How Primobolan Affects Oestradiol

Methenolone is 1-methyl-5-alpha-androst-1-en-17-beta-ol-3-one, a 5-alpha-reduced DHT derivative.

Does Primo produce oestradiol? No. Its A ring is already 5-alpha-reduced, so aromatase cannot convert it to an oestrogen. Any oestradiol on a test and Primo cycle comes from the testosterone.

Does Primo lower oestradiol made from testosterone? Unproven.

• The drug itself has never been tested against aromatase in a lab, an animal or a person.
• DHT and other 5-alpha-reduced androgens are weak competitive aromatase inhibitors in lab dishes, but only at concentrations several hundred to a thousand times higher than normal blood levels (Kirilovas, 2003).
• A 2024 study found that fungal breakdown products of methenolone acetate could inhibit human aromatase, one of them potently (Abdul Karim, 2024). Those products are not known human metabolites, and no result was reported for methenolone itself.
• The closest human analogue, mesterolone, did not change steroid levels in 9 men with testicular failure at 150 mg a day (Spitz, 1984).

Why oestradiol often falls anyway: less testosterone in the stack, further LH suppression removing the roughly 20% of male oestradiol made in the testes, immunoassay bias, and other drugs being started or stopped.

Primo added to a fixed testosterone dose: forum results range from no change (232 pmol/L before and after, 3 weeks in) to large drops (213 to 70 pmol/L over about 6 weeks, with orals stopped and a blood donation in between). No controlled data exists.

Primo replacing part of a testosterone dose: expect oestradiol to fall, because there is less substrate for aromatase. This is the most predictable effect.

Primo on top of an aromatase inhibitor: a combination commonly mentioned in forum crashed-oestradiol stories.

Timing: no human half-life has been published, but users treat the enanthate like test enanthate, so test at 5 to 6 weeks.

Test sensitive (LC-MS/MS) oestradiol 5 to 6 weeks after adding Primo, with everything else held constant, at the same point in your injection cycle as your previous draw.

Draw total testosterone, SHBG and LH at the same time. If oestradiol and total testosterone both jump after adding Primo, suspect the vial contains testosterone.

Symptoms of over-suppression: sore or dry joints, low libido, flat mood, poor sleep. Check oestradiol before assuming you need more testosterone.

Do not use Primo as a dosable AI. Its effect on oestradiol is unpredictable at best. Lowering the testosterone dose is the dependable lever.

Never stack Primo with an AI without a sensitive oestradiol result showing it is needed.

Never run Primo without a testosterone base. On its own it supplies no oestrogen and suppresses your own production.

If oestradiol has dropped too low: stop any AI first, then raise testosterone or lower Primo, and retest in 2 to 3 weeks.

Further reading: Does Primobolan lower estrogen? What users' bloodwork shows