Anti-estrogens are the class of drugs which is usually taken during anabolic steroids cycles. Extremely high concentration of anabolic steroids hormones in the body, especially testosterone, leads to the fact that the excess of testosterone does not have time to completely metabolize when passing the physiological pathway. In this case, part of the anabolic steroid hormones passes through the bypass path of metabolism - aromatization, in which testosterone is converted into estrogen by the enzyme aromatase. This leads to development of gynecomastia, edema, fat deposition by female type etc.
Anti-estrogens, especially Aromatase inhibitors are used during steroid cycles to prevent gynecomastia, increase the level of anabolic hormones in the blood, prevent hypertension, reduce estrogenic suppression of the axis of the hypothalamus-pituitary-testicles. Also by eliminating the negative effect of estrogens the drugs increase lean muscles, but also can cause some side effects such as braking muscle growth, joint pain, decrease bone strength, increase cholesterol and depression.