Formestane (trade name Lentaron) is a type I, steroidal, selective aromatase inhibitor used in the treatment of estrogen receptor-positive breast cancer in postmenopausal women. Formestane has poor oral bioavailability and thus must be administered fortnightly (bi-weekly) by intramuscular injection. Formestane is a second generation, irreversible, steroidal aromatase inhibitor. It inhibits the aromatase enzyme responsible for converting androgens to estrogens, thereby preventing estrogen production. Estrogen-sensitive breast cancer cells depend on estrogen for viability. Thus removal of estrogen from the body can be an effective treatment for hormone-sensitive breast cancers. Common side effects associated with the use of an aromatase inhibitor include hot flashes, joint pain, weakness, fatigue, mood changes, depression, high blood pressure, swelling of the arms/legs, and headache. Aromatase inhibitors may also decrease bone mineral density, which may lead to osteoporosis and an increase in fractures in susceptible patients. Formestane was the first selective aromatase inhibitor to be developed as a prescription drug, first appearing in Europe during the mid-1990s under the Lentaron Depot brand name. With the emergence of newer and more effective aromatase inhibitors, however, formestane soon lost market presence at a rapid rate. Most of the initial Lentaron preparations have since been discontinued. Currently, formestane (categorized as an anti-estrogenic agent) is prohibited from use in sports in accordance with the regulations of the World Anti-Doping Agency. The drug remains available today, but only in a small number of nations. This includes Austria, Brazil, Czech Republic, Hong Kong, and Turkey.