An anti-estrogen, simply put, is an estrogen blocker. Males and females have two major hormones in the body that is estrogen and testosterone. Guys have more testosterone and girls have more estrogen. Unfortunately for guys, estrogen carrys a strong signal to the pituitary gland in the brain, which controls your "on" and "off" switch for testosterone production. What happens when estrogen levels rise higher than normal in males, the body's testosterone production lowers, which isn’t good, because you’re trying to build lean muscle mass.

Anti Estrogen
