Aromatase inhibitors or blockers are a class of drugs that are used to lower the concentration of estrogen in the blood and increase the levels of one's own testosterone and gonadotropin hormones. Aromatase blockers are also used to treat and prevent gynecomastia in men.
 
In bodybuilding, aromatase inhibitors are used most often during an anabolic steroid cycle for:
prevention of gynecomastia
increase in the level of anabolic hormones in the blood
increase relief (by eliminating the negative effects of estrogen)
prevention of hypertension
reduction of estrogen suppression of the hypothalamic-pituitary-testicular axis, leading to faster recovery.