PharmaceuticalAromatase inhibitorAnti-estrogenSteroidal AI

Formestane

Also known as Lentaron · 4-hydroxyandrostenedione · 4-OHA

Written by enwyReviewed 15 Jul 20261 sources0 comments

A first-generation steroidal (type I) aromatase inhibitor, once marketed by injection for breast cancer. It irreversibly inactivates aromatase and has been sold in transdermal 'legal' forms and as a prohormone anti-estrogen.

01 Overview

Formestane (4-hydroxyandrostenedione, Lentaron) was the first selective aromatase inhibitor used clinically, administered as a depot intramuscular injection for postmenopausal breast cancer before oral third-generation inhibitors superseded it.

As a steroidal, suicide-substrate inhibitor it binds aromatase irreversibly. It has circulated in the supplement grey market as a transdermal anti-estrogen and appears on the WADA prohibited list. Oral bioavailability is poor due to extensive first-pass metabolism.

02 Mechanism

Steroidal (type I) aromatase inhibitor: acts as a suicide substrate that irreversibly inactivates the aromatase enzyme, lowering oestrogen synthesis from androgens.

03 Dosing

Tier	Dose	Route	Notes
Grey-market topical	100–300 mg/day	Transdermal	Supplement-era topical dosing; poorly standardised.
Breast cancer	250 mg/wk	IM	Historic depot injection every two weeks.

04 Effects

Effect	Magnitude	Evidence
Lowered estradiolIrreversible aromatase inactivation reduces oestrogen synthesis.	Marked estradiol suppression	Clinical
Reduced water retention / gyno riskLower estradiol reduces fluid retention and breast tissue stimulation on cycle.	Less oestrogenic bloat	Observational

05 Side effects

Effect	Severity	Frequency	Evidence	Countermeasures
Injection site reactionsLocal pain, sterile abscess and inflammation at depot sites.	Mild	Common	Clinical
Estrogen deficiency symptomsJoint pain, low libido and adverse lipids from over-suppressed estradiol.	Moderate	Common	Clinical

06 Legal status

United States

Not FDA approved

Formerly sold in some supplements; not a US-approved drug. · as of Jan 2026

07 References

Formestane, a new steroidal aromatase inhibitor in breast cancerJ Steroid Biochem Mol Biol

08 Discussion0 comments

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This page is reference information, not medical advice. Doses and protocols are documented as they appear in the clinical literature and in practice — describing them is not a recommendation to use them. Countermeasures listed here are not a substitute for a physician. Legal status varies by jurisdiction and changes.