S23 is a selective androgen receptor modulator (SARM) and has been studied and proven to increase muscle mass, reduce fat mass, and has a very high binding affinity to the androgen receptors. It is non-steroidal and has tissue-selective anabolic effects in muscle and bone, while sparing other androgenic effects that come from anabolic androgenic steroids (AAS). It also has been shown to decrease prostate size in studies, which is the opposite of a very common negative side effect of anabolic steroids (enlargement of the prostate).
The main side effect of S-23 use is the suppression of natural testosterone production. This is what gives it potential as a male contraceptive, and also makes it very different from other SARMS. While ostarine, andarine (s4), and Ligandrol may cause a mild shut down, S-23 is extremely suppressive. Consequently, this will be a major factor in deciding whether to use this SARM or opt for the more widespread choices. Unfortunately, there have not been any human studies, so not much is known on other potential side effects.
10-50mg per day