Section 1 of 9Overview
Overview
S-1 (also GTx S-1) is an early nonsteroidal selective androgen receptor modulator (SARM) of the arylpropionamide class. Chemically it is a small molecule — it is a true nonsteroidal SARM, NOT a peptide and NOT a steroid. It appears in this peptide reference because SARMs are frequently discussed and stacked alongside peptides in the fitness and "research chemical" community, not because S-1 is one.
S-1 was developed by James T. Dalton, Duane D. Miller and colleagues at the University of Tennessee / GTx, Inc. It is a close structural analog of andarine (S-4): the two come from the same arylpropionamide patent series (adjacent CAS registry numbers — S-1 is 401900-41-2, andarine is 401900-40-1), and S-1 differs mainly at the phenoxy B-ring substituent (a 4-fluorophenoxy ether in S-1 versus andarine's acetamido/nitro-phenoxy group). In the foundational pharmacology paper (Yin et al., 2003), S-1 was one of several arylpropionamide leads characterized in vitro and in castrated rats.
S-1 is purely preclinical. It was an early lead in the program that ultimately advanced enobosarm (ostarine) into clinical development — not S-1 itself. No human clinical trial for S-1 was identified, and there are no human safety or efficacy data.
Preclinical only — never tested in humans, not approved
S-1 has been studied only in animals and in vitro. No human trials were identified, there are no human safety or efficacy data, and it is not approved by any regulator (FDA, EMA, or otherwise) for any use. It exists as a research reagent only. As a SARM it is prohibited in sport by WADA at all times. Do not confuse this compound (PubChem CID 5288215) with the anticancer combination drug also called "S-1" (tegafur/gimeracil/oteracil, CID 146157327) — they are different chemicals. This page is educational and is not medical advice; nothing here endorses or guides human use.
Section 2 of 9Chemistry and structure
Chemistry and structure
S-1 is a nonsteroidal arylpropionamide small molecule, not an amino-acid chain and not a steroid nucleus. Its IUPAC name is (2S)-3-(4-fluorophenoxy)-2-hydroxy-2-methyl-N-[4- nitro-3-(trifluoromethyl)phenyl]propanamide.
| Property | Value |
|---|---|
| Name | S-1 (GTx S-1) |
| Class | Nonsteroidal selective androgen receptor modulator (arylpropionamide) — not a peptide, not a steroid |
| Molecular formula | C17H14F4N2O5 |
| Molecular weight | 402.30 g/mol |
| CAS number | 401900-41-2 |
| PubChem CID | 5288215 |
Section 3 of 9Mechanism of action
Mechanism of action
- Androgen receptor (AR) ligand. S-1 binds the androgen receptor as a nonsteroidal arylpropionamide ligand. In the lead series characterized by Yin et al. 2003, the arylpropionamide compounds (including S-1 and S-4/andarine) showed moderate-to-high AR binding affinity, with reported Ki values across the series in roughly the 4–37 nM range. Arylpropionamide SARMs are structural derivatives of the AR antagonist bicalutamide, re-engineered toward agonist activity. [In vitro]
- Tissue-selective anabolic agonist. In castrated rats, S-1 stimulated the levator ani (anabolic) muscle to a greater relative degree than the androgenic organs (prostate, seminal vesicles) — the defining SARM tissue-selectivity profile. Its anabolic activity was similar to or greater than testosterone propionate while its androgenic potency/efficacy was lower. This selectivity is thought to arise from tissue-specific AR conformation / coregulator recruitment rather than 5-alpha-reductase amplification. [Animal]
- Andarine (S-4) analog. S-1 differs from andarine principally at the phenoxy B-ring substituent (a fluorine / 4-fluorophenoxy group in S-1 versus andarine's acetamido-nitro group). Their adjacent CAS numbers (S-1 401900-41-2; andarine 401900-40-1) place them in the same arylpropionamide patent series. [In vitro]
- No significant LH/FSH suppression near effective doses in rats. Neither S-1 nor S-4 caused significant luteinizing hormone (LH) or follicle-stimulating hormone (FSH) suppression at doses near the ED50 in the rat model. This is an animal finding only — it does NOT establish that S-1 spares the human hypothalamic-pituitary-testicular (HPTA) axis, and the SARM class broadly is associated with testosterone/HPTA suppression in humans. [Animal]
Section 4 of 9Research and evidence
Research and evidence
| Finding | Model | Source |
|---|---|---|
| S-1 is a tissue-selective anabolic agonist in castrated rats — stimulating anabolic tissue (levator ani muscle) more than androgenic tissue (prostate/seminal vesicles) | [Animal] — castrated male rats | Yin D, Gao W, Kearbey JD, et al. (Miller DD, Dalton JT). J Pharmacol Exp Ther 2003;304(3):1334–40 (PMID 12604714) |
| Nanomolar AR binding affinity for the arylpropionamide lead series including S-1 (series Ki ~4–37 nM) | [In vitro] — AR binding assay | Yin et al. 2003 (PMID 12604714) |
| Did not significantly suppress LH/FSH near the ED50 in rats (animal finding only) | [Animal] — castrated male rats | Yin et al. 2003 (PMID 12604714) |
| No human clinical trials identified for S-1; the arylpropionamide program advanced enobosarm/ostarine rather than S-1 | [Human] — none found | ClinicalTrials.gov search returned no S-1 (arylpropionamide) trials; historical accounts describe S-1 as an early lead |
Human evidence in context. There is none. S-1 was never taken into human trials; it was an early preclinical lead in a program that clinically advanced a different compound (enobosarm/ostarine). All positive findings for S-1 come from in vitro binding work and castrated-rat pharmacology. Human pharmacokinetics (bioavailability, half-life) and even detailed rat PK for S-1 specifically were not found in these sources, and the single S-1 AR-binding Ki was not isolated from the reported series range in the reviewed sources.
Section 5 of 9Status and regulation
Status and regulation
- Regulatory approval: None. S-1 is not approved by the FDA, the EMA, or any other regulator for any indication. It is a preclinical research reagent that never entered human development.
- Anti-doping: Prohibited by the World Anti-Doping Agency (WADA) at all times (in- and out-of-competition) as a SARM under Anabolic Agents — Other Anabolic Agents (S1.2) (per USADA's summary of the Prohibited List). WADA sport-eligibility is a separate axis from legality.
- Identity caution: The anticancer combination drug "S-1" (tegafur / gimeracil / oteracil, PubChem CID 146157327) shares this compound's name but is an entirely different chemical entity. Do not conflate the two.
Section 6 of 9Safety
Safety
No human safety data — class-wide SARM signals apply
There are no human safety data for S-1. It has never been studied in humans, so any tolerability inference is extrapolated from the SARM class and from limited rat data. Absence of adverse findings in preclinical rat work is NOT evidence of human safety. [Hypothesis]
Class-wide SARM safety signals apply. The FDA has warned that SARMs can cause serious liver injury, including acute liver failure, and that SARM-containing bodybuilding products put consumers at risk of heart attack and stroke. SARMs as a class suppress endogenous testosterone and gonadotropins in humans, with reports of testicular shrinkage and infertility. In rats S-1 did not significantly suppress LH/FSH near its ED50, but this animal finding does not transfer to humans, and no human endocrine data for S-1 exist. Multiple published human case reports document drug-induced liver injury (DILI) from other marketed SARMs (e.g. LGD-4033/ligandrol, RAD140/testolone). [Human] (class-level); [Animal] (rat non-suppression, Yin et al. 2003).
Contamination reality — a label is not the contents
Products sold as "SARMs" frequently do not contain what the label claims. In an analysis of 44 products sold as SARMs (Van Wagoner et al., JAMA 2017;318(20):2004–2010, PMID 29183075), only 52% actually contained the labeled SARM, 39% contained a different unapproved drug, and 9% contained no active compound at all. The word "SARM" on a label is not a reliable statement of contents — identity, dose, and purity are unknown for research-grade or black-market material.
The overall picture: S-1 has no human data of any kind, 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.
Section 7 of 9Legal status
Legal status
Legality not assessed here
This page does not assess the legality of buying or possessing S-1 in any specific country. What is documented: it is not an approved medicine, supplement, or food ingredient anywhere, it is a preclinical research reagent only, and it is not legal to sell for human consumption. A compound like this becomes an unauthorised medicine the moment it is intended for human use. Separately, it is prohibited in sport by WADA at all times (S1.2). "Research chemical" status is a legal and safety gray zone — it is not a signal of safety or approval. This is not legal advice; check your own jurisdiction. (As of 2026-07.)
Section 8 of 9How it compares
How it compares
S-1 is best understood next to its own analog and its successor in the same program:
- Andarine (S-4) — S-1's close structural analog from the same arylpropionamide patent series (adjacent CAS numbers). Andarine progressed further — into an early, abandoned Phase 1 program halted over a visual side effect — whereas S-1 remained a preclinical lead.
- Ostarine (enobosarm) — the compound the arylpropionamide program actually advanced into human trials, in place of S-1. It is the far better-characterized clinical candidate; S-1 never reached that stage.
The honest framing: S-1 is an early, purely preclinical research reagent with no human data, and it belongs to a drug class the FDA has flagged for liver injury and testosterone suppression — it is not a "milder, safer, or legal alternative to steroids." See the muscle growth & hormone overview.
Section 9 of 9Common misconceptions
Common misconceptions
- "S-1 is a peptide." No. It is a nonsteroidal arylpropionamide small molecule (C17H14F4N2O5, 402.30 g/mol). It is covered here only because SARMs are discussed and stacked with peptides.
- "S-1 is a steroid." No. It is a true nonsteroidal SARM — it does not have a steroid nucleus.
- "S-1 is a proven or clinically used SARM." No. It was an early preclinical lead that never entered human trials; the program advanced ostarine (enobosarm) instead. All S-1 findings are in vitro or rat.
- "S-1 doesn't suppress your hormones — the rat data prove it's safe." No. The LH/FSH result is a rat finding near the ED50 only; it does not transfer to humans, and the SARM class does suppress the HPTA axis in people. There are no human data for S-1.
- "S-1 is the same as the cancer drug S-1." No. This SARM (PubChem CID 5288215) is a completely different chemical from the anticancer combination tegafur/gimeracil/ oteracil (CID 146157327), which merely shares the name.
- "If it's labeled S-1, 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.
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.