Peptide

A peptide is a linear or cyclic biological polymer consisting of 2 to approximately 50 amino acid residues joined by covalent amide bonds (peptide bonds). Peptides are chemically distinguished from proteins by their shorter length, typically lacking the complex tertiary and quaternary folded structures characteristic of larger globular proteins.

Endogenous peptides function as hormones (insulin, glucagon, oxytocin), neurotransmitters (neuropeptide Y, endorphins), and growth factors. Over 80 peptide drugs are approved worldwide for clinical treatment of diabetes, cancer, osteoporosis, and endocrine disorders.

The word “peptide” covers an enormous range of molecules, from hormones the body makes every day to approved prescription drugs to untested compounds sold online. Being a peptide says nothing on its own about whether a substance is safe or effective. Insulin, semaglutide and BPC-157 are all peptides, yet only the first two have passed regulatory review for human use. Each claim has to be judged on the evidence for that specific molecule.

Peptides behave differently from ordinary tablets. Because they are chains of amino acids, enzymes break most unmodified peptides down quickly, and they cross the gut wall poorly. That is why most peptide medicines are injected, and why developers substitute amino acids, add fatty-acid chains or attach larger carriers to make them last longer. A claim that an ordinary peptide works well as a swallowed capsule deserves extra scrutiny.

Peptides are also a real and growing part of medicine. More than 80 peptide drugs have reached the market since insulin was introduced about a century ago, for conditions including diabetes, cancer, osteoporosis, multiple sclerosis and HIV infection. Knowing that legitimate peptide therapies exist helps readers separate well-studied medicines from products that borrow the same vocabulary without the same evidence.

“Peptides are natural, so they are safe”

Many peptides are copies or modified versions of natural hormones, but natural origin does not guarantee safety. Insulin is a natural hormone and can still cause dangerously low blood sugar. Synthetic analogues are often deliberately altered to act longer or more strongly than the body’s own version. Safety has to be shown in human studies for each compound and each use.

“A peptide is basically a supplement”

Most peptides marketed for health effects act like drugs, not foods. Semaglutide and tesamorelin, for example, are prescription medicines, and many other peptides are unapproved investigational compounds. Calling a product a supplement or a research chemical does not change what it does in the body, and it does not bring the product under any pharmaceutical quality control.

“Peptides and proteins are completely different things”

They are the same kind of molecule: chains of amino acids joined by peptide bonds. The dividing line is a convention based on size, with around 50 amino acids a common cut-off. That is why insulin, with 51 amino acids, sits right on the boundary and is described as a peptide hormone by some sources and a small protein by others.

What is the difference between a peptide and a protein?

Both are chains of amino acids linked by peptide bonds. Peptides are shorter, usually between 2 and about 50 amino acids, and often lack the complex folded structure of proteins. Proteins are longer chains that typically fold into specific three-dimensional shapes. The boundary is not sharp, so a few molecules, such as insulin, are described either way.

Why are most peptide drugs injected?

Unmodified peptides are broken down by protease enzymes in the gut and cross the intestinal lining poorly, so very little of a swallowed peptide reaches the bloodstream intact. Most therapeutic peptides are therefore given by injection under the skin, into a muscle or into a vein, and a few by nasal spray. Oral semaglutide (Rybelsus) is a notable exception that relies on a special absorption enhancer.

How many peptide drugs are approved?

More than 80 peptide drugs have reached the market worldwide, according to a 2021 review in Nature Reviews Drug Discovery, and the number continues to grow. They include insulin and its analogues, GLP-1 receptor agonists such as semaglutide and liraglutide, oxytocin, and medicines for cancer, osteoporosis, multiple sclerosis and HIV infection.

Are peptides the same as steroids?

No. Steroids are lipid molecules built on a four-ring carbon skeleton, while peptides are chains of amino acids. They act through different receptors and are handled differently by the body. Some peptides, such as growth hormone secretagogues, are banned in sport alongside anabolic steroids, which may be why the two are often mentioned together.