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Primobolan

Methenolone / Primo · Anabolic-androgenic steroid

Research reference only. Pepdex does not sell, supply or track this compound, and nothing here is guidance to use it — this page exists so the evidence and the risks are visible.

What it is

An injectable (enanthate) or oral (acetate) anabolic steroid with a reputation as one of the "mildest" AAS — moderate anabolic effect, low androgenic load, no aromatization to estrogen. That reputation, plus its association with physique sport, keeps it perpetually popular and perpetually faked.

How it works

Methenolone is a dihydrotestosterone derivative that cannot aromatise, with low androgenic activity relative to a modest anabolic effect. That combination — no oestrogen conversion, mild androgenic profile, weak overall potency — is the entire basis of its reputation as the gentlest anabolic steroid, and the reputation runs well ahead of what that actually means. Weak is not safe: it still suppresses natural testosterone production, still affects lipids, and still carries the risks of the class. It exists in an oral form (acetate) and an injectable one (enanthate). The oral is NOT 17-alpha-alkylated, which may lower the classic cholestatic risk — but that is not the same as no hepatic risk, and fatal liver injury attributed to oral methenolone has been published.

What human evidence shows

Almost no modern clinical literature — it had niche medical uses decades ago (wasting, some anemias) and has largely vanished from medicine. What is known: it builds less mass than testosterone at comparable exposure, does not aromatize, and still suppresses natural testosterone production like every AAS. "Mild" describes side-effect intensity, not safety.

The studies, one by one

  • It has genuine pharmaceutical history and remains approved in some countries, so its human pharmacology is better characterised than most compounds here.
  • NONE SHOWN for physique use in a controlled trial in healthy people.
  • Its mild reputation reflects the absence of oestrogenic effects and low androgenic activity — not a demonstrated safety advantage, and no comparative ranking across steroids establishes it as the safest.
  • Substitution has been reported with this compound specifically, and black-market steroid identity is broadly unreliable — a systematic review found roughly 36% counterfeit and 37% substandard across products. No compound-by-compound ranking of counterfeiting exists, so this entry does not claim one.
  • It is prohibited in tested sport.

Half-life and how long it lasts

The oral acetate form is not 17-alpha-alkylated, which may reduce the classic cholestatic risk that defines most oral steroids — though serious and fatal liver injury has still been reported with it. The injectable enanthate has a long ester. It does not aromatise. Its low potency means people often use larger amounts to compensate, which erodes the mildness that attracted them.

Risks

Full HPTA suppression — natural testosterone shuts down, and recovery after stopping is neither fast nor guaranteed. Lipid damage (HDL crushed), cardiac remodeling risk with extended use, hair loss in the predisposed. Because real methenolone is expensive, it is among the most counterfeited steroids — much of what is sold as "primo" is testosterone or nothing. No aromatization means no estrogen bloat, which quietly tempts people into longer exposure.

Who should never touch it

  • Class-level severe outcomes reported with anabolic steroids, which no compound here is exempt from: cardiac arrest, heart attack, thickened heart muscle, heart failure and stroke; serious liver injury; and mania, paranoia, psychosis, hostility, aggression, dependence and depression on withdrawal.
  • Pregnancy and breastfeeding: androgens can cause fetal harm including virilisation of a female fetus. Avoid in both.
  • Anyone who wants children in the foreseeable future
  • Anyone with unfavourable lipids or cardiovascular disease
  • Anyone with prostate or male breast cancer
  • Women, given virilisation risk, some of which does not reverse
  • Anyone in tested sport

Interactions worth knowing

  • Anticoagulants, which anabolic steroids can potentiate.
  • Tell whoever manages your lipids that you are using it. Nothing here is a reason to start, stop or change a prescribed medicine.
  • Other steroids, where suppression is additive.
  • Tested sport, where it is prohibited.

What a clinician would watch

  • Suppression of natural testosterone production is a class effect and the mild reputation does not exempt it — though compound-specific magnitude has not been characterised.
  • Lipids, with the same caveat: adverse changes are a class effect and no methenolone-specific magnitude is established.
  • Blood pressure and haematocrit.
  • Product authenticity, which is a genuine practical concern for this compound specifically.

What stopping looks like

The injectable ester continues releasing for weeks after the last injection, so recovery starts later than expected. Endocrine recovery is variable, can take months and can be incomplete, with no reliable individual prognosis. The mild reputation does not extend to a faster or more certain recovery — that has never been established.

Myth vs evidence

“It is the safest steroid.”

No comparative ranking establishes that. It does not aromatise and its oral form is not 17-alpha-alkylated, which are real structural differences — but fatal liver injury has still been reported, and suppression, lipid and cardiovascular risks remain.

“It does not suppress because it is mild.”

It suppresses natural production like everything else in this class. Mild describes the side-effect mix, not the endocrine consequence.

“You can use more to compensate for low potency.”

Doing so removes the only thing that made it mild, which is the amount involved.

“What you bought is what the label says.”

Black-market steroid identity is broadly unreliable — a systematic review found roughly 36% counterfeit and 37% substandard — and substitution has been reported with this compound specifically.

Legal status

US: Schedule III controlled substance. Unmanufactured in the US — everything on the grey market is imported or underground.

Research on this compound

Papers about this compound, not proof of the claims above. A title says what was studied, which is sometimes a negative result or a different question entirely.

  • Treatment of refractory anemias with methenolone
    Acta Med Scand, 1979
  • Effects of testosterone and methenolone on erythropoietin activity and erythropoiesis in patients with kidney failure
    Med Welt, 1971
  • Effect of methenolone enanthate (NSC-64967) in advanced cancer of the breast
    Cancer, 1968

Evidence base

C17 research papers
early literature

Early human evidence: at least one human trial report, and not much beyond it.

phase 3/4
0
randomised
0
reviews
0
human trials
1

Counts are of published papers, not distinct trials, and they nest rather than add up: PubMed files every randomised trial as a clinical trial too. They measure how much research exists — not whether this works or is safe for you.

Research (12)

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Where next

Every compound in the catalogThe full non-peptide index, with evidence grades.→The peptide indexInjectable peptides, half-lives and what trials used.→Spotting a scam vendorRed flags, COA checks and what traceability looks like.→