Letrozole is a third generation Aromatase Inhibitor (AI) that works by attaching to -and eliminating the aromatase enzyme resposible for producing estrogen. This makes it a perfect choice to combat gynocomastia which it will completely prevent. Letrozole also quite raises LH levels which deems it suitable for post-cycle use. The human body does need estrogen to maintain proper immune function and balance cholesterol ratios in the blood. Since letrozole is so strong at eliminating estrogen, it is not suited for long term use.