S23: what this SARM is and how it works
S23 is a non-steroidal compound often grouped with the selective androgen receptor modulators, or SARMs. It appears widely in online discussion yet remains an experimental research chemical that has never been approved as a medicine. This editorial overview sets out what S23 is, how it is thought to act, what the published science actually shows, and why its legal and safety status warrants caution.
What S23 is
S23 is a synthetic, orally active small molecule developed in pharmaceutical research as a candidate selective androgen receptor modulator. It emerged from programmes exploring whether androgen-receptor activity could be harnessed for muscle and bone, and even for male contraception, without the broader effects of classical steroids. Chemically it is unrelated to testosterone; it is not a steroid at all.
Despite its visibility in the supplement and "research chemical" marketplace, S23 has never completed clinical development and is not licensed as a medicine anywhere. The great majority of what is known about it comes from animal studies and laboratory work, not from controlled human trials. That gap between popular interest and scientific evidence is the single most important fact about the compound.
Products sold under the name S23 are not regulated as medicines, which means their contents, purity and dose are not guaranteed. Independent testing of the SARM marketplace has repeatedly found mislabelled products, wrong actives and contaminants. A buyer therefore cannot be confident that a product even contains what the label claims.
This article explains the pharmacology and the evidence. It does not describe how to take S23, in what amounts, or in combination with anything else, because there is no established safe human regimen and the compound is not approved for human use.
The concept behind SARMs
The appeal of SARMs rests on the idea of tissue selectivity. Testosterone and its relatives act on androgen receptors throughout the body — muscle, bone, prostate, skin, brain — producing both the desired anabolic effects and a range of unwanted ones. The research goal behind SARMs was to find molecules that would stimulate muscle and bone strongly while acting only weakly on tissues such as the prostate.
In principle, a selective molecule could offer some benefits of androgen therapy with fewer side effects, which is why pharmaceutical companies invested in the class for conditions such as muscle wasting and osteoporosis. Several SARMs reached clinical trials, but none has yet been approved as a finished medicine, and development has repeatedly stalled over efficacy and safety questions.
S23 sits within this family conceptually, but it is important to note that different SARMs behave very differently. Some are only partial agonists; S23 is often described in the preclinical literature as a relatively full agonist at the androgen receptor, which means it may act more like a conventional androgen than some of its milder cousins.
The marketing of these compounds as "safe alternatives" to steroids runs well ahead of the evidence. Selectivity in a laboratory assay does not guarantee a clean safety profile in a living person, and the absence of clinical data means the real-world risk-benefit balance in humans is simply unknown.
Proposed mechanism of action
Like other androgen-receptor ligands, S23 is thought to work by binding to the androgen receptor inside cells. When an agonist binds, the receptor changes shape, moves into the cell nucleus and influences the transcription of genes involved in protein synthesis, muscle maintenance and bone metabolism. The "selective" idea is that the exact shape of the molecule-receptor complex differs between tissues, recruiting different co-regulator proteins and so producing different effects in muscle versus prostate.
In preclinical descriptions, S23 is characterised as having high binding affinity for the androgen receptor and good oral bioavailability. Animal work suggested effects on muscle and bone, and — notably — a suppressive effect on sperm production, which is why it was examined in the context of male contraception rather than purely as an anabolic.
A crucial caveat is that mechanism does not equal outcome. Showing that a molecule binds a receptor and changes gene activity in a rodent says little about safe, effective dosing in a human being. Receptor pharmacology is the starting point of drug development, not its conclusion.
Reports suggest S23 may suppress the body's own production of luteinising hormone and testosterone, as classical androgens do. If so, the marketing claim that SARMs leave the natural hormonal axis untouched would not hold for this compound, which is one of several reasons it cannot be assumed benign.
What the research shows
The published evidence on S23 is thin and almost entirely preclinical. The most frequently cited work comes from pharmaceutical research describing androgen-receptor binding and effects in animal models, including studies exploring male contraception. There are, to date, no robust published randomised controlled trials establishing efficacy or safety in humans.
| Evidence type | What exists for S23 | What this means |
|---|---|---|
| Receptor binding | Reported high affinity (lab) | Confirms it acts on the androgen receptor |
| Animal studies | Muscle, bone, fertility effects | Hypothesis-generating, not human-proven |
| Human clinical trials | None approved/published | Human dosing and safety unknown |
| Marketplace testing | Frequent mislabelling found | Product contents unreliable |
This matters because the absence of human data is not a neutral blank. It means that the dose-response relationship, the long-term risks, the interactions and the reversibility of effects in people are all unknown. Enthusiastic anecdotes online are not a substitute for controlled evidence and cannot establish safety.
Independent analyses of products sold as SARMs, including by regulators and academic groups, have found that a substantial proportion do not contain what they claim, sometimes containing unapproved drugs or none of the advertised compound. For S23 specifically, a consumer has no reliable way to verify a product's identity.
Taken together, the research picture is of an experimental molecule with plausible androgenic activity, meaningful signals of hormonal suppression and fertility effects in animals, and no clinical validation. That is a profile that justifies caution, not confidence.
Legal status and safety concerns
In the UK, SARMs such as S23 are not licensed medicines and cannot lawfully be sold for human consumption; the Medicines and Healthcare products Regulatory Agency (MHRA) has warned about products marketed as bodybuilding aids that contain unlicensed pharmacologically active substances. For athletes, SARMs are prohibited at all times under the World Anti-Doping Code, and positive tests from contaminated supplements have ended careers.
The health concerns follow from the pharmacology. If S23 suppresses the natural hormonal axis, then testicular function, fertility and recovery of normal hormone production could all be affected, as is seen with other androgens. Reported concerns across the SARM class include effects on cholesterol, liver enzymes and mood, and the lack of human trials means rarer risks may simply be undocumented.
- Unlicensed: not approved as a medicine; quality and contents unverified.
- Hormonal suppression: possible effects on testosterone, fertility and recovery.
- Anti-doping: prohibited in sport at all times.
- Unknown long-term risk: no human safety data.
There is also the practical hazard of the unregulated market itself: wrong dose, wrong active ingredient, contaminants and no accountability. A product that cannot be trusted to contain what it claims cannot be assessed for safety at all, which places the entire enterprise outside the bounds of informed, consented medical use.
Anyone considering such compounds for a genuine medical need — muscle wasting, low testosterone, osteoporosis — should know that licensed, monitored treatments exist for these conditions and should be discussed with a clinician rather than substituted with an experimental research chemical.
Conclusions of the editorial team
S23 is an experimental non-steroidal androgen-receptor agonist that never became a medicine. The pharmacology is real enough — it binds the androgen receptor and appears to act much like a conventional androgen, including suppressing the body's own hormones — but the human evidence that would make its use reasonable simply does not exist.
Set against that scientific emptiness is a very real set of risks: an unregulated and frequently mislabelled marketplace, probable hormonal suppression, an anti-doping ban, and unknown long-term consequences. For a reader weighing it up, the honest summary is that the hype far exceeds the evidence.
For context, our related material compares the broader SARM class with anabolic steroids, examines why supplement contamination is so common, and looks at how the androgen receptor works in muscle and bone.
References
- Bhasin S, Jasuja R. Selective androgen receptor modulators as function promoting therapies. Curr Opin Clin Nutr Metab Care. 2009;12(3):232–240.
- Jones A, Chen J, Hwang DJ, et al. Preclinical characterization of a selective androgen receptor modulator for contraception. Endocrinology. 2009;150(1):385–395.
- Van Wagoner RM, Eichner A, Bhasin S, et al. Chemical composition and labeling of substances marketed as SARMs. JAMA. 2017;318(20):2004–2010.
- Efimenko IV, Valancy D, Dubin JM, et al. Adverse effects and potential benefits among SARMs users: a cross-sectional survey. Int J Impot Res. 2022;34(7):699–705.
- Medicines and Healthcare products Regulatory Agency. Warning on unlicensed SARMs sold as bodybuilding products. London: MHRA; 2021.
- World Anti-Doping Agency. The 2024 Prohibited List, International Standard. Montreal: WADA; 2024.
