Letrozole is a third generation aromatase inhibitor. Like most aromatase inhibitors it was developed to fight breast cancer by inhibiting aromatization via the aromatase enzyme. It is similar to arimidex, however letrozole is much more potent in it's ability and is used primarily when other treatments such as selective estrogen receptor modulators have failed to produce results. This is why the majority of the research about letrozole has been conducted on post-menopausal women, but of course some of the findings can be translated over for the benefit of athletes.