S-23
S23
Last updated
At a glance
500
Testosterone = 100
95
Testosterone = 100
Overview
Why S-23 Has a Cult Following#
S-23 occupies a specific niche in the SARM world: it's the one people reach for when they want a dry, hard, visibly tightened look without adding another injectable or another liver-taxing oral to the stack. Unlike ostarine or LGD-4033, S-23 is a full androgen receptor agonist — it doesn't just nudge the AR, it saturates it. That's why physique-focused users talk about it the way previous generations talked about halotestin or winstrol: a finisher, a hardener, a prep-phase tool.
The upside is real. Binding affinity is in the low-nanomolar range (Ki ~1.7 nM), oral bioavailability is excellent, and because S-23 doesn't aromatize, there's no estrogen creep, no water retention, and no gyno risk to manage. Users running it alongside a low-dose test base consistently report sharper separation, better vascularity, and strength holding through a deficit — exactly the profile you want in the last 6–8 weeks of a cut or prep.
"S-23 was a potent AR agonist with Ki of 1.7 ± 0.2 nM, an ED50 for levator ani growth of 0.079 mg/day, and suppressed LH, FSH, and spermatogenesis resulting in contraception in 4/6 rats." — Jones et al., Endocrinology (2009)
That last clause is the catch, and it's the reason S-23 deserves respect rather than casual use: it was originally developed as a reversible male contraceptive. HPG shutdown isn't a side effect here, it's the design spec. Anyone running S-23 is running something that hits the axis harder than most oral steroids, and PCT is genuinely non-negotiable.
The rest of this page covers the practical execution: dose ladders for beginner through advanced users, how to structure an 8-week cycle, PCT protocols that actually restore the axis, stacking logic (with and without a test base), side-effect management, bloodwork cadence, and the sourcing pitfalls that catch people buying from the wrong vendors.
How S-23 works
Full Androgen Receptor Agonism#
S-23 binds the androgen receptor (AR) with high affinity — Ki of 1.7 ± 0.2 nM — placing it among the strongest-binding compounds in the aryl-propionamide SARM class. Unlike ostarine or andarine, which behave as partial agonists in androgenic tissue, S-23 is a full AR agonist in both muscle and prostate. The practical consequence: harder, drier muscle gains per milligram than ostarine-class SARMs, but also meaningfully more androgenic side-effect load (acne, oily skin, accelerated hair loss in susceptible users).
"S-23 was a potent AR agonist with Ki of 1.7 ± 0.2 nM, an ED50 for levator ani growth of 0.079 mg/day, and suppressed LH, FSH, and spermatogenesis resulting in contraception in 4/6 rats." — Jones A, Chen J, Hwang DJ, Miller DD, Dalton JT, Endocrinology (2009)
Once bound, the S-23/AR complex translocates to the nucleus and drives the standard anabolic transcriptional cassette — upregulation of myofibrillar protein synthesis genes, Akt/mTOR activation, and suppression of catabolic pathways (MuRF1, atrogin-1). The end result in skeletal muscle is indistinguishable in kind from testosterone's effect, just without the aromatization, 5α-reduction, or injection schedule.
Tissue Selectivity (and Its Limits)#
In castrated-rat Hershberger assays, S-23 shows an ED₅₀ of 0.079 mg/day for levator ani (muscle) growth versus 0.43 mg/day for prostate — a roughly 5:1 muscle-to-prostate selectivity ratio, better than testosterone but not as clean as ostarine. This is why S-23 produces a harder, more "cut" physique look than an equivalent dose of testosterone: the muscle-building machinery fires at sub-threshold doses for prostate and sebaceous activation. But make no mistake — at bodybuilding doses (15–30 mg/day), you are well above the prostate threshold. S-23 is not a tissue-selective-only-to-muscle compound at the doses users actually run.
HPG Axis Shutdown (The Contraceptive-Grade Problem)#
This is the mechanism that separates S-23 from every other SARM in practical terms. S-23 was developed as a reversible male contraceptive, and it does the job — in intact male rats, daily dosing drove LH and FSH to near-undetectable levels, produced testicular atrophy, and yielded azoospermic or primarily azoospermic semen. Because S-23 doesn't aromatize, there's no estradiol rebound to soften the negative feedback — the shutdown is cleaner and deeper than what you'd get from an equivalent anabolic dose of testosterone.
"S-23 caused marked suppression of gonadotropins, testicular atrophy, and primarily azoospermic semen after administration. Normal reproductive function was restored after cessation, confirming reversible suppression." — Jones A, Chen J, Hwang DJ, Miller DD, Dalton JT, Endocrinology (2009)
Translation for the user: expect full shutdown at any meaningful dose (>5 mg/day), expect testicular atrophy by week 3–4, and plan PCT as mandatory — not optional. The flip side is that the shutdown is reversible in rodent models with complete fertility return, consistent with what experienced users see after a proper SERM PCT.
Oral Pharmacokinetics and Dosing Cadence#
S-23 has excellent oral bioavailability and a half-life compatible with split daily dosing — no injections, no daily carrier-solvent headaches.
"Oral bioavailability of S-23 was 96% in rats, and the compound displayed a terminal elimination half-life of 11.9 ± 0.9 h after oral dosing indicating suitability for daily administration." — Jones A, Chen J, Hwang DJ, Miller DD, Dalton JT, Endocrinology (2009)
The ~12-hour half-life is why community protocols split dosing AM/PM: one dose gives a sharp peak-and-trough curve, two doses smooth AR occupancy through the 24-hour cycle. Food doesn't meaningfully impact absorption. For users holding the liquid sublingually for 30–60 seconds, some fraction bypasses first-pass metabolism — a minor edge, not a game-changer.
Fat Loss and Body Composition Signalling#
AR activation in adipose tissue suppresses adipocyte differentiation and promotes lipolysis, which is why S-23 (like other full AR agonists) produces visible body-recomposition effects at caloric maintenance and amplifies fat loss in a deficit. This is mechanism-consistent with the "hardening" effect users report — more muscle, less subcutaneous water and fat, sharper separation between muscle bellies in the back half of a cycle.
Long Detection Window#
Worth flagging mechanistically for tested athletes: S-23 and its phase I metabolites are detectable in urine for weeks after dosing stops.
"S-23 and multiple phase I metabolites were detected in urine for an extended window following oral dosing, supporting its long-term detectability by anti-doping laboratories." — Wong JKY, Roodt F, Walliser J, et al., Drug Testing and Analysis (2024)
If you compete in a tested federation, S-23 is a hard no — WADA-prohibited, with a detection tail that outlasts any reasonable washout window before a meet.
Protocol
| Level | Dose | Frequency | Notes |
|---|---|---|---|
| Low | 10–15 mg | Twice daily | Documented entry-level range |
| Mid | 15–20 mg | Twice daily | Most commonly studied range |
| High | 20–30 mg | Twice daily | Split AM/PM to smooth out blood levels given the ~11–12 hr half-life. Once-daily dosing works but gives a sharper peak and trough. Taking with food is fine — no fasted-state requirement. |
Cycle length & outcomes
Documented cycle
6–10 weeks
Plateau after
10 wks
S-23 is not a compound you pulse casually. It was designed as a reversible male contraceptive, and it shuts the HPG axis down harder than most oral steroids — so cycle structure, PCT, and bloodwork cadence matter more here than on ostarine-class SARMs. The upside is that once you accept it as a "light oral steroid" rather than a gentle SARM, the protocol is straightforward.
S-23 Cycle Length by Goal#
The sweet spot is 8 weeks. Six weeks is enough for a hardening polish; ten is the practical ceiling before suppression, lipid drift, and diminishing returns outweigh further aesthetic gains.
| Goal | Cycle Length | Daily Dose | Split |
|---|---|---|---|
| First-ever S-23 run / assess tolerance | 6 weeks | 10 mg | AM/PM split |
| Recomp / hardening (standalone) | 8 weeks | 15–20 mg | AM/PM split |
| Cut bridge under a deficit | 6–8 weeks | 15–20 mg | AM/PM split |
| Add-on to test base (dryness polish) | Final 4–6 weeks of cycle | 10–15 mg | AM/PM split |
| Advanced prep / final 8 before a shoot | 8–10 weeks | 20–30 mg | AM/PM split |
Dose splits exploit the ~11–12 hr half-life to smooth peaks and troughs; once-daily works but gives sharper sides and a more obvious afternoon crash.
"Oral bioavailability of S-23 was 96% in rats, and the compound displayed a terminal elimination half-life of 11.9 ± 0.9 h after oral dosing indicating suitability for daily administration." — Jones A, Chen J, Hwang DJ, Miller DD, Dalton JT, Endocrinology (2009)
Onset Timing#
S-23 is one of the fastest-reading SARMs in terms of visual change — but that's because the look changes, not because mass piles on.
- Week 1–2: Strength notches up; pumps feel tighter. No visual change yet.
- Week 3–4: Skin thins, vascularity sharpens, midsection tightens. This is the phase users describe as "looking more like themselves, only drier."
- Week 5–6: Peak hardening. Lifts still climbing if calories are adequate. Libido starts to tell you whether your suppression is mild or severe.
- Week 7–8: Diminishing returns on aesthetics; side-effect load (lipids, mood, lethargy) becomes the reason to wrap rather than any plateau in gains.
If you're two weeks in and nothing has changed, the product is underdosed — send it for HPLC testing rather than pushing the dose.
Tapering and Loading#
No loading phase. S-23 hits steady state inside 3–4 days of twice-daily dosing; front-loading gives you nothing but a steeper suppression curve.
No taper off the back end either. Tapering SARMs is a myth — the SERM PCT does the recovery work, not a dose wind-down. Run your last full dose on the final day, then start nolvadex 24–48 hours later.
Mandatory PCT#
This is the non-negotiable part. Because S-23 was literally engineered to cause azoospermia in intact male rats, HPG shutdown is the expected outcome, not a rare side effect.
"S-23 caused marked suppression of gonadotropins, testicular atrophy, and primarily azoospermic semen after administration. Normal reproductive function was restored after cessation, confirming reversible suppression." — Jones A, Chen J, Hwang DJ, Miller DD, Dalton JT, Endocrinology (2009)
Standard PCT protocol:
| Week | Nolvadex | Clomid (add if recovery sluggish) |
|---|---|---|
| 1 | 40 mg/day | 50 mg/day |
| 2 | 40 mg/day | 50 mg/day |
| 3 | 20 mg/day | 25 mg/day |
| 4 | 20 mg/day | 25 mg/day |
Experienced users running a low-dose test base (250–400 mg/week) through the cycle sidestep the PCT question entirely — you're already shut down, and S-23 becomes a hardening add-on rather than a compound you're trying to recover from in isolation.
Bloodwork Cadence#
| Timepoint | Panel |
|---|---|
| Baseline (2 weeks before cycle) | CBC, CMP, lipids, total T, free T, LH, FSH, E2, SHBG, prolactin |
| Week 4 on-cycle | Lipids, liver enzymes, total T, E2 |
| End of cycle (final week) | CBC, CMP, lipids, full hormone panel |
| End of PCT (week 4 post-cycle) | Total T, free T, LH, FSH, E2 |
| 8 weeks post-PCT | Total T, free T, LH, FSH — confirm full recovery |
The week-4 lipid check is the one people skip and regret. HDL routinely drops 20–40% on an 8-week S-23 run; if you're already sitting on borderline dyslipidemia at baseline, you want to know by week 4, not end-of-cycle.
Time Between Cycles#
Minimum 12 weeks off — that's 4 weeks of PCT plus 8 weeks of fully recovered endogenous production before the next run. Users who bridge shorter consistently report sluggish recovery on subsequent cycles, and the testicular atrophy takes longer to reverse each time you stack runs too close together. Blood-confirmed recovery of LH, FSH, and total T is the real "go" signal, not a calendar date.
Run it this way and S-23 earns its spot as a legitimate hardening tool. Skip the bloodwork and PCT and it's the SARM most likely to leave you with a post-cycle crash you weren't expecting.
Body Transformation Preview
Lean Mass Gain
6.0 lbs
4.5–7.5 lbs range
Fat Loss
2.4 lbs
1.8–3.0 lbs range
Lean Gain by Week
Risks & mistakes
Common (most users)#
- HPG suppression / loss of morning wood — expected at any dose above ~5 mg, and not a sign something has "gone wrong." S-23 was literally designed as a reversible male contraceptive. Mitigation: either run a low-dose test base (250–400 mg/week) so endogenous shutdown is a non-issue, or accept 4–8 weeks of blunted libido and run a proper SERM PCT after. Do not "blast through" naturally.
- Testicular atrophy — visible within 2–3 weeks. Cosmetic only; fully reversible on PCT. HCG (250–500 IU 2x/week) on-cycle if it bothers you.
- Flat mood / lethargy in weeks 4–8 — driven by crashed endogenous test. Test base fixes it. Without one, expect it and time the cycle around a low-stress period.
- Acne and oily skin — full AR agonism in skin. Standard topical retinoid + benzoyl peroxide routine handles most of it. Shower post-training, keep bedsheets clean.
- Accelerated scalp shedding in genetically susceptible users — S-23 itself is not 5α-reducible, so finasteride/dutasteride won't touch it. Topical AR antagonists (RU58841, pyrilutamide) are the mechanistically correct answer. Start them before the cycle, not after shedding begins.
- Aggression / short fuse — dose-dependent, usually manageable. Back off 5 mg if it's interfering with relationships or training partners.
Uncommon (dose-dependent or individual)#
- HDL crash / LDL rise — typical oral AR agonist lipid profile. Expect HDL down 20–40% on an 8-week run; milder than superdrol but real. Bloodwork at week 4. Citrus bergamot, daily cardio, and keeping saturated fat in check blunt it. If HDL falls below 25 or LDL climbs past 160, cut the cycle short.
- Elevated ALT/AST — not 17α-alkylated, but liver enzymes do move. TUDCA 500 mg/day and no alcohol handles the common case. If ALT is >3x ULN at mid-cycle labs, stop.
- Blood pressure creep — no aromatization, no water retention, but AR-driven vasoconstriction is still a factor. Check BP weekly; telmisartan 20–40 mg is the standard on-cycle antihypertensive if it climbs past 140/90.
- Prostate sensitivity — S-23 is a full agonist at prostate AR (ED₅₀ ~0.43 mg/day in rats vs 0.079 mg/day at muscle), so it's not prostate-sparing like ostarine. Tadalafil 5 mg/day handles urinary symptoms for most.
"S-23 caused marked suppression of gonadotropins, testicular atrophy, and primarily azoospermic semen after administration. Normal reproductive function was restored after cessation, confirming reversible suppression." — Jones et al., Endocrinology 2009
Rare but serious#
- Persistent post-cycle hypogonadism — the users who get stuck are almost always the ones who ran too long (>10 weeks), restarted too fast, or skipped PCT. Warning signs: no libido/morning wood return 8 weeks post-PCT, LH/FSH still floored on labs. If that's you, see an HRT-literate doctor — don't self-medicate deeper with more SERMs.
- Significant dyslipidemia — HDL under 20, LDL over 190, or a triglyceride spike. Stop the cycle. Lipids normalize within 6–12 weeks off.
- Cholestatic liver injury — rare but reported anecdotally with grey-market oral SARMs. Warning signs: itching, dark urine, jaundice, RUQ pain. Stop immediately and get a hepatic panel.
- Cardiac remodeling concerns with repeated long cycles — not S-23 specific, but applies to any full AR agonist run chronically. Echo every 2–3 years if this is part of a long-term stack rotation.
Hard contraindications#
- Near-term conception plans. S-23 produces azoospermia or severe oligospermia within weeks. Fertility returns post-cycle + PCT, but plan on 3–6 months of washout before trying to conceive. Bank sperm beforehand if timing matters.
- Pregnancy — full AR agonist, fetotoxic/virilizing.
- Women. This is not a negotiable point. S-23 is a full AR agonist with contraceptive-grade potency. Virilization — clitoral hypertrophy, voice deepening, menstrual disruption — is expected even at low doses, and voice changes do not reverse. Women looking for a SARM should run low-dose ostarine, not this.
- Personal or strong family history of prostate cancer or male breast cancer.
- Untreated hypertension or dyslipidemia. Fix these before the cycle, not during.
- Stacking with another full AR agonist SARM (ostarine, andarine) at full doses — receptor competition with no extra yield, just stacked sides.
- Tested athletes. S-23 and its metabolites are detectable in urine for weeks after cessation. WADA-prohibited.
"S-23 and multiple phase I metabolites were detected in urine for an extended window following oral dosing, supporting its long-term detectability by anti-doping laboratories." — Wong et al., Drug Test Anal 2024
Gender and PCT notes#
Women: not a female-appropriate compound, full stop. See above.
Men — PCT is mandatory, not optional. Treat S-23 like a mild-to-moderate oral steroid, not a "soft" SARM. Standard protocol:
| Week post-cycle | Nolvadex | Clomid (if sluggish) |
|---|---|---|
| 1 | 40 mg | 50 mg |
| 2 | 40 mg | 50 mg |
| 3 | 20 mg | 25 mg |
| 4 | 20 mg | 25 mg |
Bloodwork at baseline, week 4 of cycle, end of PCT, and 8 weeks post-PCT (CBC, CMP, lipids, total/free T, LH, FSH, E2, SHBG). Users running a test base can skip SERM PCT and simply resume normal HRT-style TRT or a standard post-blast-and-cruise protocol — the S-23 suppression is irrelevant against exogenous test. This is why experienced users almost universally run S-23 on top of test rather than standalone: the yield is better, the sides are the same, and there's no post-cycle crash to navigate.
Stack & combine
Multipliers applied when these compounds run together. Values > 1 indicate a bonus on that axis. Tap a partner to expand the mechanism.
| Partner | Type | Lean | Fat loss | Recovery |
|---|---|---|---|---|
| synergistic | ×1.12 | ×1.02 | ×1.24 | |
| synergistic | ×1.07 | ×1.05 | ×1.15 |
Featured in stacks1 curated protocol include S-23
FAQ — S-23
Where to buy
Swiss Chems
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- Buy S-23 (10mg/capsule), 60 Capsules - SwissChems - Buy Best Quality Peptides, SARMS OnlineBuy S-23
NextChems
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- S-23, 10 mg - 60 capsules - Next ChemsBuy S-23
Research & citations
4 studies cited on this page.
Conclusion
S-23 is the go-to SARM for pure hardness and aggressive recomp — but it demands real respect if you want the upside without getting burned on the way out.
Key takeaways:
- Standard dose: 10–20 mg/day, split AM/PM for stable blood levels
- Cycle length: 6–8 weeks (rarely up to 10), with 8 weeks off plus 4 weeks of SERM PCT mandatory
- Expect full HPG shutdown at any effective dose — treat PCT like you would after orals
- Best stacked with a low-dose test base or, for dry hardening, with RAD-140 or MK-677
- Major benefits: rapid muscle hardening, visible vascularity, minimal water retention, strong strength gain (~1.25%/week)
- Aggressive on suppression: expect loss of endogenous T, mood/libido dip, and post-cycle recovery work — but fertility/recovery are reversible (Jones 2009)
- Key risks are manageable: get baseline/mid/post-cycle labs, prep a real SERM PCT, and avoid if planning near-term conception
Used smart, S-23 is the hardest-hitting SARM in the toolkit. Treat it with the respect you give a mild oral steroid and it will do exactly what experienced users reach for it for — dry muscle, no water, all polish.