Anastrozole is an oral aromatase inhibitor developed by Zeneca Pharmaceuticals. Anastrozole (Arimidex) was approved by the FDA near the end of 1995. It is described as a non-steroidal, Type II inhibitor, because it inhibits the enzyme by reversible competition. This type of inhabitation does not render the enzyme permanently inactive and normal enzyme function will be restored upon clearance of the drug.

With type I inhibitors a permanent bond with the aromatize enzyme complex is formed and prolonged inhibition may be experienced even after the drug has cleared from circulation. See the exemestane profile.

Aromatase inhibitors (AI) are a class of drugs used in the treatment of breast cancer and ovarian cancer in postmenopausal women, but bodybuilders use them to stop the conversion (synthesis) of androgens into estrogens during cycles of anabolic steroids. This is useful because keeping estrogen levels under control will allow for a greatly reduced chance of estrogenic side-effects like gynocomastia and water-retention. Anastrozole is very popular amongst athletes due to its effectiveness and 0.5 mg to 1 mg a day may reduce serum estradiol by about 50% in men. In the majority of cycles 1mg is enough to prevent estrogen levels from rising high enough to cause estrogenic side-effects.

Side effects associated with the use of aromatase inhibitors include nausea, vomiting, hot flashes, joint pain, weakness, fatigue, mood changes, depression, high blood pressure and headache. Bone mineral density may also decrease, which may lead to osteoporosis. Extended periods of low estrogen levels may lead to cardiovascular disease and immune system changes, so limit usage to a maximum of two months.