S-23

Overview

S-23 (also sold as Mastorin) is a nonsteroidal selective androgen receptor modulator (SARM) of the arylpropionamide class. It is a true SARM — it is NOT a peptide and NOT a steroid. It appears in this peptide reference because it is discussed and stacked alongside peptides in the fitness and "research chemical" community, not because it is one.

S-23 was developed preclinically by GTx, Inc. and never entered a clinical trial in humans. There is no approved medical use for any indication. Everything known about it comes from rodent and in-vitro studies. In rats it binds the androgen receptor with very high affinity (Ki ~1.7 nM) — stronger than andarine — and shows a favorable ratio of anabolic (muscle, bone) to androgenic activity. Its defining preclinical finding is dose-dependent, fully reversible suppression of spermatogenesis to azoospermia: S-23 was actually studied as a candidate reversible male hormonal contraceptive.

Chemistry and structure

S-23 is a nonsteroidal arylpropionamide small molecule, not an amino-acid chain and not a steroid nucleus. Its IUPAC name is (2S)-3-(4-chloro-3-fluorophenoxy)-N-(4-cyano-3-(trifluoromethyl)phenyl)-2-hydroxy-2-methylpropanamide.

Mechanism of action

• Androgen receptor (AR) agonist. S-23 is a nonsteroidal arylpropionamide-class agonist of the androgen receptor. It is structurally related to andarine and to bicalutamide-type antiandrogens, but acts as a tissue-selective AR agonist. It is not a peptide and not a steroid. [In vitro]
• Very high AR binding affinity. In binding assays it shows Ki ≈ 1.7 nM — stronger binding than the earlier SARM andarine. This high-affinity AR agonism is the basis of both its anabolic and its gonadal-suppressive actions. [In vitro]
• Anabolic, tissue-selective effects (rodent). In castrated and intact male rats, S-23 produced dose-dependent anabolic effects — increased lean muscle mass and bone mineral density, decreased fat mass — with a favorable anabolic-to-androgenic profile. Tissue selectivity is the general SARM design goal; it has been demonstrated only in rodents for S-23. [Animal]
• HPTA / gonadal suppression (the defining finding). S-23 dose-dependently suppresses spermatogenesis, producing azoospermia, and suppresses serum LH and FSH — i.e. it suppresses the hypothalamic-pituitary-testicular (HPTA) axis. Effects were fully reversible after withdrawal, with return of fertility. It was studied as a candidate reversible male hormonal contraceptive, alone and combined with estradiol benzoate — shown in rats only, never in humans. [Animal]

Research and evidence

The evidence base is entirely preclinical (rodent and in-vitro). There are no human trials of S-23 for any indication.

Human evidence in context. There is none. S-23 never entered a clinical trial, so there are no completed human studies establishing efficacy for muscle growth, fat loss, contraception, or any medical indication. The anabolic and contraceptive claims rest on rat studies; the binding data are in vitro. Human pharmacokinetics — half-life, bioavailability, and metabolism — are not established / not found in sources, because no human data exist.

Status and regulation

• Regulatory approval: None. S-23 is not approved by the FDA, the EMA, or any other regulator for any indication. Its highest phase of development is preclinical — it never entered a clinical trial. It never advanced beyond preclinical work at GTx and later surfaced as a grey-market designer drug.
• Anti-doping: Prohibited by the World Anti-Doping Agency (WADA) at all times (in- and out-of-competition), classified under Anabolic Agents — Other Anabolic Agents (SARMs), S1.2. WADA sport-eligibility is a separate axis from legality.
• Market reality: It is sold only as a "research chemical" / grey-market compound, frequently marketed as "the strongest SARM." Such products are of unknown identity, purity, and potency (see Safety, below).

Safety

No human safety data of any kind. Because S-23 was never tested in humans, there is no dose, no toxicology, no long-term safety, and no drug-interaction profile. Hepatotoxicity specific to S-23 is not established — only the class-level SARM liver signal is documented. The absence of trials means adverse effects in humans are unquantified, not absent. [Hypothesis] The overall picture: S-23 has a known, drug-specific contraceptive-level gonadal-suppression signal in animals, sits within a drug class the FDA has flagged for liver injury and other serious harms, and — as an unregulated research chemical — carries the additional, separate hazards of contamination and mislabeling.

Legal status

How it compares

S-23 is often grouped with other SARMs and "recomposition" research chemicals, but each has its own distinct profile and none is an approved physique/performance drug:

• Andarine (S-4) — the closest chemical relative: another GTx-developed arylpropionamide SARM. Andarine at least reached early Phase 1 before being abandoned over a visual side effect; S-23 never entered human trials at all and binds the AR more tightly (Ki ~1.7 nM vs andarine).
• Cardarine (GW-501516) — frequently sold and stacked alongside SARMs, but it is not a SARM: it is a PPARδ agonist whose development was abandoned over a rodent cancer signal. Different receptor, different defining hazard.

The honest framing: S-23 is a preclinical, unapproved research reagent with a contraceptive-level gonadal-suppression signal in animals and class-wide FDA-flagged risks — it is not a "milder, safer, or legal alternative to steroids," and its "strongest SARM" reputation rests on no human evidence. See the muscle growth & hormone overview.

Common misconceptions

• "S-23 is a peptide." No. It is a nonsteroidal arylpropionamide small molecule (C18H13ClF4N2O3, 416.76 g/mol). It is not a peptide and not a steroid. It is covered here only because it is discussed and stacked with peptides.
• "S-23 is the strongest, so it's the best SARM." Its high binding affinity (Ki ~1.7 nM) is a rodent/in-vitro property. "Strongest" here is grey-market marketing, not a human efficacy or safety claim — there is zero human data.
• "SARMs like S-23 are a safe, legal alternative to steroids." No. S-23 is preclinical and never approved, its drug class is FDA-flagged for liver injury and testosterone suppression, and it strongly suppresses the reproductive axis in animals (it was studied as a contraceptive). It is prohibited in sport and sold only as an unregulated research chemical.
• "The suppression is reversible, so it's low-risk." Reversibility was shown in rats, not humans. The human recovery timeline is unknown, and contraceptive-level suppression of testosterone/LH/FSH is a serious effect, not a trivial one.
• "If it's labeled S-23, that's what's in it." Not reliably. In the JAMA 2017 analysis, only about half of products sold as SARMs contained the labeled compound; many contained a different drug or nothing active.

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This entry is educational and summarizes published research and public regulatory information. It is not medical advice, a recommendation, or a guide to obtaining or using any substance. Not legal advice — check your own jurisdiction. Last reviewed 2026-07-10.