Anti Estrogens suppements are commonly used as post cycle therapy (PCT) to help combat the effects of this increased estrogen. We see them as an "insurance policy" to protect against side effects from using highly ananbolic/androgenic pro-hormone supplements. Whenver testosterone levels are increased, estrogen has a tendancy to increase as well. This is part of the natural homeostatic feedback response mechanism that is built into the human body. Excess estrogen should be avoided as it can lead to many negative side effects like gynecomastia and mood swings.