Post-cycle therapy consists of a drug called a SERM, or Selective Estrogen Receptor Modulator. These drugs serve two purposes. First, they help to combat the effects of excess estrogen, which is beneficial in preventing estrogenic side effects like gynecomastia and weight gain after your cycle. More importantly, they stimulate the production of LH and FSH, easing your body back into natural testosterone production in mere days rather than weeks, months, or years. Failing to use a SERM could result in fat gain, muscle loss, and even testicular atrophy. For this reason, post-cycle therapy is a requirement – especially for men.