MK-2866, also known as Ostarine, is a selective androgen receptor modulator (SARM). It was originally developed by the pharmaceutical company GTx, Inc. to treat conditions like muscle wasting, osteoporosis, and other similar ailments. Ostarine works by selectively binding to androgen receptors in the body, particularly in muscle and bone tissues, which can help improve muscle mass, strength, and bone density, without the harsh side effects often associated with anabolic steroids.