Description
What Is S-23?
S-23 is a non-steroidal selective androgen receptor modulator (SARM) developed by GTx, Inc. and recognized as one of the most potent orally bioavailable SARMs characterized to date. S-23 binds to the androgen receptor with exceptionally high affinity (Ki of 1.7 nM), surpassing the binding strength of many set up SARMs and approaching that of dihydrotestosterone, the most potent endogenous androgen.
Lab studies in male rats showed that S-23 produced dose-dependent increases in lean muscle mass and decreases in fat mass while simultaneously reducing prostate weight, confirming tissue-selective androgen receptor tuning. Notably, S-23 has also been studied as a possible male hormonal contraceptive, as it produced complete and reversible suppression of spermatogenesis in animal models at higher doses. This contraceptive effect was fully reversible upon cessation of dosing, with fertility restored within about 100 days. The compound’s oral uptake and potent anabolic profile have made it a subject of intense research interest in muscle biology, body makeup, and fertility physiology.
Key Features and Specifications
| Specification |
Detail |
| Product Name |
S-23 |
| Dosage |
10 mg per capsule |
| Quantity |
60 capsules per bottle |
| Form |
Oral capsule |
| CAS Number |
1010396-29-8 |
| Cell-level Formula |
C18H13ClF4N2O3 |
| Cell-level Weight |
416.75 g/mol |
| Purity |
≥99% (HPLC verified) |
| Testing |
Third-party HPLC and MS test |
| Storage |
Room heat, cool dry place, away from light |
| Classification |
Non-steroidal SARM |
Mechanism of Action: How S-23 Works
S-23 functions as a full agonist at androgen receptors in skeletal muscle and bone tissue, starting robust anabolic transcriptional programs that drive protein synthesis, nitrogen retention, and myofiber hypertrophy. Its binding affinity (Ki = 1.7 nM) places it among the highest-affinity SARMs developed, translating to potent receptor start at comparatively low levels.
In muscle tissue, S-23 recruitment of coactivator proteins at the androgen receptor drives expression of myogenic control factors, IGF-1 splice variants, and anabolic target genes. In bone tissue, start of androgen receptors in osteoblasts boosts bone formation pathways and increases cortical bone density. Despite this potent anabolic activity, S-23 shows tissue selectivity by reducing prostate weight in intact male animals, showing antagonistic or minimal agonist activity in prostatic tissue.
The contraceptive mechanism saw in lab studies results from S-23’s potent suppression of gonadotropins (LH and FSH), leading to dramatically reduced intratesticular testosterone and arrested spermatogenesis. This effect is dose-dependent and fully reversible, making S-23 the first SARM seriously studied as a possible non-hormonal male contraceptive approach that simultaneously preserves or enhances musculoskeletal tissue.
Reported Research Applications and Benefits
Body makeup research with S-23 has produced some of the most dramatic results saw with any SARM. Lab data show potent lean mass increases accompanied by major fat mass reductions, consistent with the compound’s high androgen receptor binding affinity and full agonist activity in muscle. Researchers studying recomposition, cutting, and anabolic threshold studies often select S-23 for its strong dose-response characteristics.
Male contraception research has explored S-23’s power to produce reversible azoospermia while keeping musculoskeletal anabolic effects. The mix of gonadotropin suppression enough for contraception with tissue-selective anabolism represents a unique pharmacological profile among SARMs and has implications for fertility biology and andrology research.
Bone density studies show that S-23 increases bone mineral density and cortical bone strength in both intact and gonadectomized animal models. Fat oxidation research has documented enhanced lipid body function, with treated animals showing major reductions in adipose tissue mass even in the absence of caloric restriction or exercise protocols.
S-23 Dosage Information for Research
| Protocol |
Dosage |
Duration |
Notes |
| Standard research dose |
10–25 mg/day |
8–12 weeks |
Major suppression expected |
| Beginner protocol |
10 mg/day |
6–8 weeks |
Conservative starting dose |
| Advanced protocol |
20–25 mg/day |
8–12 weeks |
Complete LH suppression likely |
| Timing |
Split AM/PM or once daily |
— |
~12-hour estimated half-life |
S-23 is one of the most suppressive SARMs characterized, and all research protocols should include full baseline and endpoint blood panels covering testosterone, LH, FSH, and SHBG. Post-cycle therapy with SERMs such as enclomiphene is considered essential in protocols lasting six weeks or longer. The compound’s potent suppressive profile makes it unsuitable for research designs needing minimal hormonal impact.
S-23 vs RAD-140: Key Differences
| Parameter |
S-23 |
RAD-140 (Testolone) |
| AR binding affinity (Ki) |
1.7 nM |
7 nM |
| Agonist type |
Full agonist |
Full agonist |
| Anabolic potency |
Very high |
High (90:1 ratio) |
| Suppression level |
Major (contraceptive level) |
Moderate to major |
| Contraceptive possible |
Yes (reversible azoospermia) |
No |
| Fat loss effect |
Pronounced |
Moderate |
| Half-life |
~12 hours |
~60 hours (primates) |
| PCT requirement |
Essential |
Strongly recommended |
Storage and Handling Instructions
Store S-23 capsules at controlled room heat between 20°C and 25°C (68°F to 77°F) in the original sealed container. Protect from direct sunlight, heat, and moisture. Under proper storage conditions, S-23 capsules keep full potency for at least 24 months from the date of manufacture.
Why Choose PrymaLab S-23 SARM Capsules
Every batch of PrymaLab S-23 undergoes full third-party test by independent, accredited laboratories using HPLC for purity and mass spectrometry for identity confirmation. Each order includes a batch-specific Certificate of Test documenting ≥99% purity and full contaminant screening. Pharmaceutical-grade encapsulation ensures precise 10 mg dosing, and free priority shipping is included on all domestic orders.
Frequently Asked Questions About S-23
Is S-23 the strongest SARM available?
S-23 has the highest published androgen receptor binding affinity (Ki = 1.7 nM) among characterized non-steroidal SARMs, making it one of the most potent for receptor interaction. Its full agonist activity and dramatic body makeup effects in lab models support its classification as one of the strongest SARMs, though potency comparisons depend on the specific endpoint measured.
Does S-23 suppress testosterone?
Yes, greatly. S-23 produces large suppression of the hypothalamic-pituitary-gonadal axis, reducing LH, FSH, and endogenous testosterone in a dose-dependent manner. At higher doses, S-23 can produce complete azoospermia in animal models. Post-cycle therapy is considered essential for all S-23 research protocols.
Is the suppression from S-23 reversible?
Lab data confirm that S-23’s suppressive effects, including azoospermia, are fully reversible upon cessation of dosing. Fertility restoration in animal models occurred within about 100 days, with complete healing of spermatogenesis and gonadotropin levels. This reversibility was a key finding supporting S-23’s study as a possible male contraceptive.
How does S-23 compare to S4 andarine?
S-23 shows largely higher androgen receptor binding affinity (Ki 1.7 nM vs S4’s weaker binding) and functions as a full agonist rather than S4’s partial agonist profile. S-23 produces more potent anabolic and fat loss effects but carries greatly greater HPG axis suppression. S4 is often considered the milder compound suitable for lower-intensity research protocols.
What post-cycle therapy is recommended after S-23 research?
Due to S-23’s potent suppressive profile, robust PCT protocols using SERMs such as enclomiphene (12.5–25 mg/day) or clomiphene (25–50 mg/day) for four to six weeks are often incorporated into research designs. Full bloodwork tracking LH, FSH, total testosterone, and free testosterone is recommended to confirm hormonal healing.
Quality Assurance and Testing
PrymaLab enforces pharmaceutical research-grade quality standards for all S-23 products. Independent third-party laboratories conduct HPLC purity test, mass spectrometry identity check, heavy metal screening, residual solvent testing, and microbial assessment on every batch. Batch-specific Certificates of Test accompany each order for full analytical transparency.
Research Disclaimer
S-23 is sold strictly for laboratory research and scientific study purposes only. This product is not intended for human consumption, veterinary use, or treatment use. S-23 has not been approved by the FDA or any control agency for medical use. Purchasers must be qualified researchers affiliated with accredited institutions or licensed research organizations. By buying this product, you confirm that it will be used exclusively in accordance with all applicable local, state, and federal regulations governing research materials.
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