S-23, also known as Mastorin, is an investigational selective androgen receptor modulator (SARM) developed by GTX, Inc as a potential male hormonal contraceptive. It binds to the androgen receptor more strongly than older drugs such as andarine with a Ki of 1.7 nM, and in animal studies it showed both a good ratio of anabolic to androgenic effects, and dose-dependent suppression of spermatogenesis with spontaneous recovery after cessation of treatment. It was encountered as a novel designer drug by 2020.