MK-677 which is also known as Ibutamoren or Nutrobal is a Selective Androgen Receptor Modulator (SARM). Most newer SARMs are non-steroidal but have similar anabolic properties to steroids. MK-677 and most SARMs differ from steroids by how they bind to androgen receptors (ARs). Most of the popular steroids are full agonists of the ARs however, non-steroidal SARMs selectively bind to specific ARs due to their unique chemical composition. The majority of SARMs developed target primarily lean muscle mass and bone tissues. It has a high affinity for ARs in lean muscle and bone tissue and a lower affinity in other tissues.

Another interesting effect of MK-677 is its effect on growth hormone and IGF-1 release. It increases GH levels my mimiking the hormone, ghrelin and attaches to ghrelin receptors in the brain. Activated ghrelin receptors in the brain then release GH. GH has some minor anabolic properties and can help with muscle repair, decrease muscle catabolization and increase bone density. Increased GH levels have anabolic properties and can also help with muscle, joint and tendon recovery and also have other positive benefits such as: increased metabolic rate, improved mood, memory and cognition.

Nutrobal also increases IGF-1 (Insulin-Like-Growth-Factor-1) which has positive benefits similar to GH. MK-677 does not seem to affect other important hormones and does not seem to increase cortisol (stress hormone).