S-23 binds androgen receptors with extremely high affinity, triggering strong anabolic effects in muscle and bone. In terms of anabolic:androgenic ratio, S-23 is one of the highest, meaning it is very effective at building muscle and strength with relatively low androgenic (secondary sexual characteristic) impact. Users experience rapid muscle hardening and fullness, often likened to a combination of the dryness of Winstrol with the fullness of LGD-4033. Uniquely, S-23 also has profound effects on the reproductive axis: it dose-dependently suppresses follicle-stimulating hormone (FSH) and thus sperm formation. In rodent studies, S-23 caused temporary infertility in 100% of subjects, which was fully reversed after stopping the compound. This is why S-23 is being researched as a male contraceptive. Additionally, S-23 exhibits anti-catabolic effects – in studies where subjects were given a muscle-wasting stimulus (like excess glucocorticoids), S-23 prevented muscle loss, even promoting growth. It also demonstrated dose-dependent fat reduction, particularly visceral fat. Mechanistically, by binding the androgen receptor in muscle, S-23 upregulates protein synthesis and nutrient partitioning into muscle. In adipose tissue, androgen receptor activation can increase lipolysis (fat breakdown) and reduce new fat cell formation. Users should note that S-23’s strong binding causes significant HPTA suppression – natural testosterone and sperm output drop – so while on cycle, a reliable contraceptive effect is likely, and recovery protocols afterward are crucial. Despite this, the muscle-building results are among the most pronounced of any SARM, making S-23 a compound reserved for experienced researchers seeking extreme results.