Letrozol is femera, which is an anti-estrogen.
Femara is 10-30x more effective than Arimidex in it's ability to pass thru the cell membrane of lipid (fat) cells and inhibit the activity of aromatase -- in other words, Femara is far superior in lowering estrogen levels in fat cells. This has two benefits for BBs; (1) Estrogen 'attracts' water, so less water retention (2) an average male BB is around 10%BF, that's a lot of lipid cells with aromatase inside them, so a substantial percentage of aromatase is left untouched by Arimidex due to it's poor ability to enter lipid cellsArimidex is approximately 80% effective at inhibiting aromatase, Femara is around 95-97%
notes:
1. J Clin Endocrinol Metab 2000 Jul;85(7):2370-7
2. J Steroid Biochem Mol Biol 1997 Nov-Dec;63(4-6):261-7