Mesterolone, sold by Bayer as Proviron, is an oral androgen derived from dihydrotestosterone (DHT). It carries a methyl group at carbon 1 of the steroid skeleton, not the 17α-methyl group found in most oral steroids. It was introduced by Schering in the 1960s and was widely tried for male infertility in the 1970s and 1980s.
It is still registered in some European countries for male hypogonadism (low testosterone), with a current Belgian label approved in July 2024. It has never been approved in the US, where it is a Schedule III controlled substance. Outside medicine it is used alongside other steroids, including for claimed “estrogen control.” No controlled human data support that use.
This page summarizes what the European label, clinical trials and regulators report. It is for education only and is not medical advice.
| Property | Detail |
|---|---|
| Drug class | Androgen; 1α-methylated DHT derivative (ATC code G03BB01) |
| Brand | Proviron (Bayer); Belgian and Luxembourg marketing authorizations |
| Route | Oral tablet |
| Strength | 25 mg |
| Labeled adult dose (Belgium) | 25–50 mg three times daily for several months, then 25 mg two to three times daily |
| Half-life (Belgian label) | 7–8 h for mesterolone plus metabolites |
| Aromatizes to estrogen | No (5α-reduced, like DHT) |
| US status | Not FDA-approved; Schedule III controlled substance |
| WADA status | Prohibited at all times (S1.1, anabolic androgenic steroids) |
What it is
Mesterolone is 1α-methyl-5α-androstan-17β-ol-3-one: formula C₂₀H₃₂O₂, molecular weight 304.5 g/mol (PubChem). In plain terms, it is DHT with one extra methyl group at carbon 1. Animal studies of the “new androgen” were published in 1966 (Neumann et al., Arzneimittelforschung 1966), and trial reports name Schering as the maker (Gerris et al., Fertil Steril 1991).
Two structural points matter:
| Feature | What it does |
|---|---|
| 1α-methyl group with 5α-reduced A-ring | The Belgian label says these protect mesterolone from rapid breakdown, which is why it works as a tablet (Proviron SmPC). |
| No 17α-methyl group | Unlike oxandrolone, methandienone or methyltestosterone, it is not 17α-alkylated. A 1992 hematology report called it a “non-alkylated” anabolic steroid (Sadullah et al., Clin Lab Haematol 1992). |
Because it is already 5α-reduced, mesterolone cannot be converted to estradiol by aromatase. Clinical researchers grouped it with fluoxymesterone as a “non aromatizable androgen” (Spitz et al., Horm Metab Res 1984).
In a binding study using rat and rabbit tissue, mesterolone bound the androgen receptor less strongly than testosterone. It bound sex hormone-binding globulin (SHBG, the main carrier protein for testosterone in blood) about four times as strongly as DHT, the strongest SHBG binding of the steroids tested (Saartok et al., Endocrinology 1984). These are animal-tissue and lab results, not human outcome data.
Pharmacokinetics
From the Belgian Proviron label (Proviron SmPC):
| Parameter | Value |
|---|---|
| Absorption | Rapid and almost complete over oral doses of 10–100 mg |
| Peak after 25 mg | 8–25 ng/mL at 1.5–3 h |
| Half-life | 7–8 h (mesterolone plus metabolites) |
| Protein binding | ≈97% (albumin, SHBG, transcortin) |
| Main urinary metabolite | 1α-methyl-5α-androstane-3α,17β-diol, 55–70% of urinary radioactivity |
| Excretion | ≈85% in urine and ≈15% in feces, as metabolites |
| Recovery | 93% of the dose within 7 days, half of it in the first 24 h |
The short half-life is consistent with the label’s two- to three-times-daily dosing.
Approved medical use
The current Belgian label lists one indication, hypogonadism. It states that Proviron stimulates androgen-dependent organs and, when androgen deficiency starts before puberty, the development of male secondary sex characteristics (Proviron SmPC).
| Situation | Labeled dose |
|---|---|
| Development of secondary sex characteristics | 1–2 tablets (25–50 mg) three times daily for several months |
| Maintenance | Often 1 tablet (25 mg) two to three times daily |
| Absent or severely reduced androgen production | Long-term treatment needed |
The label says Proviron is for male patients only. It also states that at the recommended dose no effect on spermatogenesis (sperm production) is seen.
In the US, no mesterolone product appears in FDA’s approval database (Drugs@FDA). Mesterolone is named in the federal list of Schedule III anabolic steroids (21 CFR 1308.13).
What the research shows
Most mesterolone trials are from the 1970s–1990s and are small by modern standards. There are no trials of muscle mass or athletic performance in people.
Male infertility
Mesterolone was used for years in men with unexplained low sperm counts. Randomized trials did not show that it helped couples conceive:
| Trial | Design and n | Arms | Pregnancy result |
|---|---|---|---|
| WHO Task Force, Int J Androl 1989 | Double-blind RCT, 7 centers, 248 couples | Placebo, 75 mg/day or 150 mg/day for 6 months | 9%, 12% and 16% at 8 months; ratios vs placebo 1.3 (0.5–3.2) and 1.8 (0.7–4.4), not significant |
| Hargreave et al., Br J Urol 1984 | RCT, 368 men | Mesterolone vs vitamin C for 9 months | No difference, including in a low-testosterone, low-count subgroup |
| Gerris et al., Fertil Steril 1991 | Double-blind RCT, 52 men | 150 mg/day vs placebo for 12 months | 26% vs 48% with placebo |
| Wang et al., Fertil Steril 1983 | RCT, 46 men, five treatments | Included mesterolone 100 mg/day for 6 months | No rise in sperm concentration or pregnancy |
In the WHO trial, semen quality did not change significantly. In the Gerris trial, sperm motility and shape improved on mesterolone, but similar changes occurred on placebo.
A Cochrane review pooled 11 randomized trials with 930 patients of androgens given to “stimulate” sperm production (mesterolone or testosterone undecanoate). The pregnancy odds ratio versus placebo or no treatment was 1.10 (95% CI 0.75–1.61) (Vandekerckhove et al., Cochrane 2000). Cochrane withdrew the review in 2007 (Cochrane 2007). The current Belgian label no longer lists infertility as an indication.
Hypogonadism
The most informative modern trial randomized 55 hypogonadal men to one of four standard regimens for about 6 months. Previous androgen treatment had been stopped for at least 3 months (Jockenhövel et al., Metabolism 1999; Eur J Med Res 1997; Schubert et al., Horm Res 2003):
| Measure | Mesterolone 100 mg/day oral | Testosterone undecanoate 160 mg/day oral | Testosterone enanthate 250 mg IM every 21 days | Testosterone implant 1,200 mg |
|---|---|---|---|---|
| Serum testosterone on treatment | No significant rise | 5.7 nmol/L | 13.5 nmol/L | 23.2 nmol/L |
| Hemoglobin change | +5.6 g/L (not significant) | +12.7 g/L | +21.1 g/L | +21.7 g/L |
| Hematocrit change | +1.8% (not significant) | +3.9% | +6.4% | +6.5% |
| Bone density, primary hypogonadism | +0.8% | +3.4% | +4.8% | +7.0% |
| HDL cholesterol | −34.9% | −30.9% | −35.7% | −32.5% |
| LDL cholesterol | +46.4% | +34.3% | +65.2% | +47.5% |
Three points stand out:
- Mesterolone did not raise testosterone, which is expected for a DHT derivative. DHT levels rose in all four groups.
- It had the weakest effect on red cells and on bone density. The authors concluded that testosterone, not DHT, drives red cell production.
- Its effect on blood lipids was as large as that of the testosterone regimens.
A smaller double-blind trial compared testosterone undecanoate (n = 12) with mesterolone (n = 14) in hypogonadal men. After 4 weeks, testosterone undecanoate gave significantly better libido, erections, ejaculation and mental state scores (Luisi and Franchi, J Endocrinol Invest 1980).
Depression
In a double-blind trial in 52 depressed men aged 26–53, mesterolone at 300–450 mg/day for 6 weeks improved symptoms. Placebo did too, and the difference was not significant (Itil et al., Methods Find Exp Clin Pharmacol 1984). A second double-blind trial reported effects similar to the antidepressant amitriptyline with fewer side effects, but it had no placebo arm (Vogel et al., J Clin Psychiatry 1985). Neither result led to an approved use.
Effects on the body’s own hormones
Mesterolone is often described as not suppressing natural testosterone. The studies are mixed:
| Study | Dose and duration | Finding |
|---|---|---|
| Spitz et al., 1984, 9 men with testicular failure | 150 mg/day, 6 weeks | No change in gonadotropins (LH, FSH) or steroid levels |
| Varma and Patel, Int J Gynaecol Obstet 1988, 250 men, uncontrolled | 100–150 mg/day, 12 months | No fall in normal LH and FSH; lowered them in 25% of men whose levels were high |
| Jackaman et al., Clin Endocrinol 1977, 23 men | 6–9 months | Testosterone fell from 17.05 to 14.7 nmol/L (≈491 to ≈424 ng/dL); LH rose |
| Itil et al., 1984, 52 men | 300–450 mg/day, 6 weeks | Testosterone fell significantly |
| Dugeroglu et al., J Pak Med Assoc 2014, 34 men, uncontrolled | 50 mg/day, 2 months | Total testosterone, SHBG and estradiol fell; free testosterone unchanged |
At labeled doses, mesterolone seems to suppress LH and FSH less than testosterone does. It does not replace testosterone, though, and total testosterone fell in several studies, most clearly at doses well above the label.
About “estrogen control.” Mesterolone is widely described as an anti-estrogen or a way to prevent gynecomastia (male breast growth) during steroid use. We found no controlled human trial of this. Its strong SHBG binding and lack of aromatization are real, and one uncontrolled study found a small fall in estradiol along with a fall in total testosterone. A 1989 case series described bodybuilders who developed gynecomastia despite self-treating with drugs that included mesterolone, and concluded that the recommended treatment is stopping the drugs (Friedl and Yesalis, Phys Sportsmed 1989).
Side effects and risks
Label warnings
The Belgian label warns about the following (Proviron SmPC):
- Misuse. Mesterolone is misused, usually with other anabolic steroids, with serious risks including cardiovascular events (sometimes fatal), liver and psychiatric events, and dependence.
- Liver tumors. In rare cases benign, and more rarely malignant, liver tumors have followed hormonal substances like those in Proviron, sometimes with life-threatening bleeding into the abdomen.
- Prolonged erections. Too frequent or prolonged erections call for a lower dose or stopping, to avoid damage.
- Conditions that can worsen. Heart failure, kidney disease, high blood pressure, epilepsy and migraine need regular monitoring.
- Children. In boys before or during puberty, androgens can cause early sexual development and early closure of growth plates.
The label recommends regular prostate checks. Listed adverse effects, with frequency not known, are stomach pain, acne, hair loss, headache, more frequent erections and priapism (a painful erection that will not subside), plus salt and water retention.
Lipids
In the randomized hypogonadism trial, mesterolone 100 mg/day lowered HDL (“good”) cholesterol by 34.9% and raised LDL by 46.4% and triglycerides by 46.4% (Jockenhövel et al., Metabolism 1999). Lipids had not returned to baseline at follow-up after treatment stopped. These were hypogonadal men starting from very low androgen levels, so the size of the change may differ in men with normal testosterone.
Liver
Not being 17α-alkylated does not make mesterolone free of liver risk. The label carries the liver-tumor warning above, and a 2019 case report described drug-induced liver injury attributed to mesterolone (Pérez Palacios et al., Gastroenterol Hepatol 2019). Varma and Patel reported no significant effect on liver function in 250 men over 12 months (Int J Gynaecol Obstet 1988).
Blood clots
A 1979 report described a deep vein thrombosis in a patient who had started mesterolone. The same authors then gave 9 healthy men 100 mg/day for 21 days and found no measurable change in clotting, fibrinolysis, platelet count or hematocrit (Lowe et al., Br J Clin Pharmacol 1979).
Psychiatric effects
A 2012 case report described mania linked to mesterolone in a person with no prior psychiatric history (Gahr et al., J Clin Psychopharmacol 2012). The label lists psychiatric events among the risks of misuse.
Who should not use it
The Belgian label contraindicates mesterolone in:
- known or suspected prostate or breast cancer;
- current or past liver tumors;
- hypersensitivity to mesterolone or the tablet’s ingredients (the tablets contain lactose).
The label lists no known drug interactions. It is not intended for women.
Legal status and use outside medicine
- United States. Mesterolone is a Schedule III anabolic steroid under 21 CFR 1308.13 and has no FDA-approved product.
- Sport. It is named under S1.1 anabolic androgenic steroids and is prohibited in and out of competition (WADA 2026 Prohibited List).
- Illicit supply. Poland’s official medicines control laboratory tested 63 seized products sold for “post-cycle therapy” in 2020–2024, a group that included mesterolone. In 34.9% of these products the declared active ingredient was absent (Blazewicz et al., Front Chem 2025).
Key takeaways
- An oral DHT derivative without a 17α-methyl group. The 1α-methyl group makes it orally active. Like DHT, it cannot be converted to estrogen.
- Short half-life. The Belgian label gives 7–8 hours, which fits its two- to three-times-daily dosing.
- Failed as a fertility drug. Randomized trials, including a 248-couple WHO trial, showed no clear improvement in pregnancy rates.
- Weak testosterone replacement. In hypogonadal men it did not raise testosterone and had smaller effects on red cells and bone than testosterone, but it worsened lipids just as much.
- “Estrogen control” is unproven. No controlled human trial supports using it to prevent gynecomastia or to block estrogen.
- Regulated and prohibited. It is Schedule III and never FDA-approved in the US, and it is banned at all times in sport.
Sources
- Bayer SA-NV. Proviron 25 mg comprimés, résumé des caractéristiques du produit (approved July 2024). Federal Agency for Medicines and Health Products (FAMHP), Belgium. FAMHP.
- National Center for Biotechnology Information. Mesterolone, PubChem CID 15020. PubChem.
- Neumann F, Wiechert R, Kramer M, Raspé G. Experimental animal studies with a new androgen–mesterolone (1-alpha-methyl-5-alpha-androstan-17-beta-ol-one) [in German]. Arzneimittelforschung. 1966;16(4):455–458. PubMed.
- Sadullah S, Kruger A, Clarke N, Bell A. Use of Mesterolone, a non-alkylated anabolic steroid, in myelofibrosis. Clin Lab Haematol. 1992;14(4):335–336. PubMed.
- Spitz IM, Margalioth EJ, Yeger Y, et al. Effect of non aromatizable androgens on LHRH and TRH responses in primary testicular failure. Horm Metab Res. 1984;16(9):492–497. PubMed.
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- World Anti-Doping Agency. The 2026 Prohibited List. WADA 2026 Prohibited List.
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