Selective Androgen Receptor Modulators, they are considered the safer alternative their pro-hormone counterpart due to their likeliness of little hepatotoxicity and potential oestrogen related side effects.
Selective androgen receptor modulators contain androgens, a special kind of hormone that act as ligands (a molecule linked to another molecule) that connect to cellular androgen receptors (AR). This connection to the AR is what gives all pro-hormones and steroids their muscle growth properties.
A Selective androgen receptor modulator can both obstruct or fuel similar hormone receptors in different conditions. If it can obstruct or fuel a receptor it has the ability to replicate the important effects in one tissue and concurrently reduce the adverse effects of the natural or synthetic steroidal hormones in other tissues.