Formestane (Lentaron)

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Formestane Description:

Formestane is classified as a selective irreversible steroidal aromatase inhibitor. This agent is structurally a derivative of androstenedione, differing from this well known prohormone only by the addition of a 4-hydroxyl group. This group, however, is responsible for causing an irreversible attachment between formestane and aromatase when the two come into contact with each other. This means that formestane will bond with the enzyme and never let it go, permanently deactivating it as a result. The enzyme will need to be replaced, through normal attrition, before the body will recover its lost estrogen synthesizing capacity. This may take several days or more following cessation of therapy. Given this mode of action, formestane is also referred to as a “suicide” aromatase inhibitor, as the drug essentially sacrifices itself in the process of blocking estrogen conversion. As a class of drugs, aromatase inhibitors are used (off-label) by male bodybuilders and athletes to prevent the estrogenic side effects of certain anabolic/androgenic steroids, and to increase fat loss and muscle definition while dieting.

Because of its potent estrogen-suppressing action, formestane has been used clinically to treat breast cancer patients in a number of countries including England, Germany, Switzerland, Spain, Australia, New Zealand, Italy, and Malaysia. It has been shown to be an effective option as a second line of defense after tamoxifen, an estrogen receptor antagonist, has failed to elicit a positive response with patients, and produces an overall response statistically similar to tamoxifen when administered as the first-line therapy. In terms of overall potency, formestane is not as strong as the selective third generation inhibitors like Arimidex (anastrozole) or Femara (letrozole). One study, for example, notes a 79% level of suppression of estrogen levels with 4 weeks use of Arimidex 1 mg daily (on par with levels noted with Femara use), but only a 58% level of suppression with intramuscular formestane injections (250 mg every two weeks)¹. But next to estrogen receptor antagonists like Clomid (clomiphene citrate) and Nolvadex (tamoxifen citrate), formestane is significantly more effective at blocking the effects of estrogen in the body.


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. It was sold by Novartis, which marketed Lentaron Depot in two-dozen countries including Argentina, Austria, Belgium, Brazil, Canada, Chile, Czech Republic, Denmark, France, Germany, Greece, Hong Kong, Ireland, Israel, Italy, Malaysia, Netherlands, Portugal, South Africa, Spain, Switzerland, Turkey, and the United Kingdom. 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 Depot preparations have since been discontinued. The drug remains available today, but only in a small number of nations. This includes Austria, Brazil, Czech Republic, Hong Kong, and Turkey.

How Supplied:

Formestane is most commonly supplied in a sterile solution containing 125 mg/mL of drug in a 2 mL ampule.

Structural Characteristics:

Formestane is classified a steroidal selective irreversible aromatase inhibitor. It has the chemical designation 4- Hydroxyandrost-4-ene-3,17-dione.

Side Effects:

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. Some individuals may also respond to the medication with gastrointestinal side effects including nausea and vomiting. Aromatase inhibitors can harm the development of an unborn fetus, and should never be taken or handled during pregnancy. When taken by men (as an off-label use) to reduce estrogenicity during prolonged periods of steroid treatment, aromatase inhibitors may increase cardiovascular disease (CVD) risk by retarding some beneficial properties of estrogen on cholesterol values. Studies have demonstrated that when an aromatizable steroid such as testosterone enanthate is taken in conjunction with an aromatase inhibitor, suppression of HDL (good) cholesterol levels becomes significantly more pronounced. Since the estrogen receptor agonist/antagonist Nolvadex® generally does not display the same anti-estrogenic (negative) effect on cholesterol values, it is usually favored over aromatase inhibitors for estrogen maintenance by male bodybuilders and athletes concerned with cardiovascular health.


Formestane is indicated for the treatment of advanced breast cancer in postmenopausal women. The recommended dosage is 250 mg by intramuscular injection (buttock) every two weeks. Although not a medically approved form of the drug, studies have demonstrated that a similar level of estrogen suppression can also be achieved with oral use of formestane. Due to poor bioavailability, however, the dose needed is around 250 mg per day . When used (off-label) to mitigate the estrogenic side effects of anabolic/androgenic steroid use or increase muscle definition, male athletes and bodybuilders often take 250 mg every two weeks by injection, or 250 mg per day orally.


Formestane is not widely available as a prescription drug, and consequently is rarely circulated in the athletic community.



1. A randomised comparison of oestrogen suppression with anastrozole and formestane in postmenopausal patients with advanced breast cancer. Kleeberg UR, Dowsett M, Carrion RP et al. Oncology. 1997;54 Suppl 2:19-22.ave there been changes to the local availability of pharmaceutical anabolic steroids in your country, or can you photograph an item we don’t have? Please Contact Us so that we may update our database and let others know. is a community effort. Thank you! Be safe. – WL

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By | 2017-04-07T01:11:05+00:00 April 17th, 2015|Categories: Anti-Estrogen Profiles|0 Comments

About the Author:

William Llewellyn is a researcher in the field of human performance enhancement. He is also author of the bestselling ANABOLICS book series, most recently the ANABOLICS 10th Edition. William is an active supporter of the harm reduction community, and currently serves as honorary lecturer at the Centre for Public Health at Liverpool John Moores University.

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