Formestane is a medicine that is known as an aromastase inhibitor and it prevents the production of the female hormone called oestrogen. It does this by inhibiting the aromatase enzyme which is responsible for the conversion of androgens to oestrogens. Formestane (4-hydroxyandrostenedione) is a new mechanism based aromatase inhibitor which irreversibly inactivates the enzyme, preventing the conversion of androstenedione to oestradiol and oestrone; its antitumour effect is mediated by a reduction in oestrogen synthesis.

Formestane is a powerful suicide aromatase inhibitor. This means that it decreases estrogen production by preventing the aromatase enzyme from doing its thing with androgens such as testosterone. It also decreases the receptor counts for both estrogen and progesterone. Formestane is now approved in Europe for the treatment of metastatic breast cancer in women who have failed tamoxifen therapy. Formestane is not orally bioavailable and is given intramuscularly each month.