Estrogen is an unwanted by-product of Steroid use. In some tissues of the male body, there is a nasty little Enzyme called the “Aromatase” Enzyme. This Enzyme is responsible for turning Testosterone and other male hormones into female ones like Estrogen, and Estradiol or other girly-like hormones. An abundance of Female hormones in the male body can cause a whole list of bad side-effects, like male breast tissue growth and water retention. Bodybuilders use a lot of different drugs and compounds to combat these bad side effects. The best way to keep Estrogen from causing problems is to make sure it is never created in high amounts in the first place. Using a compound that inhibits the Aromatase Enzyme is a sure way to make sure Testosterone and other Steroids are never converted into side-effect causing Estrogen.
Aromasin is a suicidal Aromatase inhibitor, which makes it a better choice than other AIs. Exemestane binds with aromatase enzymes and then subsequently permanently disables the enzyme by destroying it.
Aromasin is a steroidal AI which keeps it from negatively affecting your lipids like other non-steroidal AIs. It is also not liver toxic, and can be ran for longer lengths of time without negative effect. This makes it a better option to run on cycle from a pure health standpoint. Aromasin will not hurt your gains on cycle like other AIs, but may actually help them. Another benefit on cycle is that steroidal AIs lower SHBG which increases the ratio of free to bound testosterone. What that means during your cycle is a very positive impact on your gains.