Androstenedione (4-Andro)
Androstenedione (4-Andro) is a naturally occurring steroid hormone and direct precursor to testosterone, marketed as a testosterone-boosting prohormone. Despite early clinical study, oral supplementation raises estradiol at least as much as testosterone and confers little anabolic benefit — it was banned as a supplement in the US in 2004.
01 Overview
Androstenedione is an endogenous 17-ketosteroid produced by the adrenals and gonads, sitting one enzymatic step from testosterone via 17β-hydroxysteroid dehydrogenase. It became famous in the late 1990s when it was linked to baseball home-run records and sold as a legal testosterone precursor. Unlike methylated designer steroids, it is a true prohormone: it must be enzymatically converted to testosterone in the body, but a large fraction is instead aromatised to estrone and estradiol.
Controlled human studies found that oral androstenedione produced modest, transient rises in serum testosterone at best and substantial rises in estradiol, with no meaningful gain in lean mass or strength beyond resistance training alone. It was added to the US Controlled Substances Act as a Schedule III anabolic steroid in 2004. It remains prohibited in sport and detectable on standard steroid screens.
02 Mechanism
Serves as a substrate for 17β-HSD, which converts it to testosterone, and for aromatase, which converts it to estrone/estradiol; net androgenic effect is limited by preferential aromatisation and first-pass metabolism.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Common (marketed) | 100–300 mg/day | Oral | Historic supplement dosing; poor oral bioavailability meant most doses were largely wasted or aromatised. |
Ester comparison
| Ester | Half-life | Injection freq. | Testosterone by mass |
|---|
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Transient testosterone riseSerum testosterone rises briefly in some individuals, often not exceeding the normal physiologic range. | small/short-lived | Clinical | |
| Minimal lean mass gainRandomised trials showed no significant advantage over resistance training with placebo. | ≈0 beyond training | Clinical |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Reduced HDL cholesterolChronic use lowered HDL in study participants. | Mild | Common | Clinical | |
| Elevated estradiolAromatisation raises estrone and estradiol, which can drive gynecomastia and water retention. | Moderate | Common | Clinical |