Formestane (lentaron, chemical name: 4-hydroxyandrostenedione) is a type I steroidal aromatase inhibitor (AI). It was developed for use in treatment of estrogen-receptor-positive breast cancer in post-menopausal women. For medical purposes, it is made in intramuscular depot form and injected. It is not used in oral form due to poor bioavailability, which makes the oral version inferior to aromasin, letrozole, and arimidex.
Supplement companies, with a bodybuilder customer base, have made it in topical form. It was available as part of gel stacks, which are rubbed onto the skin with no need for injection. That allowed transdermal delivery, bypassing the liver. In case you are wondering, the reason I use “was” is because formestane is now illegal and a scheduled drug.
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History and Ban
Formestane was derived from androstenedione, with the only difference being the addition of a 4-hydroxy group. From the late 2000's until its ban in late 2014, it grew in popularity among bodybuilders. Its success came from the ability to easily use it in topical gel form.
On December 18, 2014, the Designer Steroid Anabolic Control Act of 2014 was signed into law, making formestane illegal. Since the signing of the act, it has disappeared from the supplemental market onto the black market.
Formestane and Cancer
Some cancers require estrogen to grow, and aromatase is an enzyme that synthesizes estrogen. Formestane is a 2nd generation aromatase inhibitor that blocks this synthesis. Thereby, it lowers estrogen levels and slows cancer growth in the breasts of women.
There are 2 main uses for formestane, which help bodybuilders who use anabolic-androgenic steroids (AAS).
According to studies, formestane can lower SHBG up to 34%, which will increase free testosterone levels. SHBG (sex hormone binding globulin) is a protein that binds to sex steroids, and renders them inactive, for as long as SHBG is bound to them. There is a difference between 'free' testosterone, and 'bound' testosterone. Free testosterone is what matters, so lowering SHBG will increase 'free' testosterone. Higher testosterone means more strength, muscle mass, less body fat, better sense of well being, and libido.
Many anabolic steroids aromatize in the body into estrogen. The most common are deca durabolin, equipoise, testosterone, and dbol. If an AI is not used, estrogen will increase and potentially cause major problems with estrogenic side effects. The most common side effects of estrogen are gynecomastia (bitch tits), excessive water retention (moon face), high blood pressure (hypertension), and insomnia.
In addition, coming off cycle with estrogen levels that are too high can hinder recovery of the body's HPTA, causing gains on cycle to be lost. Formestane, while being used, will permanently disable the aromatase enzyme, so that it does not rebound after cycle. Other AI's, such as letrozole and arimidex will not stop estrogen rebound.
Suicidal AIs vs Non-Suicidal AIs
There are two types of aromatase inhibitors.
- Non-steroidal aromatase inhibitors such as arimidex, and the very potent letrozole, which inhibit the aromatase enzyme by reversible competition.
- Suicidal (irreversible) steroidal aromatase inhibitors, such as aromasin, which will form a permanent bond with the aromatase enzyme that permanently deactivates it. The reason we call these suicide AI's is because these compounds sacrifice their structural independence to disable the aromatase (estrogen) enzyme permanently.
Formestane falls into the 2nd category of suicide aromatase inhibitors, so no estrogen rebound should occur. After formestane use is discontinued, it will take time for aromatase to become active again.
Formestane does not cause many side effects unless abused. Sometimes bodybuilders will complain of dry joints, mood swings, libido changes, or rashes, which are typical of all AI's. Another side effect is lower HDL (good) cholesterol, so you have to watch your diet.
The most common version of formestane sold was in a topical gel form and it was dosed at 100-200mgs per application. Users would apply it onto clean, dry skin and rub in thoroughly; usually on the thighs or upper arms. It took 1 to 5 minutes to dry.
The injectable version for both breast cancer purposes and bodybuilders is 125mgs twice per week or 250mgs once per week.
Formestane half life is about 4 days, which is in line with other aromatase inhibitors.
Want to read about Formestane on our forums? check out these threads: