Short answer: A peptide is a short chain of amino acids. Your body makes hundreds of them, like insulin and oxytocin, and uses them as messages between cells. Most peptides people use are injected under the skin because the gut breaks them down. Much of the research on newer peptides is in animals, so set your expectations and track what you notice.
Amino acids are single building blocks. Link a few of them together and you get a peptide. Link a lot of them and fold the chain into a 3D shape and you get a protein. The usual cut-off is about 50 amino acids, though it is a convention more than a hard line.
Your body already runs on peptides. Insulin, glucagon and oxytocin are all peptides. Many medicines are peptides too, like semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound). Other peptides, like BPC-157, are sold for research and are not approved as medicines.
Most peptides act as signals. A peptide travels to a cell, fits into a matching receptor on the cell surface, and that sets off a response inside the cell, such as releasing a hormone.
Because a receptor only responds to signals that fit it, a peptide tends to act on one system. That is part of why researchers are interested in them. It does not mean they have no side effects: approved peptide drugs like semaglutide have well-known ones.
See the sub-Q injection guide for how an injection under the skin is done.
Anabolic steroids such as testosterone are built from cholesterol, not amino acids. They pass into the cell and act on receptors inside it. Taking testosterone lowers your own production.
Growth hormone (somatropin) injections give the hormone itself.
Growth-hormone-releasing peptides such as ipamorelin or sermorelin ask your own pituitary gland to release growth hormone, so the release still comes in pulses.
Different does not mean safer. Every compound has its own risks, and many peptides have far less human safety data than approved drugs.
Approved peptide drugs have clinical trial data that tells you what to expect and when. Most research peptides do not. Much of what you read online is personal reports, and effects people describe are often slow and hard to feel day to day.
That is why a baseline helps. Write down how you sleep, your weight, your training and any lab results before you start, then log as you go. With records, you can tell what changed instead of guessing.
See also: Compound library · Glossary