S-23
1 product from 1 brand · free UK delivery · EU 3–7 days
S-23, also written S23 and known chemically as (S)-N-(4-cyano-3-trifluoromethyl-phenyl)-3-(3-fluoro-4-chlorophenoxy)-2-hydroxy-2-methyl-propanamide, is a non-steroidal selective androgen receptor modulator (SARM) of the aryl-propionamide family developed by GTx. It was investigated in rats as a candidate for reversible hormonal male contraception rather than for muscle wasting. This page lists every brand of S-23 stocked at Free Muscle, compared on strength per capsule, capsules per pack and batch testing, and summarises the published research and its limits.
What is S-23?
S-23 is a close structural relative of ostarine and andarine, sharing their aryl-propionamide backbone but carrying a chloro- and fluoro-substituted phenoxy ring. In the original characterisation it bound the androgen receptor (AR) with an inhibitory constant of about 1.7 nanomolar and behaved as a full agonist in vitro, which distinguishes it from partial agonists such as andarine. In castrated rats it was markedly more potent in the levator ani muscle than in the prostate, the pattern that defines a SARM.
What sets S-23 apart is the reason it was studied. Because a full AR agonist strongly suppresses luteinising hormone and follicle-stimulating hormone from the pituitary, and therefore suppresses sperm production, GTx explored S-23 combined with an oestrogen as a reversible male contraceptive in rats. The compound never entered published human trials for any indication. S-23 has never been approved as a medicine by the MHRA, EMA, FDA or any other regulator, and its development appears to have stopped at the preclinical stage.
What the published research reports
Preclinical characterisation for male contraception (2009). Jones and colleagues characterised S-23 in male rats. In castrated animals the ED50 in the levator ani muscle was roughly five times lower than in the prostate, a separation in favour of muscle. In intact rats treated for two weeks, S-23 alone suppressed LH by more than 50 per cent, with a corresponding decrease in prostate size and increase in levator ani size. In a ten-week study combining S-23 with oestradiol benzoate, four of six animals in one group had no sperm in the testis and none of six produced pregnancies in mating trials; fertility was fully restored after 100 days off treatment. S-23 also increased bone mineral density and lean mass and reduced fat mass dose-dependently. These are rat data only.
Review of SARM discovery (2005). Chen, Kim and Dalton reviewed the aryl-propionamide programme that produced S-4, S-22 (later ostarine) and related compounds, describing the structure-activity relationships that convert an anti-androgen scaffold into a tissue-selective agonist and the therapeutic rationale for pursuing muscle, bone and pituitary effects with reduced prostate stimulation.
UK market analysis of SARM products (2021). Leaney and colleagues, working in a UK laboratory, analysed supplement products marketed as SARMs to UK consumers by high-resolution mass spectrometry. They found products with no active ingredient, products containing undeclared prohibited substances, and measured concentrations that frequently disagreed with the label. They also noted that toxicity data for SARMs, particularly in combination, are incomplete.
All efficacy findings above come from rats or from review of preclinical work. There are no published human trials of S-23, and none of these findings is a claim about the products listed on this page.
Brands of S-23 we stock
- iMuscle S-23, 15mg, 60 capsules. Capsule form, 900mg of compound per pack. A certificate of analysis is shown on the listing only where iMuscle has supplied one for the batch we hold; if none is displayed, none is available for that batch. View the iMuscle S-23 listing.
Free Muscle does not currently produce its own S-23, so there is no Janoshik-assayed Free Muscle batch on this page at present. Any Free Muscle-brand line added later will carry batch-level Janoshik testing like the rest of our range.
How to compare them
Multiply strength per capsule by capsules per pack to get total milligrams (15mg by 60 gives 900mg). Divide the pack price by that figure for cost per milligram so that a 15mg pack can be compared fairly against a 10mg or 25mg one from another supplier. Then weigh the certificate. S-23 is a less common compound than ostarine or LGD-4033, so an independent identity assay against a verified reference standard is the only objective evidence of what a capsule contains. Our guide to price per milligram covers the arithmetic.
Batch testing and certificates of analysis
A certificate of analysis (COA) answers the two questions that matter about an S-23 capsule: is the compound what the label says, and how much is present. A genuine batch certificate is issued by an independent laboratory, names the lot number, and reports identity (by HPLC or mass spectrometry against a reference standard) and assayed purity or content.
Every Free Muscle-brand batch is assayed by Janoshik Analytical, an independent laboratory, and the certificate for the batch you receive is sent on request by email. For other brands we show a COA only where the manufacturer has supplied one for the batch we hold. A generic PDF with no batch number, or one dated long before the product was made, is not a batch certificate and tells you nothing about the capsules in your hand. Our lab tests page explains how to read a Janoshik certificate and match it to your batch.
Legal status in the UK
S-23 is not a licensed medicine in the United Kingdom. It has never been granted a marketing authorisation by the MHRA, and it is not an authorised food supplement or a permitted novel food ingredient. It cannot legally be sold or labelled for human consumption. Free Muscle sells S-23 to adults aged 18 or over for laboratory and research purposes only, and every listing states this. S-23 falls within section S1.2 of the World Anti-Doping Agency Prohibited List (Other Anabolic Agents), which covers all SARMs, and is prohibited at all times. Anyone subject to anti-doping rules should treat any product in this class as prohibited at all times, in and out of competition. For a fuller overview of the class, see our research guide to SARMs.
Storage and handling
Keep S-23 capsules in the original sealed container in a cool, dry place away from light and heat. Do not decant into unlabelled containers, and record the batch number so that a COA request can be matched to the exact material. Keep out of reach of children and pets, and follow standard laboratory good practice: gloves when handling, no eating or drinking in the work area, and disposal in line with local rules for research chemicals.
Delivery and returns
Free Muscle (FreeMuscle LTD, Scunthorpe) is a UK company. Every order is sent with free tracked UK delivery in plain packaging, and orders placed before 2pm on a weekday are processed the same working day. EU delivery is 3 to 7 working days. Unopened items can be returned within 30 days, and customers must be 18 or over. Read the shipping page and guarantee page for full terms.
Related reading
- What are SARMs? The 2026 research guide
- SARMs explained: how selective androgen receptor modulators work
- How to read a certificate of analysis
- What is Janoshik Analytical?
Related collections: Andarine (S-4), Ostarine (MK-2866), iMuscle, all SARMs.
S-23 is sold strictly for laboratory research use only. It is not a licensed medicine, has not been evaluated or approved by the MHRA, EMA or FDA, is not a food supplement and is not for human consumption. Buyers must be 18 or over. Nothing on this page is medical advice, dosing advice or a recommendation to use this compound in any way; the published findings summarised above describe study participants and laboratory models, not the products listed.
Frequently asked questions
Is S-23 legal in the UK?
S-23 is not a controlled drug in the UK and may be sold to adults for laboratory research, which is the only basis on which Free Muscle supplies it. It has never been licensed by the MHRA, is not an authorised food supplement, and cannot be sold or labelled for human consumption. Customers must be 18 or over.
Is S-23 a SARM or a steroid?
S-23 is a non-steroidal selective androgen receptor modulator from the aryl-propionamide family, closely related to ostarine and andarine. It has no steroid skeleton and is not converted to oestrogen or dihydrotestosterone. In the original rat study it acted as a full agonist at the androgen receptor. WADA classes it as an anabolic agent.
What is the difference between S-23 and S-4 (andarine)?
Both are aryl-propionamide SARMs from the same programme. S-23 carries a chloro- and fluoro-substituted phenoxy ring and was reported as a full androgen receptor agonist with strong pituitary suppression, which is why it was studied for male contraception. Andarine is a partial agonist with a nitro group and was explored for muscle wasting and prostate conditions. Neither reached approval.
Has S-23 been tested in humans?
No. The only published efficacy data are from rats, principally a 2009 study of S-23 alone and combined with an oestrogen as a reversible male contraceptive. That study reported suppression of LH and sperm production, increased muscle and bone mass, and full recovery of fertility after treatment ended. There are no human trials and no human adverse event data.
How do I verify the batch I received?
Check the iMuscle S-23 listing for a batch certificate. We publish one when iMuscle has supplied it for the batch in stock and leave the listing without one when they have not; a certificate for a different batch would tell you nothing about your capsules. Verify that the lot number on the certificate is the one printed on your container, then read the identity and purity results. The lab tests page explains how.
Is S-23 detectable in sports drug testing?
Yes. S-23 is a SARM, and all SARMs are prohibited at all times under section S1.2 of the WADA Prohibited List. Anti-doping laboratories have characterised its metabolites and include it in routine screening for anabolic agents. Anyone subject to anti-doping rules should treat S-23 as prohibited in and out of competition.
Which strength of S-23 should I choose?
We cannot advise on strength, quantity or use of S-23, which is sold for laboratory research only. Researchers choose material according to their own protocol. To help with comparison, the listing states 15mg per capsule and 60 capsules per pack, giving 900mg in total, from which cost per milligram can be calculated. Nothing on this site constitutes dosing or medical advice.
How is S-23 shipped, and can I return it?
We send S-23 with free tracked UK delivery in plain packaging, processing weekday orders received before 2pm the same working day. EU orders usually arrive in 3 to 7 working days. Should you need to return a pack, we accept unopened items within 30 days of delivery; opened containers are excluded. Free Muscle is a UK company (FreeMuscle LTD, Scunthorpe) and sells only to customers who are 18 or over.