg where anastrozole is 1 mg despite letrozole being the stronger of the two per unit of enzyme inhibition. It is the compound usually reached for when a triazole is wanted and anastrozole has proved insufficient, and the completeness of its suppression is the reason it is also the one most often described as easy to overshoot.

How Letrozole compares

Against anastrozole: same mechanism, same chemical family, materially greater potency and more complete suppression. Against exemestane: letrozole binds reversibly and exemestane does not, so their durations behave differently regardless of dose. Milligram-for-milligram comparison between any of the three is meaningless because their potencies differ several fold.