Peptides are molecules designed to replicate the action of some hormones. Melanotan, for example, is a "suntanning" peptide, imitates the action of melanocitostimulating hormone. It was noted by the users that melanotan sometimes produces solid erections. This led to the development of bremelanotide, which is as far as I remember is a fragment of melanotan. How Brem works is uncertain.
Peptide biochemistry is complicated. In order to replicate the action of GH you have to use the combination of GHRH and GHRP. There are side-effects, and the effects will be apparent only on top of a solid protocol - exercise and diet. And there are also debates in regards to IGF-1 and it's role in cancer growth.
There used to be an incredible website, Datbetrue.co.uk run by a very clued up guy. His literature reviews used to freak me out, they were worthy of being put on PubMed. Few years ago he sent every member the message that he had been diagnosed with terminal illness and was going to close the site. Since then sources of information are scarce.
So, like with any drugs, including steroids, you have to know what you are doing. And obviously why.