S-23 SARM: what the research says, the contraception studies and the UK legal position (2026)

Last reviewed 15 September 2026. Products on this site are supplied strictly for laboratory research and are not for human consumption. This guide summarises published research; it is not medical advice.

What S-23 is

S-23 is a non-steroidal selective androgen receptor modulator developed by GTx Inc in the late 2000s as a successor to Andarine (S-4). It belongs to the aryl-propionamide family, the same chemical class as Ostarine and Andarine, and was designed to bind the androgen receptor with higher affinity than the earlier compounds. In the published binding data S-23 shows one of the strongest androgen receptor affinities of any SARM studied, which is why it is often described in the research community as "the strongest SARM". That description refers to receptor binding in the laboratory, not to any outcome in a person.

What the research studied

S-23 is unusual among SARMs in that its primary research purpose was not muscle wasting but male contraception. The principal study is Jones et al. (2009), published in Endocrinology, in which rats were dosed with S-23 for ten weeks. The researchers reported three findings that have shaped every later discussion of the compound: a dose-dependent suppression of spermatogenesis (sperm production) that reversed after the compound was withdrawn, an increase in lean body mass and bone density comparable to testosterone, and suppression of the hormones LH and FSH that drive natural testosterone production. In other words, in animals S-23 behaved like a strong androgen in muscle and bone while shutting down the body's own hormonal signalling.

A second strand of research looked at S-23 in models of muscle and bone loss, again in rodents, where it increased muscle mass and bone mineral density at low doses. GTx did not take S-23 into human trials; the company's clinical programme focused on Ostarine (enobosarm), and S-23 was shelved when the contraceptive application did not progress.

What has not been studied

There are no published human trials of S-23 of any kind. Everything known about its effects in people comes from case reports and anecdote, which is not evidence. The animal data show complete suppression of natural testosterone at effective doses, with recovery after withdrawal in rats; whether the same recovery happens in humans, and over what time, is unknown. Liver effects, lipid effects and long-term safety have not been characterised. For a compound with this binding profile, the absence of human data is the most important fact about it.

How it compares

The legal position in the UK, 2026

S-23 is not a controlled drug under the Misuse of Drugs Act 1971. It is not a licensed medicine, so it cannot be sold for human consumption; the MHRA treats any product supplied with dosing instructions or health claims as an unlicensed medicine. It may be sold as a research chemical. In sport S-23 is prohibited at all times under WADA class S1.2. Buyers outside the UK should check their own country's rules; Australia schedules SARMs as prescription-only and several EU states restrict import. Full guide: Are SARMs legal in the UK?

What to check on a research product

Because S-23 has no human safety data, specification matters more than for any other compound in the range. Check the strength per capsule, the capsule count and the batch number on the bottle, and match the batch to its certificate of analysis when published. Independent batch certificates of analysis for Dexterz Labs products are being commissioned and will be published on each product page as each batch is tested. See how to read a certificate of analysis.

Key references

Jones A, Chen J, Hwang DJ, Miller DD, Dalton JT. Preclinical characterization of a (S)-N-(4-cyano-3-trifluoromethyl-phenyl)-3-(3-fluoro, 4-chlorophenoxy)-2-hydroxy-2-methyl-propanamide: a selective androgen receptor modulator for hormonal male contraception. Endocrinology. 2009;150(1):385-395. Bhasin S, Jasuja R. Selective androgen receptor modulators as function promoting therapies. Current Opinion in Clinical Nutrition and Metabolic Care. 2009;12(3):232-240. World Anti-Doping Agency. Prohibited List 2026, S1.2.

Dexterz Labs S-23 10 mg, 90 capsules (research use only)   Back to SARMs 101

Products are supplied strictly for laboratory research and are not for human consumption. 18+ only.