“Oral Primobolan” is the tablet form of metenolone (also spelled methenolone), an anabolic-androgenic steroid built on the dihydrotestosterone (DHT) skeleton. The drug in the tablet is metenolone acetate, the short-chain ester of metenolone. The long-acting injection sold as Primobolan Depot is a different ester, metenolone enanthate, so the single brand name covers two products with different routes and dosing.
The only current regulated oral product we found is Bayer’s Primobolan 5 mg tablet in Japan, a prescription drug labeled for osteoporosis, severe wasting and aplastic anemia (Japanese package insert). It is unusual among oral steroids because it has no 17α-methyl group. It has never been FDA-approved in the US, where metenolone is a Schedule III controlled substance. No published study has measured its oral bioavailability or half-life in people, and no controlled trial has tested it for muscle gain.
This page summarizes what the Japanese product label, the published studies and regulators report. It is for education only and is not medical advice.
| Property | Detail |
|---|---|
| Active ingredient | Metenolone acetate (the 17β-acetate ester of metenolone), ATC code A14AA04 |
| Product | Primobolan tablets 5 mg (Bayer Yakuhin, Japan), prescription-only; 100 tablets per pack |
| Route | Oral |
| Labeled adult dose (Japan) | 10–20 mg/day, split into 2–3 doses |
| Metenolone per tablet | ≈ 4.4 mg (the acetate is ≈ 88% metenolone by weight) |
| Half-life and bioavailability | Not stated in the label; no published human values found |
| 17α-methyl group | No (unlike most oral steroids) |
| Conversion to estrogen | None, per the Spanish product information for metenolone |
| US status | Not FDA-approved; Schedule III controlled substance |
| WADA status | Prohibited at all times (class S1.1, anabolic androgenic steroids) |
What it is
Metenolone is 1-methyl-17β-hydroxy-5α-androst-1-en-3-one (21 CFR 1308.13). Compared with DHT, it has a double bond between carbons 1 and 2 and a methyl group at carbon 1. In the tablet, the 17β-hydroxyl group carries an acetate (two-carbon) ester. The Japanese label classifies the product as an “oral protein-anabolic steroid,” and metenolone acetate is an official substance of the Japanese Pharmacopoeia (KEGG DRUG).
This page is about the tablet product. The same molecule is covered from the ester’s point of view on the metenolone acetate page, and the injectable is covered on the metenolone enanthate page. Some of the earliest clinical work on the oral drug dates from 1962 (Weller, Arzneimittelforschung 1962).
Oral Primobolan vs Primobolan Depot
The two products release the same steroid, metenolone, but differ in nearly every practical respect (Japanese package insert; AEMPS SmPC; PubChem, metenolone acetate; PubChem, metenolone; PubChem, metenolone enanthate):
| Feature | Oral Primobolan | Primobolan Depot |
|---|---|---|
| Ester | Acetate (C₂₂H₃₂O₃, 344.5 g/mol) | Enanthate (C₂₇H₄₂O₃, 414.6 g/mol) |
| Share that is metenolone | ≈ 88% | ≈ 73% |
| Route | Tablet by mouth | Intramuscular injection in oil |
| Where authorized (current record) | Japan, 5 mg tablet | Spain, 100 mg/mL ampoule |
| Labeled adult dose | 10–20 mg/day in 2–3 doses | 100 mg every 2 weeks, then every 3–4 weeks after a good response |
| Published kinetic data | Urinary and fecal excretion only | Peak 6–11 days after injection; complete bioavailability |
| Labeled uses | Osteoporosis; wasting from chronic kidney disease, cancer, trauma or burns; aplastic anemia | Wasting, cachexia, advanced breast or female genital cancer |
Because the tablet’s oral bioavailability has never been published, milligram doses of the tablet and the injection cannot be converted into each other.
Why it works by mouth
Most oral steroids, such as oxandrolone and methandienone, carry a 17α-methyl group that slows their breakdown in the liver. Metenolone acetate does not, yet it is labeled and was studied for oral use. What the sources show about how it is handled:
- Ester cleavage. The Spanish product information states that both the acetate and the enanthate are completely cleaved by esterases (ester-splitting enzymes) after oral or injected dosing, releasing metenolone (AEMPS SmPC).
- Gut tissue favors short esters. In human tissue samples in the lab, gastric mucosa split short-chain steroid esters such as the acetate faster than long-chain enanthates. Fat tissue showed the opposite pattern (Bellmann et al., Acta Endocrinol 1976). This is in-vitro data and does not measure absorption.
- Absorption amount unknown. The label’s only human data are excretion figures (see below). They do not show what fraction of a tablet reaches the bloodstream.
About “oral Primobolan has very low bioavailability.” This is often repeated online, sometimes with a specific percentage. We found no human study that measured the oral bioavailability of metenolone acetate, so no figure can be given. The drug’s labeled oral dose is 10–20 mg/day.
How metenolone acts
Metenolone works through the androgen receptor. In rat and rabbit tissue, its binding ranked below nandrolone but above testosterone, and it bound only weakly to sex hormone-binding globulin (SHBG), the blood protein that carries testosterone (Saartok et al., Endocrinology 1984). Binding was similar in muscle and prostate for most steroids tested, so receptor data do not show a muscle-selective steroid. The Japanese label describes the drug as promoting protein synthesis, reducing protein breakdown, and promoting calcium and phosphorus deposition in tissues. The Spanish product information states that metenolone is not converted to estrogenic metabolites (AEMPS SmPC).
Pharmacokinetics
The Japanese label gives only excretion data, from a study with a radiolabeled dose (Japanese package insert):
| Parameter | What the label states |
|---|---|
| Study | 4 healthy men, single oral dose of 114 mg ¹⁴C-metenolone acetate (above the approved 10–20 mg/day) |
| Urinary excretion | 21–47% of the dose within 7–13 days |
| Fecal excretion | 14–22% of the dose within 7–13 days |
| Bioavailability, half-life, peak level, time to peak | Not stated |
We found no published human study that measured the tablet’s half-life. The label splits the daily dose into 2–3 doses but does not give a pharmacokinetic reason.
Metabolism and detection after oral doses
After a 50 mg oral dose in two men, researchers identified twelve urinary metabolites (Goudreault and Massé, J Steroid Biochem Mol Biol 1990):
- The main metabolite was 3α-hydroxy-1-methylen-5α-androstan-17-one.
- Unchanged metenolone was found in urine for up to 90 hours, accounting for only 1.63% of the dose.
- The metabolites came from oxidation of the 17-hydroxyl group and reduction of the A-ring, with or without added hydroxyl groups.
Anti-doping methods have extended detection over time:
| Study | Dose | Longest detection |
|---|---|---|
| Björkhem and Ek, J Steroid Biochem 1983 | Single oral 10 mg Primobolan | Main metabolite measurable for 5 days |
| Albertsdóttir et al., Drug Test Anal 2020 | Two excretion studies (form not stated in the abstract) | Sulfated metabolite up to 17 days |
| He et al., Steroids 2016 | One volunteer, oral metenolone | Sulfated metabolite up to 40 days |
Approved medical use
In Japan, Primobolan tablets are a prescription medicine with these labeled indications (Japanese package insert):
- osteoporosis;
- marked wasting caused by chronic kidney disease, cancer, trauma or burns;
- bone marrow depletion from aplastic anemia (a failure of the marrow to make blood cells).
| Population | What the label says |
|---|---|
| Adults | 10–20 mg/day of metenolone acetate (2–4 tablets), split into 2–3 oral doses, adjusted for age and symptoms |
| Children | Use with care and close observation; can cause early closure of growth plates and precocious puberty |
| Older adults | Use with care; an androgen-dependent tumor may be present undetected |
| Breastfeeding | Weigh the benefit of treatment against the benefit of breastfeeding |
| Pregnancy | Contraindicated |
The label also tells prescribers to check the prostate regularly in men, to warn women about possible voice change, and to run regular liver tests, especially during long-term use.
In the US, a search of FDA’s Drugs@FDA data returned no metenolone product (openFDA). In Spain, the only authorized metenolone product is the injectable Primobolan Depot (AEMPS CIMA).
What the research shows
Early metabolic studies
The Japanese label summarizes small early studies of the oral drug in healthy people and in patients with osteoporosis, hypogonadism or muscle disease. They were not placebo-controlled:
| Measure | Oral dose | Reported effect |
|---|---|---|
| Nitrogen retention (a marker of protein build-up) | 20–30 mg/day, 330 mg in total | 47.1 mg/kg/day of nitrogen retained |
| Calcium and phosphorus retention | 20–30 mg/day, 300–360 mg in total | Calcium 1.26 mg/kg/day; phosphorus 2.75 mg/kg/day |
| Red cells | 20 mg/day for 28 days | Hemoglobin and red cell count increased |
Aplastic anemia and other marrow failure
The best controlled data for the oral drug come from aplastic anemia. In a randomized trial, 30 patients (18 with severe disease) received antithymocyte globulin (ATG), an immune-suppressing treatment, with or without the oral androgen metenolone at 3 mg/kg (Kaltwasser et al., Eur J Haematol 1988):
| Group | Patients | Responses | Survival |
|---|---|---|---|
| ATG + oral metenolone | 15 | 11 (8 complete, 3 partial) | 87% |
| ATG alone | 15 | 4 (2 complete, 2 partial) | 43% |
The difference in response was significant (p = 0.01), but the survival difference was not (p = 0.15). The authors noted that a controlled American study had found no benefit from adding androgens to ATG. The 3 mg/kg dose is far above the 10–20 mg/day in the current label.
In Japan, the tablet is still used as an add-on for anemia from marrow failure, mostly documented in case reports where it was given alongside other treatments:
| Report | Condition | Oral metenolone acetate | Given with | Outcome |
|---|---|---|---|---|
| Murai et al., Rinsho Ketsueki 1995 | Severe aplastic anemia after gold therapy, 65-year-old woman | 40 mg/day | GM-CSF (a white-cell growth factor) and prednisolone | Recovery of all three blood cell lines |
| Katayama et al., Int J Hematol 1996 | Refractory anemia (a form of myelodysplastic syndrome), 45-year-old woman | Dose not stated in abstract | G-CSF and erythropoietin | No effect alone for 19 weeks; recovery after growth factors were added |
| Ushimaru et al., JCEM Case Rep 2024 | Myelodysplastic syndrome with stage 4 kidney disease, 73-year-old man | 20 mg/day | Darbepoetin | Hemoglobin rose from 7.6 to 9.1 g/dL at 1 month and 12.6 g/dL at 2 years |
In the 2024 report, the authors wrote that the drug is still used in Japan for anemia due to marrow failure, unlike in Europe and the US. They saw no liver dysfunction or PSA rise during treatment. Because every patient also received a growth factor, these cases cannot show how much of the response came from metenolone.
Osteoporosis
Osteoporosis is a labeled use, but the evidence is thin. A 2008 Japanese review reported that anabolic steroids raised bone mineral content or density at the forearm and spine, that their use for osteoporosis had been declining, and that no prospective study had tested whether they prevent fractures (Adachi and Takayanagi, Clin Calcium 2008). The 2024 case report above described a slower loss of bone density during treatment, and its authors noted that the drug had rarely been used for osteoporosis (Ushimaru et al., JCEM Case Rep 2024). A single case cannot show cause and effect.
Muscle and performance
We found no controlled trial of oral metenolone acetate on muscle mass or strength in healthy or trained adults. The only modern controlled muscle data for metenolone come from small stroke-rehabilitation studies of the injectable enanthate, covered on the enanthate page.
Blood clotting
In men with ischemic heart disease, five 17α-alkylated oral steroids, including oxymetholone and methandienone, increased plasma fibrinolytic activity (the breakdown of blood clots). Metenolone acetate, the only non-alkylated steroid tested, did not (Walker et al., Thromb Diath Haemorrh 1975).
Side effects and risks
The Japanese label lists side effects without frequencies (Japanese package insert).
Liver
No 17α-methyl group does not mean no liver risk:
- The label lists liver dysfunction with marked rises in AST, ALT and GGT (liver enzymes), and jaundice, as serious adverse reactions.
- It calls for regular liver tests, especially during long-term use, and caution in people with liver disease.
- It notes reports of liver tumors in patients, such as those with aplastic anemia, who took anabolic steroids at high doses for long periods.
In one case report, a 75-year-old man with severe aplastic anemia developed sharply raised liver enzymes after starting metenolone acetate, followed by liver failure and death. Autopsy findings fit drug-induced liver injury, and metenolone acetate was judged the most likely cause (Tsukamoto et al., Ann Hematol 1993). The authors noted that liver injury from this drug had been thought to be mild, reversible and infrequent, and urged extreme caution in debilitated patients.
Virilization in women
The label tells prescribers to warn women about voice changes. Listed effects in women include:
- hoarseness, which can become hard to reverse as it progresses and is often preceded by menstrual changes;
- excess hair growth (hirsutism) and acne;
- skin pigmentation and menstrual abnormalities;
- clitoral enlargement and increased libido.
Effects in men and fertility
Listed effects in men include acne, penile enlargement, impotence and priapism (a prolonged, painful erection). The label states that large, continued doses suppress testicular function, with reduced sperm and semen. No study has measured how much the tablet lowers LH, FSH or the body’s own testosterone at labeled doses.
Other cautions
The label advises care in people with:
- an enlarged prostate;
- heart or kidney disease, because of sodium and fluid retention;
- cancer that has spread to bone, because of the risk of high blood calcium;
- diabetes, because glucose tolerance can worsen.
Nausea, vomiting and hypersensitivity reactions are also listed.
Interactions
- Corticosteroids: glucose tolerance may worsen when combined.
- Coumarin anticoagulants such as warfarin: their effect may be increased, so close monitoring is needed.
Who should not use it
The label lists two contraindications:
- androgen-dependent cancers, such as prostate cancer, or suspicion of one;
- pregnancy or possible pregnancy, because of the risk of virilizing a female fetus.
Legal status and use outside medicine
- US: metenolone, including its esters, is a Schedule III anabolic steroid (21 CFR 1308.13(f)). No FDA-approved product exists, so any oral metenolone sold in the US comes from outside the regulated drug supply.
- Sport: metenolone is named under S1.1 (anabolic androgenic steroids) on the 2026 list and is prohibited in and out of competition (WADA 2026 Prohibited List).
- Food residues: in an experiment, none of the men who ate chickens fed metenolone acetate by mouth (1 mg/day for 21 days) tested positive, while 50% of urine samples taken 24 hours after eating chickens injected with the depot ester were positive (Kicman et al., Clin Chem 1994).
Key takeaways
- The tablet is metenolone acetate. Oral Primobolan is the acetate ester, about 88% metenolone by weight; the injectable Primobolan Depot is the enanthate.
- Still a prescription drug in Japan. The 5 mg tablet is labeled for osteoporosis, severe wasting and aplastic anemia at 10–20 mg/day in 2–3 doses. It is not FDA-approved and is Schedule III in the US.
- Oral without a 17α-methyl group, but kinetics unknown. Esterases release metenolone after oral dosing, yet no human bioavailability or half-life has been published.
- Thin controlled evidence. The strongest trial is a 30-patient aplastic anemia study at 3 mg/kg. No controlled trial has tested the tablet for muscle in healthy adults, and none has shown fewer fractures.
- Not liver-safe by default. The label lists serious liver injury and jaundice, and a fatal case has been reported.
- Banned in sport. Prohibited at all times under WADA class S1.1, with metabolites detectable for weeks after an oral dose.
Sources
- Bayer Yakuhin. Primobolan tablets 5 mg (metenolone acetate) package insert [in Japanese], revised August 2020 (1st edition). KEGG MEDICUS.
- Kanehisa Laboratories. KEGG DRUG: D01375, methenolone acetate (USAN); metenolone acetate (JP19); Primobolan (TN). KEGG.
- Agencia Española de Medicamentos y Productos Sanitarios (AEMPS). Primobolan Depot 100 mg solución inyectable: ficha técnica (summary of product characteristics), Bayer Hispania. AEMPS.
- Agencia Española de Medicamentos y Productos Sanitarios (AEMPS). CIMA registry search for active ingredient metenolona. AEMPS CIMA.
- US Food and Drug Administration. Drugs@FDA dataset, search for active ingredient methenolone. openFDA.
- Electronic Code of Federal Regulations. 21 CFR 1308.13, Schedule III, paragraph (f) Anabolic steroids. eCFR.
- World Anti-Doping Agency. The 2026 Prohibited List, International Standard. In effect January 1, 2026. WADA 2026 Prohibited List.
- National Center for Biotechnology Information. PubChem Compound Summary: methenolone acetate (CID 252372). PubChem.
- National Center for Biotechnology Information. PubChem Compound Summary: methenolone (CID 3037705). PubChem.
- National Center for Biotechnology Information. PubChem Compound Summary: methenolone enanthate (CID 248271). PubChem.
- Weller O. Clinical research with orally administered methenolone acetate [in German]. Arzneimittelforschung. 1962;12:234–240. PubMed.
- Bellmann O, Duhme HJ, Gerhards E. In vitro studies on enzymatic cleavage of steroid esters in the female organism. Acta Endocrinol (Copenh). 1976;81(4):839–853. PubMed.
- Saartok T, Dahlberg E, Gustafsson JA. Relative binding affinity of anabolic-androgenic steroids: comparison of the binding to the androgen receptors in skeletal muscle and in prostate, as well as to sex hormone-binding globulin. Endocrinology. 1984;114(6):2100–2106. PubMed.
- Goudreault D, Massé R. Studies on anabolic steroids–4. Identification of new urinary metabolites of methenolone acetate (Primobolan) in human by gas chromatography/mass spectrometry. J Steroid Biochem Mol Biol. 1990;37(1):137–154. PubMed.
- Björkhem I, Ek H. Detection and quantitation of 3 alpha-hydroxy-1-methylen-5 alpha-androstan-17-one, the major urinary metabolite of methenolone acetate (Primobolan) by isotope dilution–mass spectrometry. J Steroid Biochem. 1983;18(4):481–487. PubMed.
- Albertsdóttir AD, Van Gansbeke W, Coppieters G, et al. Searching for new long-term urinary metabolites of metenolone and drostanolone using gas chromatography-mass spectrometry with a focus on non-hydrolysed sulfates. Drug Test Anal. 2020;12(8):1041–1053. PubMed.
- He G, Yang S, Lu J, Xu Y. New long term metabolite in human urine for metenolone misuse by liquid chromatography quadrupole time-of-flight mass spectrometry. Steroids. 2016;105:1–11. PubMed.
- Kaltwasser JP, Dix U, Schalk KP, Vogt H. Effect of androgens on the response to antithymocyte globulin in patients with aplastic anaemia. Eur J Haematol. 1988;40(2):111–118. PubMed.
- Murai K, Michimata M, Shimosegawa K, et al. Initiation of hematopoietic recovery by recombinant human granulocyte-macrophage colony-stimulating factor in a case of severe aplastic anemia induced by gold salt [in Japanese]. Rinsho Ketsueki. 1995;36(1):23–28. PubMed.
- Katayama Y, Kojima K, Omoto E, Harada M. Androgen therapy in combination with granulocyte colony-stimulating factor and erythropoietin in a patient with refractory anemia. Int J Hematol. 1996;65(1):89–92. PubMed.
- Ushimaru S, Sumi H, Aso M, et al. Attenuation of bone mineral density decline during anemia treatment with methenolone acetate in myelodysplastic syndrome. JCEM Case Rep. 2024;2(4):luae055. PubMed; PMC.
- Adachi M, Takayanagi R. Effect of anabolic steroids on osteoporosis [in Japanese]. Clin Calcium. 2008;18(10):1451–1459. PubMed.
- Walker ID, Davidson JF, Young P, Conkie JA. Plasma fibrinolytic activity following oral anabolic steroid therapy. Thromb Diath Haemorrh. 1975;34(1):236–245. PubMed.
- Tsukamoto N, Uchiyama T, Takeuchi T, et al. Fatal outcome of a patient with severe aplastic anemia after treatment with metenolone acetate. Ann Hematol. 1993;67(1):41–43. PubMed.
- Kicman AT, Cowan DA, Myhre L, et al. Effect on sports drug tests of ingesting meat from steroid (methenolone)-treated livestock. Clin Chem. 1994;40(11 Pt 1):2084–2087. PubMed.