Peptides are short chains of amino acids that mostly act as signals, for example telling the body to release its own growth hormone or to repair tissue. Anabolic steroids are synthetic forms of testosterone that build muscle strongly but carry serious risks to the heart, liver, fertility and mood. SARMs are unapproved experimental drugs, often sold mislabelled, and linked to liver injury. Peptides generally have milder effects and fewer side effects, but many are still experimental. None of the three should be used without a physician, and all can be banned in tested sport.

IGF-1 LR3 and MGF are muscle-focused peptides related to insulin-like growth factor 1, the hormone that carries out much of growth hormone's effect on muscle. IGF-1 LR3 is a long-acting modified form, and MGF is a version that muscle makes after training or injury. Both show muscle growth and repair effects in cell and animal studies, but neither has been tested in human trials for muscle building. They also carry real risks, mainly low blood sugar, so they should only be used under close medical supervision and never by tested athletes.