Does Primobolan Lower Estrogen? What Users' Bloodwork Shows

Ask on any steroid forum and you will hear the same line: Masteron doesn't really lower your E2, but Primo does, the same way EQ does. People run Primo specifically to keep estrogen down while pushing their test dose up. Others swear they added 300 mg and their E2 didn't move.

We read the threads, pulled every bloodwork number we could find, and checked them against the research. This is what users' labs actually show, what the science can and can't explain, and how to read your own numbers on a test and Primo cycle.

Does Primobolan lower estrogen?

There are two different questions hiding in that one. Can Primo become estrogen? And does it pull your existing estrogen down? The answers are not the same.

Primo cannot become estrogen

Methenolone is 1-methyl-5α-androst-1-en-17β-ol-3-one (PubChem): a DHT backbone with an extra methyl group and a double bond in the first ring (Saartok et al., 1984). The "5α" part matters. The A ring has already been reduced, so it has none of the 4-ene-3-one arrangement aromatase needs to build an estrogen. Primo adds nothing to your E2. That much is chemistry, and nobody disputes it.

Does it block aromatase?

This is where the forum claim lives, and the evidence is thin.

• The drug itself has never been tested. We found no study that puts methenolone, oral or injectable, against the aromatase enzyme in a test tube, an animal or a person.
• Related steroids block aromatase in a dish, at doses far above real life. In rat ovarian cells, DHT and other 5α-reduced androgens were potent competitive inhibitors of aromatase (Hillier et al., 1980). In human ovarian cells, DHT only suppressed aromatase at 1 to 10 micromolar, and actually stimulated it at lower levels (Kirilovas et al., 2003). Blood DHT in men sits several hundred to a thousand times lower than that.
• A 2024 lab study is often cited, but it does not say what forums think. Researchers fed methenolone acetate to fungi and tested the breakdown products on human aromatase. One product was a potent inhibitor, one was moderate, and others were inactive (Abdul Karim et al., 2024). The potent one has not been reported as a human metabolite, and the abstract gives no result for methenolone itself. All it shows is that close relatives of Primo can block aromatase in a dish.
• The closest human analogue found no effect. Mesterolone (Proviron, 1α-methyl-DHT) carries the same "anti-estrogen" reputation. In 9 men with testicular failure given 150 mg a day for six weeks, it "produced no changes in steroids" (Spitz et al., 1984).

Compare that with EQ, whose metabolites have lab data showing they inactivate aromatase. Primo has a plausible story and a lot of user belief behind it, but no measured mechanism.

What users' bloodwork shows

We went through Evolutionary.org, EliteFitness and Anabolex logs looking for posted numbers. Here is every case with usable figures.

Two things stand out. First, the biggest drop, 213 to 70 pmol/L, is real and worth taking seriously. Second, not one case held everything else constant. The cleanest case, where only Primo changed, showed no change at all, although three weeks is probably too short for the ester to reach steady state.

Four ordinary reasons E2 falls after adding Primo

Before you credit Primo with an AI-like effect, rule these out:

1. Less testosterone. Many people cut their test to make room for Primo. Estradiol rises and falls with your test dose (Lakshman et al., 2010), so a smaller test dose alone lowers E2.
2. Your own testosterone switching off. About 20% of a man's estradiol comes straight from the testes (Vermeulen et al., 2002). Adding another androgen suppresses LH further, which can remove what is left of that output, especially for someone on a modest TRT dose.
3. The assay. Standard estradiol immunoassays read men's E2 high by 6% to 74% (Handelsman et al., 2014), probably because of cross-reaction with other steroids. Change the steroids in your blood and the reading can shift without your true E2 moving. In one forum thread, the same blood draw read 184 and 221 pmol/L on two standard tests and 171 by LC-MS/MS.
4. Something else stopped or started. An AI, orals coming off, weight loss and blood donations all appear in the cases above.

Any of these can produce a "Primo lowered my E2" story. None of them rules out a real Primo effect either, which is why some men may genuinely be what one experienced forum member called "hyper responders" while others see nothing.

Primo vs Masteron vs EQ on estrogen

Click to expand

Picture testosterone as the main bucket. One pipe, aromatase, fills your estradiol bucket. In that picture:

• Primo and Masteron both pour into the DHT side. Neither adds any estrogen. Masteron does not appear to touch the aromatase pipe in bloodwork, and our Masteron and estrogen guide covers why it mostly changes how E2 feels. Primo is the open question: forums say it narrows the pipe a little, the research has not tested it.
• EQ has lab evidence for clamping the aromatase pipe through its metabolites, and is the compound most likely to crash E2 without an AI. See our EQ and estrogen guide.
• An AI clamps the pipe hardest. Stacking one on top of EQ or Primo is how most crashes happen.

What if my E2 reads...?

"I'm on 300 mg test, E2 is 213 pmol/L, and I'm thinking of adding Primo instead of an AI." That is about 58 pg/mL, high but not alarming. Adding Primo may bring it down, or may not. If you go ahead, change nothing else and retest at 5 to 6 weeks by LC-MS/MS. Lowering your test dose is the more predictable lever; see estradiol on TRT.

"E2 dropped to 70 pmol/L after adding Primo. Is that too low?" 70 pmol/L is about 19 pg/mL, at the bottom of the 20 to 40 pg/mL range many TRT clinicians aim for. Fine if you feel fine. If your joints ache or libido drops, watch it: some users report it keeps falling for a few more weeks.

"E2 is under 40 pmol/L on test, Primo and an AI." That is about 11 pg/mL, close to the 10 pg/mL below which bone breakdown rose in a trial of healthy men (Finkelstein et al., 2016). Stop the AI first. Our crashed E2 guide has recovery times and a decision tree.

"E2 didn't move at all after adding Primo." That can be normal; many users see no change. But if nothing else moved either, no drop in SHBG or HDL and no change in how you look or feel, read the fake Primo section below.

SHBG and free testosterone on Primo

Methenolone binds SHBG less strongly than testosterone or DHT (Saartok et al., 1984). That still means it competes for the protein, and androgens in general lower SHBG: very low SHBG and HDL are listed as clues to steroid use (Anawalt, 2019). One user on test and Primo reported SHBG that "really dropped".

Expect your SHBG to fall, and read calculated free testosterone rather than total testosterone. A falling SHBG also changes free estradiol, which is one more reason total E2 alone does not tell the whole story. No study gives a Primo-specific number.

Primo, haematocrit and lipids

Haematocrit

Primo has an unusual history here: it was a prescription treatment for anaemia. In 30 patients with aplastic anaemia, adding oral methenolone to antithymocyte globulin raised the response rate from 31% to 73% (Kaltwasser et al., 1988), and it was used for other refractory anaemias in the 1970s and 80s (Lockner, 1979). It had mixed results in failing bone marrow, and Lockner saw no survival benefit. No study measures what it does to haematocrit in healthy men, and one case report saw oral Primo do nothing where testosterone later raised haemoglobin from 11.7 to 15.2 g/dL (Iijima et al., 2011).

What we do know is that androgens raise haematocrit in a dose-dependent way (Coviello et al., 2008), by raising erythropoietin and suppressing hepcidin (Bachman et al., 2014). On a test and Primo cycle, your total androgen load is what to watch. "Milder than EQ" may be true, but it is not haematocrit-neutral.

Lipids

Primo's reputation as the mildest steroid for cholesterol is reputation, not data. The only human lipid study of methenolone we found gave it to 28 women with breast cancer; 12 developed raised LDL-type lipid patterns, all reversible (Garbrecht et al., 1981). We found no study measuring HDL in men on Primo.

Class data gives the general shape. Injectable testosterone at 200 mg a week cut HDL by 9%, while oral stanozolol cut it by 33% (Thompson et al., 1989). In athletes running steroid stacks, HDL fell from 1.08 to 0.43 mmol/L, ApoB rose, and nothing had recovered six weeks after stopping. In the same paper, a controlled 8-week trial of injectable nandrolone alone did not change HDL (Hartgens et al., 2004), which suggests orals and stacks do most of the damage.

User reports fit "mild but not zero": HDL down 12 points after adding Primo in one log, 39 to 37 mg/dL on 300 test and 100 Primo in another, and HDL of 0.7 mmol/L with haematocrit 0.54 in a 58-year-old on test, Primo and NPP. Our cholesterol on steroids guide covers what to do about it.

Hair loss on Primo

Primo is already 5α-reduced, so it reaches your scalp's androgen receptors without the enzyme that finasteride blocks. Finasteride works by stopping testosterone becoming DHT (Kaufman et al., 1998), so it can lower the DHT made from your test, but it cannot touch Primo itself. No trial has tested this; it follows from the structure.

Forum reports range widely. One user says Masteron and Primo together "shed the hell out of my hair". Another says Primo is fine for most people but "absolutely nukes" his hair, while Masteron does not. If you are prone to hair loss, expect Primo to add to it, not to be the safe option. See our TRT and hair loss guide.

Shutdown and recovery

We found no modern study measuring how much methenolone suppresses your own testosterone production (a 1970 German study on methenolone and the pituitary has no abstract available). "Mild on the HPTA" is marketing, not a finding. What we have is class data from steroid users:

• Two and a half years after stopping, 27% of former users still had total testosterone below 12.1 nmol/L, against none of the controls (Rasmussen et al., 2016).
• Recovery took a mean of 10.7 months for LH and 14.1 months for sperm output, and longer use slowed sperm recovery, though the authors found recovery was effectively complete for most men within 6 to 18 months (Shankara-Narayana et al., 2020).

If you are on TRT, your own production is already switched off, and Primo adds to the androgen load. If you plan to come off, assume Primo suppresses like any other steroid. Our PCT bloodwork guide covers recovery.

Oral Primo acetate vs injectable enanthate

The oral form, methenolone acetate, is often described as "17α-alkylated" and liver toxic. It is not 17α-alkylated. Its oral activity is generally attributed to its 1-methyl group, a different structure from the 17α-alkyl group behind classic oral liver damage, which is well documented (Nieschlag and Vorona, 2015). We found no case reports clearly describing liver injury from methenolone, although one 1995 report of a "severe adverse effect" from oral methenolone gives no detail in its abstract (Geissler et al., 1995). That is not a guarantee: liver damage appeared in a 1979 series of anaemia patients on both alkylated and non-alkylated steroids (Androgen Therapy Study, 1979), so check ALT and AST on any oral. Our liver enzymes guide explains which rises matter.

The bigger practical problem with oral Primo is authenticity. It is rare and expensive, and an experienced forum member warns most "Primo tabs" are faked with Winstrol. That would explain the reputation for being liver-harsh.

There is no published human half-life for either form. In two men given 50 mg of oral acetate, the drug itself was detectable in urine for 90 hours (Goudreault and Massé, 1990). Users treat the enanthate like test enanthate, with a steady state after about 5 to 6 weeks. Draw your bloods at the same point in your injection cycle every time; our blood test timing guide has the method.

Fake and underdosed Primo

Primo is widely reported as one of the most faked steroids, because it is expensive and often scarce. Across 19 studies of black-market steroids, 36% were counterfeit and a further 37% were substandard (Magnolini et al., 2022). The same review lists methenolone esters among those swapped for testosterone or trenbolone. Under 20% of doping products seized at the Swiss border contained the claimed drug at the claimed dose (Weber et al., 2017).

The forums are full of examples. A Janoshik test found a lab's "Primo" was testosterone propionate. A user peeled the Primo label off a vial to find a Deca label underneath. A university HPLC run on another product found no steroid at all.

Your bloodwork can hint at what is in the vial:

• E2 and total testosterone both jump after adding "Primo": suspect testosterone.
• Bloating, a big appetite rise and higher prolactin: suspect nandrolone.
• Nothing moves at all, not SHBG, not HDL, not LH, after 6 weeks at 400 mg or more: suspect an underdosed or inert product.

Our guide to testing UGL steroids covers how people check what they are injecting.

Test to Primo ratio: set it from your numbers

Forum ratios range from 2:1 test to Primo through 1:1 and beyond, with some running Primo well above test. Rather than pick a number, let your bloodwork decide.

Never run Primo without a testosterone base. On its own it shuts down your own production and supplies no estrogen at all, which is a reliable route to genuinely low E2.

What users report on the forums (anecdotal)

This section is anecdotal: self-reports, unverified products, and plenty of vendor representatives. It is useful for spotting patterns and useless for proving anything.

Camp A: "Primo lowers E2." "Primo will act as an AI and will reduce your e2. On 1,000mg of primo your estrogen will crash" (EliteFitness). "Primo should simply be used as a tool to help control and manage E2 while increasing test, same concept as EQ" (Evo). One user found that 400 mg of Primo on 800 mg of test crushed his libido and energy, and that E2 recovered when he backed off (Evo).

Camp B: "It's just less test." "Primo does not reduce how much testosterone converts to estrogen, but if I have less testosterone from the 400 mg total, overall the level of conversion should be less" (Evo). A long-time user on Anabolex: "back in the day we used to stack tons of primo with little testosterone. We never had estrogen problems" (Anabolex). Another veteran had "always had to utilise AI to keep e2 at bay" on test and Primo (Evo).

The middle ground may be closest to the truth. One experienced member put it as "some guys have about 5% cut e2 on primo, some nothing and some are hyper responders" (Evo). The same member, in other threads, says Primo is "not for e2 control". Even the people defending it don't treat it as a reliable AI.

Other patterns that came up again and again: libido loss at Primo-heavy ratios, hair shedding as the main reason people stop, painful injections, and Primo being scarce and frequently fake.

The Primo bloodwork panel

Frequently asked questions

Does Primo act like an AI?

Not reliably. It cannot become estrogen, and some users see their E2 fall after adding it, but no study has shown it blocks aromatase, and plenty of users see no change. If you need predictable estrogen control, lowering your test dose is the dependable lever.

Is Primo better than Masteron for estrogen?

Users report Primo lowering E2 more often than Masteron, which mostly seems to change how estrogen feels rather than the number. Neither has been studied properly. Our Masteron vs Primobolan comparison covers the rest of the differences.

Can Primo crash your estrogen?

It can happen, mostly at Primo-heavy ratios, with a small test base, or with an AI on top. If your E2 is low and you feel flat, achy and uninterested in sex, see our crashed E2 guide.

Is oral Primo liver toxic?

Methenolone acetate is not 17α-alkylated, so it should be far easier on the liver than classic orals, and we found no liver injury reports. Many oral "Primo" products are reportedly something else, though, so check ALT and AST anyway.

Does Primo raise haematocrit?

Probably, to some degree. It was used medically to treat anaemia, and every androgen raises red cell production. No study has measured how much in healthy men.

Key takeaways

• Primo is a DHT derivative. It cannot become estrogen
• No study has shown Primo blocks aromatase or lowers serum E2. A 2024 lab study found related compounds can, but not the drug itself
• User bloodwork is split: drops like 213 to 70 pmol/L next to no change at all, and every case had something else changing
• Less test, LH suppression, assay bias and stopping other drugs can all lower E2 after adding Primo
• Expect SHBG and HDL to fall and haematocrit to rise. Mild is not zero
• Oral Primo acetate is 1-methylated, not 17α-alkylated, but fakes are common
• Primo is heavily counterfeited. If nothing in your bloodwork moves, suspect the vial
• Never run it without a testosterone base, and test E2 by LC-MS/MS

---

References

1. Androgen therapy of aplastic anaemia: A prospective study of 352 cases. (1979). Scandinavian Journal of Haematology, 22(4), 343-356. PubMed
2. Abdul Karim, A., Atia-Tul-Wahab, Aziz, A., Shaikh, N. N., & Choudhary, M. I. (2024). Biotransformation of metenolone acetate and epiandrosterone by fungi and evaluation of resulting metabolites for aromatase inhibition. Steroids, 202, 109345. PubMed
3. Anawalt, B. D. (2019). Diagnosis and management of anabolic androgenic steroid use. Journal of Clinical Endocrinology & Metabolism, 104(7), 2490-2500. PubMed
4. Bachman, E., Travison, T. G., Basaria, S., et al. (2014). Testosterone induces erythrocytosis via increased erythropoietin and suppressed hepcidin: Evidence for a new erythropoietin/hemoglobin set point. Journals of Gerontology Series A, 69(6), 725-735. PubMed
5. Coviello, A. D., Kaplan, B., Lakshman, K. M., et al. (2008). Effects of graded doses of testosterone on erythropoiesis in healthy young and older men. Journal of Clinical Endocrinology & Metabolism, 93(3), 914-919. PubMed
6. Garbrecht, M., Lehmann, U., O'Brien, S., Stolzenbach, G., & Müllerleile, U. (1981). Hyperlipoproteinaemia during additional methenolone administration in metastasizing breast carcinoma. Deutsche Medizinische Wochenschrift, 106(13), 400-403. PubMed
7. Goudreault, D., & Massé, R. (1990). Studies on anabolic steroids 4. Identification of new urinary metabolites of methenolone acetate (Primobolan) in human by gas chromatography/mass spectrometry. Journal of Steroid Biochemistry and Molecular Biology, 37(1), 137-154. PubMed
8. Handelsman, D. J., Newman, J. D., Jimenez, M., McLachlan, R., Sartorius, G., & Jones, G. R. (2014). Performance of direct estradiol immunoassays with human male serum samples. Clinical Chemistry, 60(3), 510-517. PubMed
9. Hartgens, F., Rietjens, G., Keizer, H. A., Kuipers, H., & Wolffenbuttel, B. H. (2004). Effects of androgenic-anabolic steroids on apolipoproteins and lipoprotein (a). British Journal of Sports Medicine, 38(3), 253-259. PubMed
10. Finkelstein, J. S., Lee, H., Leder, B. Z., et al. (2016). Gonadal steroid-dependent effects on bone turnover and bone mineral density in men. Journal of Clinical Investigation, 126(3), 1114-1125. PubMed
11. Geissler, K., Ganser, A., Kaltwasser, J. P., & Hoelzer, D. (1995). Partial remission and severe adverse effect caused by metenolone acetate in a male patient with aplastic anemia. European Journal of Haematology. PubMed
12. Hillier, S. G., van den Boogaard, A. M., Reichert, L. E., Jr, & van Hall, E. V. (1980). Alterations in granulosa cell aromatase activity accompanying preovulatory follicular development in the rat ovary with evidence that 5alpha-reduced C19 steroids inhibit the aromatase reaction in vitro. Journal of Endocrinology, 84(3), 409-419. PubMed
13. Iijima, M., et al. (2011). Myelodysplastic syndrome treated effectively with testosterone enanthate. International Journal of Urology, 18(6), 469-471. PubMed
14. Kaltwasser, J. P., Dix, U., Schalk, K. P., & Vogt, H. (1988). Effect of androgens on the response to antithymocyte globulin in patients with aplastic anaemia. European Journal of Haematology, 40(2), 111-118. PubMed
15. Kaufman, K. D., et al. (1998). Finasteride in the treatment of men with androgenetic alopecia. Journal of the American Academy of Dermatology, 39(4), 578-589. PubMed
16. Kirilovas, D., Naessen, T., Bergström, M., et al. (2003). Effects of androgens on aromatase activity and 11C-vorozole binding in granulosa cells in vitro. Acta Obstetricia et Gynecologica Scandinavica, 82(3), 209-215. PubMed
17. Lakshman, K. M., Kaplan, B., Travison, T. G., et al. (2010). The effects of injected testosterone dose and age on the conversion of testosterone to estradiol and dihydrotestosterone in young and older men. Journal of Clinical Endocrinology & Metabolism, 95(8), 3955-3964. PubMed
18. Lockner, D. (1979). Treatment of refractory anemias with methenolone. Acta Medica Scandinavica, 205(1-2), 97-101. PubMed
19. Magnolini, R., Falcato, L., Cremonesi, A., Schori, D., & Bruggmann, P. (2022). Fake anabolic androgenic steroids on the black market: A systematic review and meta-analysis. BMC Public Health, 22, 1371. PubMed
20. Nieschlag, E., & Vorona, E. (2015). Doping with anabolic androgenic steroids (AAS): Adverse effects on non-reproductive organs and functions. Reviews in Endocrine and Metabolic Disorders, 16(3), 199-211. PubMed
21. Rasmussen, J. J., Selmer, C., Østergren, P. B., et al. (2016). Former abusers of anabolic androgenic steroids exhibit decreased testosterone levels and hypogonadal symptoms years after cessation: A case-control study. PLoS ONE, 11(8), e0161208. PubMed
22. Saartok, T., Dahlberg, E., & Gustafsson, J. A. (1984). 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, 114(6), 2100-2106. PubMed
23. Shankara-Narayana, N., et al. (2020). Rate and extent of recovery from reproductive and cardiac dysfunction due to androgen abuse in men. Journal of Clinical Endocrinology & Metabolism. PubMed
24. Spitz, I. M., Margalioth, E. J., Yeger, Y., Livshin, Y., Zylber-Haran, E., & Shilo, S. (1984). Effect of non aromatizable androgens on LHRH and TRH responses in primary testicular failure. Hormone and Metabolic Research, 16(9), 492-497. PubMed
25. Thompson, P. D., Cullinane, E. M., Sady, S. P., et al. (1989). Contrasting effects of testosterone and stanozolol on serum lipoprotein levels. JAMA, 261(8), 1165-1168. PubMed
26. Vermeulen, A., Kaufman, J. M., Goemaere, S., & van Pottelberg, I. (2002). Estradiol in elderly men. The Aging Male, 5(2), 98-102. PubMed
27. Weber, C., Krug, O., Kamber, M., & Thevis, M. (2017). Qualitative and semiquantitative analysis of doping products seized at the Swiss border. Substance Use & Misuse, 52(6), 742-753. PubMed
28. PubChem. Methenolone (CID 3037705). PubChem