History

S23 was developed by GTx, Inc. (the Dalton/Miller group at the University of Tennessee) as a nonsteroidal aryl-propanamide androgen receptor ligand, investigated chiefly as a candidate male hormonal contraceptive rather than as a muscle-building drug. The key rodent work was published by Jones and colleagues in Endocrinology in 2009, building on earlier medicinal-chemistry characterization of this metabolically stable SARM class (Marhefka et al., J Med Chem, 2004). It later appeared as a "designer drug" in the research-chemical and bodybuilding markets.

S23 is a synthetic selective androgen receptor modulator (SARM) — explicitly not a peptide — that binds the androgen receptor and acts as a tissue-selective agonist, aiming for anabolic effects in muscle and bone with relatively less prostate/seminal activity. It was designed as a male contraceptive candidate, and the only solid evidence for it comes from rats. Despite vendor and biohacking marketing as a potent recomposition compound, there are no published controlled human trials of S23 for any purpose.

What it is

S23 (also called “Mastorin”; CAS 1010396-29-8; formula C18H13ClF4N2O3) is a small synthetic molecule, a nonsteroidal aryl-propanamide androgen receptor ligand. As a SARM, it binds the androgen receptor and acts as a tissue-selective agonist. Like other AR agonists, it suppresses the hypothalamic–pituitary–gonadal axis (lowering LH and FSH) — the mechanism behind both its intended contraceptive effect and its endocrine side effects. It was developed by GTx, Inc. as a candidate male hormonal contraceptive, not as a muscle drug.

The claims

Research-chemical vendors and bodybuilding/biohacking circles market S23 as a potent “lean mass / strength / recomp” SARM. These claims are marketing and anecdote, not clinical findings. Vendor sites typically carry a “not for human consumption / research use only” disclaimer while the product is nonetheless purchased for personal use.

What the evidence actually shows

Evidence on S23 is preclinical only (rodent); there are no published controlled human trials for any indication.

The key primary study (Jones A, Chen J, Hwang DJ, Miller DD, Dalton JT. Endocrinology. 2009;150(1):385–395) found that in male rats, S23 produced dose-dependent suppression of spermatogenesis — “four of six animals showed no sperm in the testis.” Combined with low-dose estradiol benzoate it produced azoospermia in most animals and zero pregnancies in mating trials. Crucially, the infertility was fully reversible — a 100% pregnancy rate returned after about 100 days of recovery off-drug — which is why S23 was studied as a contraceptive. An earlier paper (Marhefka CA, Gao W, Chung K, et al. J Med Chem. 2004;47(4):993–998) characterized this class of metabolically stable SARMs.

Bottom line on evidence: the claimed bodybuilding benefits in humans are unproven; the only robust data are rodent reproductive and endocrine studies.

Legal and regulatory status

FDA: SARMs are not approved drugs and are not lawful dietary-supplement ingredients. The FDA classifies SARM-containing bodybuilding products as unapproved, misbranded drugs and warns of risk of heart attack, stroke, and serious liver damage. The FDA issued warning letters to Infantry Labs (Oct 23, 2017), IronMagLabs (Oct 22, 2017), and Panther Sports Nutrition (Oct 23, 2017), plus a consumer advisory aimed at teens and young adults.

DEA scheduling: SARMs as a class are not currently federal controlled substances. S23 has been encountered as a designer drug (by 2020). The SARMs Control Act (S.2742, 2018; S.2895, 2019; and later reintroductions) sought to place SARMs in Schedule III, but no version has been enacted into law as of mid-2026 — treat federal scheduling as proposed, not law.

For contrast, oxandrolone is an FDA-approved anabolic-androgenic steroid and a Schedule III controlled substance — but it is a steroid, not a SARM, and not the same compound class as S23.

Anti-doping status

All SARMs, including S23, are prohibited at all times (in- and out-of-competition) under Section S1.2 “Other Anabolic Agents” of the WADA Prohibited List. Named examples include andarine, ostarine/enobosarm, LGD-4033, RAD140, S-23, and YK-11. (Note: GW-501516 [cardarine] and SR9009 are not SARMs — GW-501516 is a PPARδ agonist listed separately under S4 Metabolic Modulators.)

Safety

Human safety data for S23 are essentially absent; risk inferences come from the SARM class and rodent data.

Endocrine suppression: The best-documented effect. S23 suppresses LH/FSH and spermatogenesis in rats (reversible in those studies). Expected human correlates of AR agonists are testosterone suppression and impaired fertility, though no human data quantify this for S23.
Lipids: SARMs as a class are repeatedly associated with lowered HDL cholesterol — a class signal, not specifically quantified for S23 in humans.
Hepatotoxicity: FDA and case-report literature link SARM-containing products to drug-induced liver injury, some requiring hospitalization. Real-world products are also frequently mislabeled or contaminated.
Cardiovascular: FDA cites increased risk of heart attack and stroke with SARM products.
Class context (not established for S23): Andarine (S4) causes yellow-tinted vision and impaired light/dark adaptation (reported reversible) — specific to andarine. GW-501516, a PPARδ agonist (not a SARM, not S23), had development halted by GlaxoSmithKline around 2007 after long-term rodent carcinogenicity studies showed tumors in multiple organ systems.
Honest gaps: There is no reliable evidence of permanent blindness or QT prolongation specifically attributable to S23.

Bottom line

S23 is a SARM, not a peptide. It was created as a male contraceptive candidate and has been studied only in rodents, where it reversibly suppressed sperm production. There are no human trials, so the lean-mass and recomp claims are unverified marketing. It is an unapproved, misbranded drug per the FDA, carries documented class risks (liver injury, cardiovascular events, lowered HDL, hormonal suppression), and is banned at all times in sport. The honest summary: minimal evidence of benefit in humans, real and partly unquantified risk.

Evidence grade: Animal only.