Formestane belongs to a class of drugs known as 2 aromatase inhibitors. It is used in the treatment of estrogen-receptor positive breast cancer in post-menopausal women. It is available as an intramuscular depot injection (Lentaron).

Formestane is often used to suppress estrogen

Estrogen

Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...

 production from anabolic steroids or prohormones. It also acts as a prohormone to 4-hydroxytestosterone

4-Hydroxytestosterone

4-Hydroxytestosterone is an anabolic steroid. It was first patented by G.D. Searle & Company in 1955 and is testosterone with a hydroxy group at the four position. 4-OHT has moderate anabolic, mild androgenic, and anti-aromatic properties and is similar to the steroid clostebol....

, an active steroid which displays weak androgenic activity in addition to acting as a mild aromatase inhibitor.

Formestane has poor oral bioavailability and as such is no longer popular as many orally active aromatase inhibitors have been identified.

It is selective.