Ostarine

Ostarine (enobosarm) is the most clinically studied SARM — a 'selective androgen receptor modulator.' Like testosterone it acts on the androgen receptor, but it's designed to be tissue-selective.

The goal of that selectivity is to switch on the muscle- and bone-building effects of androgen signaling while having less effect on tissues like the prostate and skin. The selectivity is thought to come from the shape the receptor takes when this drug binds and which helper proteins it recruits, and in trials it did increase lean body mass.

The selectivity is only partial, though: trials and reports still show suppression of the body's own testosterone (the HPTA axis), lowered HDL cholesterol, and sometimes raised liver enzymes. It is not approved for any use and is banned in sport.