Testolactone
A first-generation aromatase inhibitor derived from testosterone, formerly marketed as Teslac for breast cancer. Despite steroidal structure it is non-androgenic and irreversibly inhibits aromatase; used historically for gynecomastia and precocious puberty.
01 Overview
Testolactone (Teslac) is a d-homo steroidal lactone derived from testosterone. It was one of the earliest aromatase inhibitors, licensed for advanced breast cancer, and later used off-label for pubertal gynecomastia and, with other agents, in precocious puberty.
Although steroidal, it lacks meaningful androgenic activity. It has been superseded by more potent oral inhibitors and requires relatively high, frequent dosing, but remains historically important as a low-side-effect anti-estrogen.
02 Mechanism
Steroidal aromatase inhibitor: irreversibly inhibits the aromatase enzyme, reducing conversion of androgens to oestrogens; essentially non-androgenic.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Breast cancer | 250–1000 mg/day | Oral | Historic dose was 250 mg four times daily. |
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Lowered estrogenAromatase inhibition lowers oestrogen synthesis. | Estradiol reduction | Clinical | |
| Reduced pubertal gynecomastiaUsed historically to treat adolescent gynecomastia. | Regression of breast tissue | Observational |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| ParesthesiasNumbness or tingling of extremities reported in older labelling. | Mild | Uncommon | Clinical | |
| Gastrointestinal upsetNausea and anorexia with the high, frequent dosing required. | Mild | Common | Clinical |