S-23 SARM: the strongest SARM for cutting and definition

27 Mar

S-23 SARM is regarded within the bodybuilding community as the most potent selective androgen receptor modulator (SARM) available. The substance was originally developed by GTx Inc. as a potential male contraceptive — yes, you read that right — due to its ability to severely suppress spermatogenesis. The fact that it also produces tangible effects on muscle mass, fat burning and hardness has made it popular among advanced users who are looking for an alternative to anabolic steroids during cutting phases. Here we review what the research shows, which S-23 dosage is used, which side effects you should expect and how the substance compares to RAD-140.

How does S-23 work and why is it considered the strongest SARM?

S-23 binds to androgen receptors with extremely high affinity — higher than most other SARMs including RAD-140 (Testolone), Ostarine (MK-2866) and Ligandrol (LGD-4033). This strong binding means a strong activation of the androgen receptors in muscle and bone tissue, which drives muscle protein synthesis and reduces fat storage.

What separates S-23 from milder SARMs is the degree of selectivity — or rather, the lack thereof. While SARMs such as Ostarine largely limit their effect to muscles and bones, S-23 exhibits a broader activation that partially affects the prostate and reproductive system. In animal studies (rats), S-23 caused complete suppression of spermatogenesis at doses that produced anabolic effects — a profile more similar to anabolic steroids than milder SARMs.

The binding affinity is measured as Ki value, where lower number means stronger binding. S-23 has a Ki value of about 1.7 nM — compared to RAD-140 at 7 nM and Ostarine at 3.8 nM. That difference explains why S-23 produces faster and more noticeable visual changes, but also why the side effects are more pronounced. High affinity means that the substance “competes out” the body’s own testosterone more effectively, which paradoxically leads to stronger suppression of the natural hormone production.

What effects are reported with S-23 use?

No human studies with S-23 have been published as of 2026. All data on human effects are based on anecdotal reports from the community and preclinical animal studies. With that caveat, experienced users report:

• Increased muscle hardness and vascularity — the most common and most consistent effect reported. The muscles are experienced as harder and more defined, probably through a combination of increased muscle protein synthesis and reduced subcutaneous water retention.

• Fat burning during cutting — many describe a noticeable reduction in body fat, especially around the waist, without changing diet. In rat studies, S-23 showed a dose-related reduction in fat mass.

• Strength gain — moderate compared to anabolic steroids, but measurable. Most people report a 5-10% increase in maximal strength over an 8-week cycle.

• Aggressiveness and drive — reported by some users and may be linked to the androgenic profile. This effect is experienced as positive by some (better focus in the gym) and negative by others (irritability in everyday life).

These effects make S-23 a popular substance during lean cutting phases, where the goal is to maintain maximum muscle mass while reducing body fat. The timeline for visible results varies, but most people report noticeable changes in muscle quality as early as 10-14 days, while the full effect on body composition is typically reached around week 5-6.

S-23 dosage — what does the community use and what do the animal studies show?

Dosage recommendations for S-23 are based entirely on anecdotal experience within the bodybuilding community, as human studies are lacking. The most common protocols look like this:

|Experience level|Dose|Cycle length|PCT recommended?|

|Beginners (not recommended)|10 mg/day|6 weeks|Yes, always|

|Means|15–20 mg/day|8 weeks|Yes, always|

|Advanced|20–30 mg/day|8 weeks|Yes, always|

S-23 has a half-life of about 12 hours, which means that most people divide the dose into two doses per day (morning and evening) for more even blood concentrations. A dose of 20 mg is thus divided into 10 mg at breakfast and 10 mg at dinner. The substance is orally bioavailable and is taken as a liquid (dissolved in PEG-400 or ethanol) or in capsule form, depending on the supplier.

Higher doses (>25 mg) increase the risk of side effects exponentially without proportionally increasing efficacy — a typical dose-response curve that plateaus. Experienced users who have tried 30 mg rarely report better results than at 20 mg, but almost always worse night sweats and faster hair loss.

Post Cycle Therapy (PCT) is not optional with S-23. The substance strongly suppresses endogenous testosterone production — complete suppression of LH and FSH was measured in animal studies, which places S-23 in the same category as anabolic steroids in terms of hormonal influence. Standard PCT with Nolvadex (tamoxifen) 20–40 mg per day for 4 weeks or Clomid (clomiphene) 50–100 mg per day for 4 weeks is the most common protocol. Without PCT, you risk prolonged hypogonadism with all the symptoms that follow — fatigue, decreased libido, muscle loss and depression. Read more about PCT and SARM stacks in our guide on SARM stacks 2026.

S-23 side effects — what you risk

The side effect profile of S-23 is the most aggressive among commercially available SARMs and is similar in many respects to mild anabolic steroids.

Testosterone suppression is the most serious side effect. In rat studies, S-23 caused complete suppression of spermatogenesis—sperm production completely ceased during treatment. In humans, users report testosterone levels of 1–3 nmol/L (normal: 8–30 nmol/L) after an 8-week cycle without PCT. This degree of suppression produces all the classic symptoms of testosterone deficiency.

Other reported side effects include night sweats — one of the most common complaints, likely linked to hormone fluctuation and altered thermoregulation. Many users describe waking up soaked several nights in a row, especially during the first two weeks.

Hepatotoxicity is milder than with oral anabolic steroids (17-alpha-alkylated), but ALT and AST levels rise measurably in most people. Blood tests typically show 1.5-2.5 times the normal value at doses around 20 mg per day — not alarming in itself, but a clear signal that the liver is being stressed. Limiting alcohol intake during cycling and taking hepatoprotective supplements such as TUDCA (250–500 mg/day) or NAC (600–1200 mg/day) are recommended in the community, although clinical evidence is lacking.

Hair loss in the genetically predisposed is a recurrent report, consistent with the androgenic profile. Reduction of HDL cholesterol by 20-40% during cycle is documented in animal studies and consistent with anecdotal blood samples from users. This shift in the lipid profile increases cardiovascular risk, especially with longer cycles or repeated courses without adequate recovery.

S-23 vs RAD-140 — which SARM is best for cutting?

The comparison between S-23 and RAD-140 (Testolone) pops up in every SARM forum. Both are strong SARMs, but their profiles differ in important ways.

RAD-140 provides a stronger anabolic effect relative to its androgenic profile. It means more muscle growth with fewer androgen-related side effects (acne, aggression, hair loss). S-23 provides better hardness, vascularity and fat burning but with stronger hormonal suppression and more side effects.

The recovery after cycling differs markedly. RAD-140 requires PCT but suppression is generally milder — most recover within 4-6 weeks with normalized testosterone levels. S-23 suppression may take 6–10 weeks to reverse, even with aggressive PCT. Blood tests during recovery often show that LH and FSH remain suppressed longer after S-23 compared to RAD-140, suggesting a deeper disruption of the hypothalamic-pituitary-gonadal axis.

Sarm cutting with S-23 is best suited for experienced users who have already tried milder SARMs and understand the risks of hormonal suppression. RAD-140 is the better choice for those who want a strong SARM with a more manageable side effect profile. Read our RAD-140 Testolone in-depth guide for a full review. And if you are wondering about the legal status: check out our article on whether SARMs are legal in Sweden 2026 before making any decision.

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