IGF-1 LR3, or Long Arginine 3-IGF-1, is a synthetic analog of human insulin-like growth factor 1 (IGF-1). It's an 83 amino acid polypeptide chain hormone that binds to muscle cell IGF-1 receptors, activating a pathway that promotes protein synthesis and inhibits protein breakdown. This can lead to increased muscle mass and strength over time.

IGF-1 LR3 is primarily used to increase the biological activity of IGF inside of our bodies. When IGF-1 LR3 is mobile in the body, it functions differently in individual types of tissues. For example, muscle tissue forces the muscle to be more sensitive to insulin's effects, such as the decrease in fat storage.