Peptides are basically protein chains. Proteins are instructions to certain things in our bodies, proteins can activate/deactivate certain processes in our bodies among many other things.

If you artificially inject the correct protein into your body to mimic a naturally producing one, it would do the job and in that way peptides works. However, most of peptides injected by people are hardly human tested for long term effects. Human body is remarkable at adjusting and maintaining a baseline and we don’t know what would be the unintended side effects of these peptides because the body would try to counterbalance it. At best it could be just a mild headache or stomachache and at worst it could be cancer