How Do Peptides Work?

If peptides feel like something new, that is a marketing artefact. The first peptide drug was given to a patient on 11 January 1922. It was insulin, and it predates antibiotics.⁹

What follows is what a peptide actually is, how many of them there are, what your own body does with them, and — because it matters — the difference between the three very different categories of thing that get sold under the word.

What A Peptide Is

A peptide is a short chain of amino acids joined by peptide bonds. The bond forms by condensation: the carboxyl group of one amino acid reacts with the amino group of the next, releasing a molecule of water and leaving a covalent amide bond.¹

Because that bond has partial double-bond character, it is rigid and planar — which is a large part of why chains of amino acids hold stable shapes rather than flopping about. Each amino acid in the chain, once the water is gone, is called a residue. Chains have direction: the free amino end is the N-terminus, the free carboxyl end the C-terminus, and sequences are written N to C.¹

Human proteins and peptides are built from 20 standard amino acids. Nine of them are essential for adults — histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine — meaning your body cannot make them and you have to eat them.²

How Many Peptides Are There?

This question gets answered with confident round numbers on a lot of websites. Those numbers are not traceable to anything. The honest answer is that it depends entirely on what you are counting — so here is what the actual curated databases hold.

For peptide drugs specifically: a 2021 review in Nature Reviews Drug Discovery counted more than 80 peptide drugs on the market across diabetes, cancer, osteoporosis, multiple sclerosis, HIV and chronic pain, with hundreds more in clinical development.³ Dozens more have been approved since. Counts published elsewhere range from roughly 60 to over 100 depending on the cutoff year and whether protein therapeutics are folded in.

And How Many Are Possible?

This is where the scale becomes obvious. With 20 amino acids, the number of distinct sequences of length n is 20ⁿ. A five-amino-acid peptide has 3,200,000 possible sequences. A ten-amino-acid peptide has over 10 trillion. A modest 46-residue peptide has more possible sequences than there are atoms in the solar system.

Nature has explored a vanishing fraction of that space. So has medicine.

The Peptides You Already Run On

Peptides are not an intervention layered on top of normal physiology. They are how a great deal of normal physiology is conducted. A partial list of the ones working in you right now:

Semaglutide is 94% homologous to native GLP-1. That is the whole idea of the drug class: take a signal the body already uses, and make it last longer than the minutes the natural version survives.

Why Peptides Make Good Medicines — And Where They Struggle

The Nature Reviews Drug Discovery assessment is worth quoting in substance.³

The advantages. Peptides mimic the body's own signalling molecules, so they bind their targets with high specificity and potency. They generally show low off-target toxicity and low tissue accumulation, because the body breaks them down into amino acids rather than storing foreign metabolites.

The limitations. Peptides are digested. Gut proteases destroy them and the intestinal wall absorbs them poorly, which is why most peptide drugs are injected rather than swallowed — oral semaglutide exists, but only because of a specialised absorption-enhancing co-formulation. Native peptides are also cleared fast: somatostatin's natural half-life is one to three minutes.¹⁰ Getting from that to a once-weekly injection is chemistry — fatty-acid conjugation to bind albumin, non-natural amino acids to resist proteases, cyclisation to hold shape.

Three Different Things Are Sold As “Peptides”

This is the part that matters most for deciding what to trust, and it is the part most of the internet blurs.

1. FDA-Approved Peptide Drugs

Insulin, glucagon, octreotide, leuprolide, semaglutide, tirzepatide. Reviewed by FDA for safety, effectiveness and manufacturing quality. Prescription-only. Made under Good Manufacturing Practice.

2. Compounded Peptides

Prepared by a licensed compounding pharmacy for an individual patient against a valid prescription, under Section 503A of the Federal Food, Drug, and Cosmetic Act. These do not go through FDA premarket review. FDA states plainly: “Compounded drugs are not FDA-approved. This means that FDA does not verify the safety, effectiveness, or quality of compounded drugs before they are marketed.”¹² FDA's position is that compounded drugs should be used by patients whose needs cannot be met by an FDA-approved drug.

3. Gray-Market “Research Use Only” Peptides

Sold online, labelled “for research purposes” or “not for human consumption.” That label is a workaround, not an approval. FDA has warned companies illegally selling unapproved drugs containing semaglutide, tirzepatide or retatrutide under exactly that framing, and describes such products as “of unknown quality and may be harmful.”¹¹

So Should You Feel Comfortable?

Comfortable with the chemistry: yes, and the reason is not reassurance, it is a century of clinical use. Peptide therapeutics are one of the oldest classes of biologic medicine still in continuous use. The molecules are made of the same twenty amino acids as everything else in your body, and your body disposes of them the same way.

Comfortable with any specific product: that depends entirely on which of the three categories above it falls into, who prescribed it, which pharmacy made it, and whether anyone with a licence looked at your medical history first. Those are answerable questions. Ask them.

At Rūl, every therapy is prescribed by a licensed U.S. physician after an assessment and dispensed by a 503A-licensed U.S. compounding pharmacy. We do not sell research-use-only products. And we will tell you when a therapy you are considering is not FDA-approved, because it usually is not, and you should know that before you decide.

References

1. Biochemistry, Peptide — StatPearls, NCBI Bookshelf (NIH). https://www.ncbi.nlm.nih.gov/books/NBK562260/
2. Biochemistry, Essential Amino Acids — StatPearls, NCBI Bookshelf (NIH). https://www.ncbi.nlm.nih.gov/books/NBK557845/
3. Muttenthaler M, King GF, Adams DJ, Alewood PF. Trends in peptide drug discovery. Nature Reviews Drug Discovery. 2021;20:309–325. https://www.nature.com/articles/s41573-020-00135-8
4. Usmani SS, et al. THPdb: Database of FDA-approved peptide and protein therapeutics. PLOS ONE. 2017;12(7):e0181748. https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0181748
5. UniProtKB/Swiss-Prot release statistics — ExPASy. https://web.expasy.org/docs/relnotes/relstat.html
6. Antimicrobial Peptide Database (APD) — University of Nebraska Medical Center. https://aps.unmc.edu/faq
7. Kang X, et al. DRAMP 2.0, an updated data repository of antimicrobial peptides. Scientific Data. 2019;6:148. https://www.nature.com/articles/s41597-019-0154-y
8. Insulin Biosynthesis, Secretion, Structure — Endotext, NCBI Bookshelf (NIH). https://www.ncbi.nlm.nih.gov/books/NBK279029/
9. The Nobel Prize. The “miracle” discovery that reversed the diabetes death sentence. https://www.nobelprize.org/the-miracle-discovery-that-reversed-the-diabetes-death-sentence/
10. Physiology, Somatostatin — StatPearls, NCBI Bookshelf (NIH). https://www.ncbi.nlm.nih.gov/books/NBK538327/
11. U.S. Food and Drug Administration. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. https://www.fda.gov/drugs/drug-alerts-and-statements/fdas-concerns-unapproved-glp-1-drugs-used-weight-loss
12. U.S. Food and Drug Administration. Compounding and the FDA: Questions and Answers. https://www.fda.gov/drugs/human-drug-compounding/compounding-and-fda-questions-and-answers
13. U.S. Food and Drug Administration. Bulk Drug Substances Used in Compounding Under Section 503A. https://www.fda.gov/drugs/human-drug-compounding/bulk-drug-substances-used-compounding-under-section-503a-fdc-act

Written by the Rūl editorial team. Every clinical claim on this page is sourced to the primary literature and listed in the references below. This article is educational and is not medical advice.