HGH (Somatropin)

Mecasermin is recombinant human IGF-1, identical to the 70-amino-acid hormone your liver makes in response to growth hormone. Sold as Increlex, it was FDA approved in 2005 for a narrow group: children with severe primary IGF-1 deficiency (mostly Laron syndrome) or a growth hormone gene deletion with antibodies against GH. In those kids it is the only thing that works, because GH cannot do its job. It is not a GH substitute, and its real risks (low blood sugar, tonsil overgrowth, cancer warnings) are why it stays a specialist drug.

MK-677 (ibutamoren) is not actually a peptide, it is a small, orally active non-peptide molecule that mimics ghrelin, your hunger and growth-hormone hormone. Taken as a daily pill, it reliably pushes up growth hormone and IGF-1 levels, which is why it is popular for muscle and recovery. It has been through real human trials but was never approved as a drug, and the trials that mattered most for older adults and Alzheimer's came up short.

GHRP-6 is a synthetic six-amino-acid peptide (His-D-Trp-Ala-Trp-D-Phe-Lys-NH2) that tricks the body into releasing its own growth hormone. It was one of the first growth hormone secretagogues ever made, and the hunt to find out why it worked led scientists straight to the discovery of ghrelin. It has no approval as a drug anywhere and is used only as a research compound.

GHRP-2 (generic name pralmorelin) is a synthetic hexapeptide growth hormone secretagogue and a second-generation cousin of GHRP-6. It prompts the pituitary to release growth hormone and is the one peptide in this family with an actual regulatory approval: it is licensed in Japan as a diagnostic agent for growth hormone deficiency. Outside that narrow diagnostic use it has no approval and is sold elsewhere only as a research compound.

MGF is the name given to a splice variant of the IGF-1 gene (IGF-1Ec in humans, IGF-1Eb in rodents) that muscle switches on after loading or damage. The "MGF" sold as a peptide is usually a synthetic 24-amino-acid copy of that variant's unique tail end (the E-domain, sequence YQPPSTNKNTKSQRRKGSTFEEHK), often with a modified C-terminus. It got famous through early lab work from Geoffrey Goldspink's group suggesting it kicks muscle stem cells into action. The honest picture is messier: a careful 2014 pharma replication found no effect on muscle cells at all, nobody has shown the free peptide actually exists in the body, and there are zero human trials.

IGF-1 DES is insulin-like growth factor 1 with its first three amino acids chopped off, leaving a 67-residue peptide. That small cut makes it bind poorly to the IGF binding proteins that normally keep IGF-1 in check, so more of it reaches the receptor and it acts stronger, but shorter. It occurs naturally in small amounts (it was first isolated from human fetal brain tissue) and is sold online as a muscle-building research peptide. There are no human trials of it at all, so everything said about it in people is extrapolation.