"Peptide" basically just means a particular amino acid chain. You can't take the orally as your digestive system will just break them up and use them as food (like protein), so they're all injected, with all that implies. (It's not good.)
Each one either does something or does nothing. If it does nothing it's a waste of money, plus the injection risks. If it does do something, it generally mimics an existing body hormone. Hormones are complicated and have feedback loops (basically to self regulate, so that you don't have too much or too little). Once you start taking one, you have to account for that, possibly for the rest of your life. So be very careful with the ones that actually do anything.
This doesn't mean you can't take them. Just be aware that there are serious risks.