EMA / German Federal Institute for Drugs (BfArM)
Bayer Proviron-25 SmPC (Summary of Product Characteristics)
Bayer AG
EmergingResearch compound
Proviron (Mesterolone)
Mesterolone, 1α-methyl-DHT oral androgen Bayer Rx (Proviron-25, registered in DE/AT/PL markets). NOT 17α-alkylated → low hepatotoxicity. During AAS cycle a SHBG suppressor and "AI mimicker" adjunct; NOT a standalone bulker.
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WHAT IS PROVIRON (MESTEROLONE)?
Detailed overview
Proviron (Mesterolone) is a 1α-methyl-dihydrotestosterone derivative oral androgen, synthesized by Schering AG in 1966 and marketed under the Proviron brand (Bayer since 2006). Bayer Rx in Germany, Austria, Poland (Proviron-25, 25 mg tablet) – original clinical indications: male subfertility, delayed puberty, hypogonadism, libido decline in older men. Anabolic:androgenic ratio ~100:30-40 (weak anabolic, moderate androgenic). NOT 17α-alkylated (only 1α-methyl) → low hepatotoxicity. Does NOT aromatize (NOT a CYP19 substrate). HIGH SHBG affinity → "AI mimicker" effect: Proviron bound to SHBG reduces the SHBG-T fraction, so free-T (and free-E2) rises alongside stack-partner Test. In bodybuilding context typically AAS-cycle adjunct (25-50 mg/day), NOT a standalone bulker. WADA-banned year-round.
Mechanism
AR agonist (weak) + SHBG suppressor + mild AI
Anabolic:Androgenic
100:30-40
Half-life
12-13 hours
Onset
1-2 h (oral)
Legal status
Bayer Rx (DE/AT/PL), WADA-banned
Data console
Lab data
/lab/molecular-data.jsonLIVE
> Androgenic:Anabolic100:30-40
> AR bindingAR affinity weak (~10% vs testosterone). HIGH SHBG affinity…
> Active half-life12-13 h
> Detection windowMesterolone metabolites (1α-methyl-5α-androstan-3-one metabolite family) urinary 4-5 weeks (Schänzer 1996 GC-MS methods). WADA-accredited.
> AromatizationNo – 1α-methyl-DHT structure, NOT a CYP19 substrate; clinically also shows mild AI activity via its high SHBG affinity (Nieschlag-Behre 2012 testosterone monograph, Pope-Kanayama 2014 PMID 24423981)
> HepatotoxicityLow – 1α-methyl group (NOT 17α!) provides weak hepatic first-pass stress; clinical ALT/AST rise rarely documented, much milder than Anavar or Dianabol. Long-term use (Bayer SmPC chronic Rx indication) hepatic profile relatively safe (Hartgens-Kuipers 2004 PMID 15233599)
Safety
Side effects, stop signs, contraindications
Side effects · 8
- Androgenic effects: acne, oily skin, increased body hair; as mesterolone is a DHT-structural analog, the androgenic load is strong.
- Acceleration of androgenetic hair loss in genetically predisposed users; finasteride does NOT protect, because mesterolone is an already-reduced DHT analog (not a 5α-reductase substrate).
- Virilization in women: voice deepening, hirsutism, clitoral enlargement, menstrual disturbance; due to the low-anabolic/high-androgenic ratio the virilization risk is significant even at low doses and partly irreversible.
- Adverse lipid profile: drop in HDL cholesterol (typically ~10-15%), which as an oral androgen raises atherosclerosis risk with prolonged use.
- Prostate stimulation: the DHT analog directly drives prostate tissue, which can worsen benign prostatic hyperplasia symptoms (urinary difficulty) and raise PSA, especially over age 40.
- HPTA suppression: on its own a mild-to-moderate suppression of native testosterone and spermatogenesis; at supraphysiological doses or as part of an AAS stack the HPTA suppression is pronounced.
- Prolonged, painful erection (priapism) can occur as a sign of androgenic overstimulation; it requires urgent medical care.
- Anticoagulant interaction: it can potentiate the blood-thinning effect of warfarin, leading to increased bleeding risk (INR monitoring required).
Contraindications · 7
- Prostate carcinoma or known/suspected breast carcinoma in men: androgen-dependent tumors, mesterolone is absolutely contraindicated (Bayer SmPC).
- Pregnancy and breastfeeding: the androgenic action can virilize the fetus (notably masculinization of a female fetus's external genitalia); absolute contraindication.
- Women using it for aesthetic/performance goals: the low-anabolic/high-androgenic ratio makes the virilization risk disproportionate to the benefit, and partly irreversible.
- Symptomatic benign prostatic hyperplasia (urinary obstruction): the DHT-analog androgen further stimulates the prostate and can worsen symptoms.
- Personal/family history of androgenetic alopecia: mesterolone directly accelerates hair loss and finasteride offers no protection.
- Pre-existing adverse lipids / cardiovascular risk (dyslipidemia, coronary disease): the HDL drop further worsens the risk, warranting caution/contraindication.
- Severe hepatic or renal impairment: although mesterolone is not 17α-alkylated and has low hepatotoxicity, it should be avoided with existing organ failure due to altered hormone metabolism and fluid retention.
Related Performance Compounds
Same therapeutic category
Studies
Related research and clinical findings
Endocr Rev. 2014;35(3):341-75.
Adverse health consequences of performance-enhancing drugs: an Endocrine Society scientific statement
Pope HG Jr, Wood RI, Rogol A, Nyberg F, Bowers L, Bhasin S.
PMID: 24423981Open
Sports Med. 2004;34(8):513-554.
Effects of androgenic-anabolic steroids in athletes
Hartgens F, Kuipers H.
PMID: 15248788Open
Acta Endocrinol (Copenh). 1968;59(3):497-507.
Androgen treatment without inhibiting effect on hypophysis and male gonads. Biochemical and morphological studies after treatment with Mesterolone
Petry R, Rausch-Stroomann JG, Hienz HA, Senge T, Mauss J.
PMID: 4880965Open
Andrologia. 1983;15 Spec No:531-5.
Double-blind cross over treatment with mesterolone and placebo of subfertile oligozoospermic men value of testicular biopsy
Aafjes JH, van der Vijver JC, Brugman FW, Schenck PE.
PMID: 6364886Open
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MolekulaX Editorial Team·Source-verified · PubMed · FDA · EMA
Updated: June 19, 2026
The information here is for educational and scientific purposes only. Performance-enhancing compounds (AAS, prohormones, stimulants, doping agents) are illegal without prescription in Hungary and most of the EU, and carry serious health and legal risks. WADA bans them in competitive sport. This is NOT a usage guide, and we do not encourage any illegal use. If you do use them, medical supervision and regular bloodwork are ESSENTIAL. Severe endocrine, cardiovascular, hepatic and psychiatric side effects are possible.