Formestane (Lentaron) is a type I, steroidal aromatase inhibitor. It is used in the treatment of estrogen-receptor positive breast cancer in post-menopausal women. It is available as an intramuscular depot injection.

Formestane is often used to suppress estrogen production from anabolic steroids or prohormones. It also acts as a prohormone to 4-hydroxytestosterone, 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.