S23

S23 is a lab-made drug in a family called SARMs (selective androgen receptor modulators — molecules that switch on the same cell “docking points” as testosterone, but only in certain tissues). It is not a peptide. The idea behind it was to build up muscle and bone while having relatively little effect on the prostate. It was actually created as a possible male birth-control pill, and the only solid evidence we have for it comes from rats. Sellers and biohackers promote it as a powerful body-recomposition compound, but there are no published, carefully controlled studies in people for any use.

What it is

S23 (also called “Mastorin”; CAS 1010396-29-8; formula C18H13ClF4N2O3) is a small synthetic molecule — chemically a nonsteroidal aryl-propanamide androgen receptor ligand (a non-steroid compound that latches onto testosterone’s receptor). Because it’s a SARM, it turns on that receptor in some tissues but not others. Like other compounds that switch this receptor on, it dials down the brain-to-testicle hormone signaling system (the hypothalamic–pituitary–gonadal axis), lowering LH and FSH (two hormones that tell the testicles to make sperm and testosterone). That same effect is both why it could work as a contraceptive and why it causes hormone-related side effects. It was developed by GTx, Inc. as a possible male birth-control drug, not as a muscle drug.

The claims

Research-chemical sellers and the bodybuilding and biohacking crowd promote S23 as a strong SARM for “lean mass / strength / recomp.” Those claims are marketing and personal anecdotes, not findings from clinical studies. Seller websites usually slap on a “not for human consumption / research use only” label, even though people clearly buy the product to use themselves.

What the evidence actually shows

The evidence on S23 is animal-only (rodent). There are no published, controlled human trials for any use.

The main study (Jones A, Chen J, Hwang DJ, Miller DD, Dalton JT. Endocrinology. 2009;150(1):385–395) found that in male rats, S23 cut down sperm production more and more as the dose went up — “four of six animals showed no sperm in the testis.” When paired with a low dose of estradiol benzoate (a form of estrogen), most animals stopped producing sperm entirely (azoospermia) and there were zero pregnancies in mating trials. Importantly, the infertility was fully reversible — a 100% pregnancy rate came back after about 100 days off the drug — which is exactly why S23 was studied as a contraceptive. An earlier paper (Marhefka CA, Gao W, Chung K, et al. J Med Chem. 2004;47(4):993–998) described this group of SARMs, which break down slowly in the body (metabolically stable).

Bottom line on evidence: the bodybuilding benefits claimed for humans are unproven. The only strong data are rodent studies on reproduction and hormones.

Legal and regulatory status

FDA: SARMs are not approved drugs, and they are not legal ingredients in dietary supplements. The FDA treats SARM-containing bodybuilding products as unapproved, mislabeled (misbranded) drugs and warns they can raise the risk of heart attack, stroke, and serious liver damage. The FDA sent warning letters to Infantry Labs (Oct 23, 2017), IronMagLabs (Oct 22, 2017), and Panther Sports Nutrition (Oct 23, 2017), and put out a consumer alert aimed at teens and young adults.

DEA scheduling: As a group, SARMs are not currently federal controlled substances. S23 has shown up as a designer drug (by 2020). The SARMs Control Act (S.2742, 2018; S.2895, 2019; and later reintroductions) tried to add SARMs to Schedule III, but no version has been signed into law as of mid-2026 — so treat federal scheduling as a proposal, not the law.

For comparison, oxandrolone is an FDA-approved anabolic-androgenic steroid and a Schedule III controlled substance — but it’s a steroid, not a SARM, and it is not in the same drug family as S23.

Anti-doping status

All SARMs, including S23, are banned at all times (both in and out of competition) under Section S1.2, “Other Anabolic Agents,” of the WADA Prohibited List. The named examples include andarine, ostarine/enobosarm, LGD-4033, RAD140, S-23, and YK-11. (One note: GW-501516 [cardarine] and SR9009 are not SARMs — GW-501516 is a PPARδ agonist [it acts on a different cell switch] and is listed separately under S4 Metabolic Modulators.)

Safety

There is basically no human safety data for S23. What we can say about risk is drawn from the SARM family in general and from the rat studies.

• Hormone suppression: This is the best-documented effect. S23 lowers LH and FSH and reduces sperm production in rats (reversible in those studies). In people, compounds that switch on the androgen receptor would be expected to suppress testosterone and harm fertility, though no human data put numbers on this for S23.
• Cholesterol (lipids): SARMs as a group are repeatedly linked to lower HDL (the “good” cholesterol) — that’s a pattern across the class, not a figure measured specifically for S23 in humans.
• Liver damage (hepatotoxicity): FDA reports and case reports tie SARM-containing products to drug-caused liver injury, sometimes serious enough to land people in the hospital. Real-world products are also often mislabeled or contaminated.
• Heart and blood vessels (cardiovascular): The FDA notes a higher risk of heart attack and stroke with SARM products.
• What we know about the broader class (not shown for S23 itself): Andarine (S4) causes a yellow tint to vision and trouble adjusting between light and dark (reported to reverse) — that’s specific to andarine. GW-501516, a PPARδ agonist (not a SARM, and not S23), was dropped by GlaxoSmithKline around 2007 after long-term rat studies showed tumors in several organ systems.
• Honest gaps: There is no reliable evidence that S23 specifically causes permanent blindness or QT prolongation (a dangerous change in the heart’s electrical rhythm).

Bottom line

S23 is a SARM, not a peptide. It was built as a possible male contraceptive and has only been tested in rodents, where it temporarily shut down sperm production. There are no human trials, so the lean-mass and recomp claims are unproven marketing. The FDA considers it an unapproved, mislabeled drug, it carries documented class risks (liver injury, heart events, lower HDL, and hormone suppression), and it is banned at all times in sport. The honest summary: little to no evidence of benefit in people, and real risk that is partly unmeasured.

Evidence grade: 3/10 · Animal only.

Sources

• Jones A, et al. Endocrinology. 2009;150(1):385–395 (PMID 18772237)
• Marhefka CA, et al. J Med Chem. 2004;47(4):993–998 (PMID 14761201)
• FDA: Certain Bodybuilding Products Put Consumers at Risk
• FDA: SARMs Among Teens, Young Adults (consumer update)
• FDA warning letter — Infantry Labs LLC (10/23/2017)
• FDA warning letter — IronMag Labs (10/22/2017)
• FDA warning letter — Panther Sports Nutrition (10/23/2017)
• USADA: SARMs — Prohibited Class, Anabolic Agents
• WADA Prohibited List
• Congress.gov — SARMs Control Act of 2018 (S.2742)
• Congress.gov — SARMs Control Act of 2019 (S.2895)
• GW501516 — Wikipedia
• S-23 (drug) — Wikipedia