B tier · viable
Mesterolone retains a current marketing authorisation with a manufacturer SmPC carrying real pharmacokinetic data, and because it is 1alpha-methylated rather than 17alpha-alkylated it lacks the signature hepatotoxicity of the oral alkylated steroids — but efficacy evidence for its labelled indications remains thin by modern standards.
mesterolone
Mesterolone is an orally active dihydrotestosterone derivative that achieves oral bioavailability through 1alpha-methylation rather than 17alpha-alkylation, and it remains an approved prescription androgen in several European and other markets. Its manufacturer label describes it as well tolerated by the liver, though it carries the androgen-class contraindications and the usual caveats about tumour risk with hormonal substances.
the explanation
Mesterolone is a prescription male hormone tablet still sold in some countries under the name Proviron. Unlike most steroid pills it is not built in the way that typically damages the liver, so it is considered one of the gentler members of this family.
regulatory status
US Schedule III controlled substance; approved medical indication retained outside the US
Enumerated at 21 CFR 1308.13(f)(45) as 'mesterolone (1alpha-methyl-17beta-hydroxy-5alpha-androstan-3-one)'. Mesterolone has never been marketed as an approved medicine in the United States, but Proviron 25 mg tablets hold current marketing authorisation from Bayer in a range of European and international markets, with a November 2023 SmPC listing indications in androgen deficiency, hypogonadism and male infertility associated with oligozoospermia or Leydig-cell insufficiency. Anabolic steroids as a class are Class C under the UK Misuse of Drugs Act 1971, and mesterolone is prescription-only where authorised. Prohibited at all times in sport under WADA.
how it works · proposed mechanism
Mesterolone is a non-aromatisable androgen receptor agonist made orally durable by A-ring rather than D-ring modification.
1alpha-methyl, not 17alpha-alkyl
Oral activity is achieved by methylation at C1alpha on the A-ring, which slows hepatic inactivation without the C17alpha alkyl group. This structural choice is the reason the label states the compound is well tolerated by the liver, in contrast to alkylated oral steroids.
5alpha-reduced, non-aromatisable
As a dihydrotestosterone derivative the A-ring is already saturated, so mesterolone cannot be converted to an oestrogen by aromatase. Receptor signalling is therefore purely androgenic with no oestrogenic metabolite.
Androgen deficiency substitution
The SmPC positions mesterolone as balancing a deficiency of endogenous androgen formation, targeting symptoms such as reduced libido, fatigue and mood disturbance. It states that at recommended therapeutic exposure spermatogenesis is not impaired, distinguishing it from suppressive androgen replacement.
what’s reported
evidence shape
5 sourcesThe composition of the cited literature by study type. This describes the shape of the evidence, not its quality — and it does not by itself set the grade.
⚠ the catch
Being liver-friendly relative to its class is not the same as being benign: mesterolone is still a controlled anabolic steroid with prostate cancer as an absolute contraindication and rare reports of benign and malignant liver tumours attributed to hormonal substances of this type. The efficacy evidence behind its labelled indications, particularly male infertility, is historic and would not satisfy contemporary regulatory standards.
key published findings
- Human PK (Bayer Proviron SmPC, Nov 2023): serum mesterolone peaks approximately 1.6 h after oral administration and declines with a terminal half-life of 12-13 hours.
- Human regulatory (SmPC): approved indications cover androgen-deficiency symptoms (fatigue, impaired concentration, reduced libido and potency, mood disturbance), hypogonadism and development of secondary sexual characteristics, and infertility associated with low sperm count or Leydig-cell insufficiency.
- Human label statement: the SmPC explicitly states that at the recommended therapeutic dosage Proviron will not impair spermatogenesis, and separately that the drug is well tolerated by the liver — a claim not made for 17alpha-alkylated orals.
- Human label safety: prostate carcinoma and previous or existing liver tumours are listed as absolute contraindications, and the label notes that benign and malignant liver tumours have rarely been observed after use of hormonal substances of this type, with potential for life-threatening internal bleeding.
- Regulatory (21 CFR 1308.13(f)(45)): despite ex-US approval, mesterolone is a US Schedule III anabolic steroid with no US marketing authorisation.
limitations of the evidence
- Efficacy evidence for the labelled infertility and androgen-deficiency indications predates modern randomised controlled trial standards; the SmPC is not itself trial evidence.
- Published pharmacokinetic data are limited essentially to the manufacturer's own summary, with little independent replication in the peer-reviewed literature.
- The 'well tolerated by the liver' characterisation is a manufacturer statement at therapeutic exposure and says nothing about the supratherapeutic exposures encountered in non-medical use.
documented safety signals
- Absolute contraindication in prostate carcinoma; androgen therapy of any kind can stimulate growth of androgen-dependent prostate tissue, and the label mandates prophylactic prostate examination.
- Contraindicated in previous or existing liver tumours; the label records rare benign and malignant hepatic tumours after hormonal substances of this class, with risk of life-threatening intra-abdominal haemorrhage.
- HPG-axis suppression: although the SmPC states spermatogenesis is not impaired at recommended therapeutic dosage, any exogenous androgen exposure above replacement can suppress LH/FSH, endogenous testosterone production and spermatogenesis.
- Non-aromatisable DHT-class androgens do not generate oestrogenic metabolites, removing the oestrogen-mediated protective effect on HDL cholesterol and bone; adverse lipid shifts remain a class concern for androgens generally.
- Virilisation risk in women and children: the label restricts administration to male patients; androgenic effects such as voice deepening, hirsutism and clitoral enlargement can be irreversible in females.
- Androgenic dermatological and urological effects: acne, seborrhoea, androgenic alopecia, benign prostatic hyperplasia and urinary symptoms.
- In prepubertal exposure, androgens can cause premature epiphyseal closure and accelerated bone maturation.
- Not exempt from class hepatotoxicity monitoring in non-medical high-exposure settings, where product identity and purity are also unverified.
identity
| full name | Mesterolone (1alpha-methyl-17beta-hydroxy-5alpha-androstan-3-one) |
| category | Anabolic & Androgenic |
| modality | steroid |
| formula | C20H32O2 |
| molar mass | 304.5 g/mol |
| cas | 1424-00-6 |
| half-life | 12-13 h (terminal, oral; Bayer SmPC) |
laboratory handling
| storage | Solid; store tightly closed in a light-resistant container at controlled room temperature per the manufacturer label. Analytical reference material is typically held refrigerated or frozen. |
| solubility | Practically insoluble in water; soluble in ethanol and DMSO, typical of neutral 5alpha-reduced C19 steroids. |
Handling information describes laboratory practice for a research reagent. It is not a preparation guide for use in humans.
the receipts
5 citedBayer AG · 2023 · official
Bayer AG · 2023 · official
Electronic Code of Federal Regulations (DEA) · 2026 · official
PubChem, US National Library of Medicine · 2026 · official
British Journal of Pharmacology · 2008 · review
others in Anabolic & Androgenic