S-23 — 60 caps, 15mg

iMuscle S-23 is a pack of 60 capsules stated at 15 mg each, 900 mg of S-23 in total. S-23 is a potent non-steroidal selective androgen receptor modulator that was studied in rats as a candidate for hormonal male contraception. It has never been tested in a published clinical trial; the only human data come from anti-doping excretion studies. Free Muscle stocks this iMuscle product through Muscle Market for laboratory research by adults aged 18 and over.

About S-23

S-23 is a non-steroidal selective androgen receptor modulator from the same aryl-propionamide family as andarine and ostarine. It was investigated not for muscle wasting but as a candidate for hormonal male contraception, because of how effectively it suppressed the pituitary-gonadal axis in animals.

The key paper is a 2009 preclinical characterisation in rats (Jones et al., 2009). S-23 bound the androgen receptor with an inhibitory constant of about 1.7 nanomolar and behaved as a full agonist in cell assays. In the animals it increased levator ani muscle weight and, when combined with an oestrogen, suppressed spermatogenesis and produced infertility that reversed once treatment stopped. The same work recorded a strong decrease in luteinising hormone and follicle-stimulating hormone. In castrated rats it acted on the prostate as well as on muscle, though at higher amounts, while in intact rats prostate size fell as luteinising hormone was suppressed. No clinical trial of S-23 has been published, and no clinical development has been reported. Its suppressive effect on the animals' own reproductive hormones is therefore its best-documented property. These rodent findings are not claims about this listing.

Name and chemical identity

S-23 never reached clinical development, so it is known only by its series code, also written S23, and is easily confused with S-22 (an early code for ostarine) and S-4 (andarine). A certificate for this pack should match these PubChem identifiers:

• Systematic name: (2S)-3-(4-chloro-3-fluorophenoxy)-N-[4-cyano-3-(trifluoromethyl)phenyl]-2-hydroxy-2-methylpropanamide
• CAS number: 1010396-29-8
• Formula and molecular weight: C18H13ClF4N2O3, 416.8 g/mol
• InChIKey: SSFVOEAXHZGTRJ-KRWDZBQOSA-N
• PubChem CID: 24892822

This listing

• Compound: S-23
• Brand: iMuscle
• Form: capsules
• Pack size: 60 capsules
• Stated strength: 15 mg per capsule (manufacturer's label claim)
• Stated total per pack: 900 mg
• Intended use: laboratory research only, not for human consumption
• Sold to adults aged 18 and over

The strength is the manufacturer's label claim; only a batch certificate confirms it. To compare brands of S-23, use total milligrams per pack, as explained in price per milligram.

S-23 in the UK: buying, legal status and what to check

Searches such as "S23 SARM UK" and "S-23 SARM for sale" ask two things: whether S-23 can be bought lawfully in Britain, and how to tell a listing that can be checked from one that must be taken on trust.

The legal position

S-23 is not a licensed medicine in the UK: it has no MHRA marketing authorisation, and no regulator anywhere has granted one. Nor is it an authorised food supplement, so it cannot lawfully be sold for human consumption, although it is not a controlled substance. Free Muscle supplies it to adults aged 18 or over for laboratory research only. WADA prohibits it at all times under section S1 (anabolic agents) of its Prohibited List. Buyers are responsible for the rules that bind them, including any anti-doping code and, for EU orders, local import rules. The wider class is covered in What are SARMs? The 2026 research guide.

What a listing should show, and what this one shows

A label alone proves little. Anti-doping researchers in Cologne name contaminated dietary supplements as one route by which SARMs have reached athletes' samples (Alhalabi et al., 2025): a tub can carry a substance it does not declare. Reviews of "S23" record what writers felt, not what they took. Full terms are on our delivery and guarantee pages, and further checks are in how to choose a SARMs supplier in the UK.

Capsules compared with other forms

Besides capsules, S-23 circulates as tablets and as liquids labelled in milligrams per millilitre. The molecule does not change between them; what changes is where the stated amount is fixed and what a certificate can confirm. No published study has compared formats of S-23.

In a capsule or tablet the amount per unit is fixed at the factory, so a laboratory can report milligrams per capsule, the figure that tests the 15 mg claim here, although a pooled assay gives only the average and unit-to-unit variation shows only when capsules are tested singly. A liquid's certificate reports concentration on the test date, and the amount in any portion then depends on how accurately a volume is drawn, whether the compound stays dissolved in the cold, and solvent lost from a bottle opened many times.

As a small synthetic molecule rather than a peptide, S-23 is not open in the same way to the objection that digestive enzymes break swallowed peptides apart (see do oral peptides work). The oral excretion studies described under detection below gave it by mouth, one as a tablet, and we know of no published study of it by injection. A dry powder in a shell is also kept from the water and solvents in which breakdown usually runs fastest, though that is general chemistry, not a measured result for S-23.

S-23 compared with RAD-140

"S23 vs RAD140" is the comparison searched most often for this compound, usually as a question about which is stronger. Chemistry and regulation answer part of it; the rest has never been studied.

S-23's reputation for strength rests on the 2009 cell work, where it bound the receptor tightly and switched it on fully; Radius Health likewise called RAD-140 potent. Those results come from different laboratories, assays and species, and no study has tested the two side by side, so no ranking is supported. Receptor potency would in any case say nothing about effects or risks in a person.

The evidence is unequal: RAD-140 has adverse events and blood levels recorded in supervised patients, while S-23 has rat pharmacology and urine data from anti-doping volunteers. No equivalence between their milligram labels has been established, and both are sold here for laboratory research only. RAD-140 brands, including iMuscle Testolone RAD-140, are compared in the RAD-140 (Testolone) collection, and its research record is set out in RAD-140 (testolone): what the research says.

Reading the certificate of analysis for this product

iMuscle is a third-party brand, so any certificate of analysis (COA) is the manufacturer's, not ours. Where iMuscle has supplied one for the batch we hold, we show it; where none exists, we say so rather than show a generic document. Our Janoshik Analytical batch testing covers Free Muscle-brand compounds only, not iMuscle. At a minimum, a real batch certificate carries the lot number printed on the pack, the test method (HPLC or LC-MS) and a purity figure; a full S-23 certificate lets you check each of these:

1. The right compound. S-23 by name, ideally with CAS 1010396-29-8. A report for S-22, S-4 or an unnamed "SARM" does not describe this pot.
2. Identity, with the method stated. HPLC matches a peak to an S-23 reference standard; LC-MS adds a mass that should fit C18H13ClF4N2O3 (416.8 g/mol). With one chlorine atom, S-23 should also give a companion signal two mass units higher, roughly a third the height of the main one.
3. Purity. The percentage of chromatogram peak area that is S-23. A high figure says the material is clean, not that each capsule holds 15 mg.
4. Content per capsule. A measured milligram figure to set against the 15 mg label claim; sixty times it is the real pack total.
5. Which mirror-image form. S-23 is the (2S) form. Its mirror image has the same formula and mass, which ordinary HPLC and LC-MS cannot tell apart; unless a chiral method is named, this has not been checked.
6. Lot number, test date and laboratory. The lot must match your pot; the date shows the result's age; a named laboratory can confirm it issued the report.

If an iMuscle document falls short on any of these, contact us with your lot number and we will tell you what exists for the stock we hold. See also how to read a COA and our lab tests page.

Detection, sport and testing

S-23 falls under section S1.2 (other anabolic agents) of the WADA Prohibited List, which covers SARMs as a class, and is banned in and out of competition. It lay behind five adverse analytical findings reported in 2022 (Alhalabi et al., 2025), and under strict liability an athlete answers for any prohibited substance in their sample, however it got there.

Oral excretion studies give detection times. At the Institute of Legal Medicine in Strasbourg, a single volunteer took one oral amount in the milligram range; unchanged S-23 was found in hydrolysed urine from 2 hours to 28 days afterwards, as was a hydroxylated metabolite (Ameline et al., 2022). At the German Sport University Cologne, volunteers took single and repeated amounts of 1, 10 or 50 micrograms, the scale of a contaminated product. With a method able to detect 1 picogram per millilitre, a single microgram was traced for an average of up to 253 hours, about ten days, and a single 50 micrograms for up to 544 hours, about 23 days; 18 metabolites were identified (Alhalabi et al., 2025).

Those figures come from one person and from small groups given very small amounts. Detection after repeated or larger milligram quantities has not been studied, so for this product: detectable, window not established. Anyone subject to anti-doping rules should treat any contact with S-23 as a risk of an adverse finding.

Storage, stability and handling

We know of no published stability study of S-23 in any form, so iMuscle's printed expiry or best-before date is the working limit, and a certificate describes the batch on its test date, not on the day the pot is opened.

Keep the capsules in their original pot with the lid closed, because its lot number links them to any certificate. Store it cool, dry and dark, in a cupboard rather than a kitchen, bathroom or windowsill; humidity softens shells and makes powder cake. Let a cold pot reach room temperature before opening it so condensation does not form inside. Shells that have darkened, gone soft or fused together suggest the contents have changed since the batch was tested; keep that pot apart and tell us its lot number.

Wear gloves whenever a capsule is opened or powder is spilled, and work away from food and drink. Label anything taken out of the pot with compound, lot and date. Store the pot where children and pets cannot reach it, and treat unwanted material as chemical waste under local rules rather than tipping it into a household bin or drain. Unopened items can be returned within 30 days, so a pot left sealed keeps that option open.

Glossary

• Aryl-propionamide: the structural series of S-23, ostarine (S-22) and andarine (S-4), source of the S-numbered codes.
• Full agonist: a compound that activates a receptor as completely as the body's own hormone, as S-23 did in 2009 cell assays; some related SARMs are only partial agonists.
• Inhibitory constant (Ki): how tightly a compound binds a receptor in a competition assay; lower means tighter, about 1.7 nanomolar for S-23.
• Levator ani: a small androgen-sensitive pelvic muscle weighed in rat studies as a marker of anabolic activity.
• LH and FSH: luteinising hormone and follicle-stimulating hormone, released by the pituitary gland to drive testosterone and sperm production in the testes.
• Spermatogenesis: sperm production in the testes.
• Reference standard: material of certified identity used as the benchmark for identifying and measuring a sample.
• Hydrolysed urine: urine treated with an enzyme that strips attached groups such as glucuronic acid, freeing a compound and its metabolites for measurement.
• Adverse analytical finding: a laboratory's formal report that a prohibited substance or its metabolite is present in an athlete's sample.
• Strict liability: the anti-doping rule that athletes answer for whatever is found in their sample, whatever its source.

Related

• All brands of S-23, compared on strength, pack size and batch testing
• All iMuscle products stocked at Free Muscle
• Related SARMs: RAD-140 (Testolone), Andarine (S-4), Ostarine (MK-2866) and all SARMs
• Delivery and returns

This product is supplied for laboratory research use only. It is not a medicine, has not been evaluated by the MHRA, EMA or FDA, and is not for human consumption. Sold to adults aged 18 and over. Nothing on this page is medical, dosing or usage advice.

References

1. Jones A, et al. Preclinical characterization of a (S)-N-(4-cyano-3-trifluoromethyl-phenyl)-3-(3-fluoro, 4-chlorophenoxy)-2-hydroxy-2-methyl-propanamide: a selective androgen receptor modulator for hormonal male contraception. Endocrinology. 2009;150(1):385-95. PMID: 18772237.
2. Ameline A, et al. In vitro characterization of S-23 metabolites produced by human liver microsomes, and subsequent application to urine after a controlled oral administration. J Pharm Biomed Anal. 2022;212:114660. PMID: 35182830.
3. Alhalabi H, et al. Investigations Into the Urinary Metabolite Elimination Profile of the Selective Androgen Receptor Modulator S-23 in Studies Mimicking Contaminated Product Ingestion for Doping Control Purposes. Biomed Chromatogr. 2025;39(6):e70090. PMID: 40277337.
4. LoRusso P, et al. A First-in-Human Phase 1 Study of a Novel Selective Androgen Receptor Modulator (SARM), RAD140, in ER+/HER2- Metastatic Breast Cancer. Clin Breast Cancer. 2022;22(1):67-77. PMID: 34565686.