In comparison tables of steroids, Stanozolol is almost always marked with "no estrogenic activity, no progestogenic activity." This is generally true, but behind the short label there are several nuances that are worth knowing about: the indirect effect on the balance of sex hormones, the role of SHBG, and widespread Internet claims about the "antiprogestin" effect. The editors analyze what is confirmed from this.
Why Stanozolol does not convert to estrogen
Estrogens in the body are formed from androgens thanks to the enzyme aromatase (CYP19A1). It turns the A ring of the steroid molecule into an aromatic one: this is how estradiol is formed from testosterone, and estrone from androstenedione. This reaction requires a certain structure of ring A, in particular a double bond and a 3-keto group.
Stanozolol is based on dihydrotestosterone, in which the A ring is saturated (5α-reduced). In addition, it has a pyrazole ring in place of the 3-keto group. Such a molecule is not a substrate of aromatase, so stanozolol is not converted into estrogens. This is also confirmed by the fact that there are no aromatic compounds among its metabolites described by Schänzer et al.
Stanozolol does not have an established clinically significant direct estrogenic effect. This differs from testosterone and metandienone, which can generate estrogenic metabolites. Oxymetholone is a separate case: it can produce estrogen-like adverse effects without aromatization, and should not be grouped with those two mechanisms.
Practical consequence — on the background of stanozolol, fluid retention associated with estrogens, characteristic of aromatizable androgens, does not occur. Hence the idea of a "dry" look, which we analyze separately in the article on myths.
| Drug | Aromatization | Progestogenic activity |
|---|---|---|
| Testosterone | Yes | Practically none |
| Nandrolone | Weak | Pronounced |
| Metandienone | Yes | Minor |
| Stanozolol | No | Clinically significant has not been described |
Progestogenic activity: what is known
Progestogenic activity means activation of progesterone receptors and is described for several 19-nortestosterone derivatives, including nandrolone and trenbolone. This should not be treated as proof that they invariably raise prolactin or that prolactin alone explains breast symptoms; those require clinical investigation.
Stanozolol does not belong to the 19-norsteroids: it has a methyl group in position 19. There are no data on the significant activation of progesterone receptors by stanozolol in human studies, and reviews of the pharmacology of anabolic steroids do not attribute this property to it.
There is a widespread claim on the Internet that stanozolol is a "progesterone receptor antagonist" and therefore "protects" against the progestogenic effects of other drugs. The editors are not aware of a single human clinical study that would confirm this. Even if a certain interaction was observed in individual experimental models, it is incorrect to transfer it to a person without clinical data.
Hence the practical conclusion: from the point of view of pharmacology, stanozolol is a "pure" androgen without an estrogenic and progestogenic component. Any claims about its ability to neutralize the effects of other steroids should be considered unverified.
Indirect effect on estrogens
The absence of its own estrogenic activity does not mean that stanozolol does not affect estrogens at all. The first mechanism is suppression of the hypothalamus-pituitary-gonadal axis. When one's own testosterone falls, the substrate for aromatase also decreases, so the level of estradiol in a man can decrease.
Low estradiol in men is not a welcome "bonus", but a problem. Estrogens are needed to maintain bone mineral density, normal lipid metabolism, libido, and erectile function. A long-term deficiency can be manifested by a decrease in sexual desire, fatigue, and mood deterioration.
The second mechanism is related to SHBG. Stanozolol strongly reduces the synthesis of this protein in the liver. SHBG binds not only testosterone, but also estradiol, so its reduction increases the free fraction of both hormones. If stanozolol is combined with aromatizable androgens, free estradiol may increase, and then the estrogenic effects will be more pronounced, although stanozolol itself is not converted to estrogen.
In women, the picture is different. During stanozolol, a violation of the menstrual cycle, up to its termination, has been described. This is due to gonadotropin suppression and direct androgenic action rather than estrogenic or progestogenic activity.
Gynecomastia: is it possible on stanozolol
Gynecomastia is a benign growth of glandular breast tissue in men. Its main cause is a violation of the balance between estrogens (which stimulate the tissue) and androgens (which suppress it). A review by Braunstein (2007) in the New England Journal of Medicine lists anabolic steroids among the known medical causes of this condition.
Stanozolol itself should not cause gynecomastia, as it is not an estrogen and acts as an androgen. However, in real conditions it is rarely used in isolation. The combination with aromatizable drugs and the reduction of SHBG can increase estrogen stimulation, and in the period after withdrawal, when exogenous androgens have already been eliminated, and own testosterone has not yet recovered, the balance can shift in favor of estrogens.
Another factor is prolactin. It is not directly related to stanozolol, but when combined with 19-norsteroids, its role in the development of changes in the breast gland is discussed. Therefore, the doctor should find out the causes of gynecomastia in each specific case, and not "guess" from the list of drugs.
Signs that require consultation with a doctor:
- a firm area or tenderness under the nipple on one or both sides;
- nipple discharge;
- asymmetric or rapid breast enlargement;
- solid, immobile mass — it should be examined to rule out other causes.
Editorial conclusions
Stanozolol is not aromatized and has no clinically significant estrogenic activity. It also does not have a progestogenic effect, and claims about "antiprogestin" properties are not confirmed by clinical data.
At the same time, the drug indirectly changes the hormonal balance: it reduces its own testosterone, and together with it, estradiol, and reduces the level of SHBG, which increases the free fraction of other hormones.
Gynecomastia directly related to stanozolol is not characteristic, but is possible in combinations and in the period after withdrawal. The lack of estrogenic activity does not make the drug safe — its risks lie in other areas.
We also recommend that you read our articles on the effects of stanozolol on your own testosterone production, myths about this drug, and a full overview of its side effects.
References
- Kicman AT. Pharmacology of anabolic steroids. Br J Pharmacol. 2008;154(3):502–521.
- Schänzer W. Metabolism of anabolic androgenic steroids. Clin Chem. 1996;42(7):1001–1020.
- Braunstein GD. Gynecomastia. N Engl J Med. 2007;357(12):1229–1237.
- 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.
- Pope HG Jr, Wood RI, Rogol A, et al. Adverse health consequences of performance-enhancing drugs: an Endocrine Society scientific statement. Endocr Rev. 2014;35(3):341–375.
- Nieschlag E, Behre HM, Nieschlag S (eds). Testosterone: Action, Deficiency, Substitution. 4th ed. Cambridge University Press; 2012.
