Peptide Therapeutics: A Primer for Medical Students

For a medical student, peptides can feel like a footnote wedged between proteins and small-molecule drugs. In practice they are a large and growing part of the therapeutic landscape, and understanding them early pays off across endocrinology, oncology, and beyond. This primer offers a concise orientation: what peptides are, why they make useful drugs, where their limitations lie, and how to think critically about the many peptide compounds now circulating outside approved medicine.

What a peptide is

A peptide is a chain of amino acids linked by peptide bonds. The distinction between a peptide and a small protein is conventional rather than absolute, with proposed boundaries varying by source. Mature human insulin, for example, contains 51 amino acids and is commonly described as a peptide hormone. A peptide’s amino-acid sequence and three-dimensional structure influence its biological activity and receptor interactions. Peptides function throughout the body as hormones, neurotransmitters and signalling molecules; familiar examples include insulin, oxytocin and glucagon.

A large and established field

Peptide therapeutics are not new. Insulin, first used in patients in the 1920s, was among the earliest, and the field has expanded steadily since. A widely cited review in Nature Reviews Drug Discovery noted that more than 80 peptide drugs had reached the market worldwide, with many more in clinical development (Muttenthaler and colleagues, 2021). Familiar examples span specialties: GLP-1 receptor agonists such as semaglutide in metabolic medicine, gonadotropin-releasing hormone analogs such as leuprolide in oncology, the somatostatin analog octreotide in endocrinology, and oxytocin in obstetrics.

Why peptides make useful drugs

Peptides occupy a useful middle ground between many small molecules and large biologics such as antibodies. Individual peptide drugs may offer high receptor affinity, potency and target selectivity, and their structures can often be modified systematically. These are potential advantages, not guaranteed class properties. Peptides can still activate multiple receptors, produce off-target or serious on-target effects, cause immune reactions and acquire prolonged exposure through half-life extension. Their eventual breakdown into amino acids does not by itself establish low toxicity.

The limitations are just as instructive

Peptides also have characteristic weaknesses that shape drug design and delivery. Oral delivery is difficult because digestive enzymes can degrade peptides and intact peptides generally cross the intestinal lining poorly. Many are therefore injected. Some also have short circulating half-lives because of enzymatic degradation and kidney clearance. Strategies to address these problems include resistance to proteolysis, modifications that promote reversible albumin binding, conjugation to larger molecules, depot formulations and specialised oral formulations that protect the peptide or enhance absorption. Oral semaglutide, for example, uses an absorption-enhancing excipient; tablet stability alone does not solve the delivery problem.

The research-grade category, and why it matters

Here is a distinction that matters for anyone entering clinical practice. Alongside approved peptide medicines, a large online market sells compounds labelled for research purposes only. Such products are not FDA-approved drugs, and their evidence and manufacturing quality vary enormously. BPC-157, promoted for tissue repair, has encouraging animal findings, but rigorous human musculoskeletal trials are lacking and it is not FDA-approved (Vasireddi and colleagues, 2025). FDA has also identified potential immunogenicity, impurity and characterisation concerns. GHK-Cu is used in topical cosmetic products and has laboratory and limited clinical research concerning skin biology and wound repair, but that should not be confused with strong proof of clinical anti-ageing effects. Injectable GHK-Cu is not an approved treatment, and FDA identifies limited human safety data and potential immunogenicity concerns.

The skill of appraisal

The lesson for a student is one of evidence hierarchy. A plausible mechanism and positive animal data are a starting point, not proof of clinical benefit or safety in humans. Approved peptide drugs have undergone regulatory review based on evidence appropriate to their indication, including clinical safety, effectiveness and manufacturing-quality data. Research-only compounds have not passed that process. Some are prohibited in competitive sport or have been flagged by regulators over safety and product-quality concerns. Explaining that distinction clearly to patients is a valuable clinical skill.

Where the field is heading

The trajectory is worth appreciating. Advances in synthesis, formulation, and half-life extension keep widening what peptides can do, and areas such as metabolic disease have seen rapid recent progress. For the current generation of students, peptides are likely to feature more prominently in practice over a career, not less, both as legitimate medicines and as compounds patients will ask about.

A working grasp of peptide therapeutics, covering both approved medicines and far less certain research compounds, is therefore a sound investment. Students should begin with standard pharmacology texts, current regulatory documents and primary literature. Supplier reference material, including content from Doctor Peptide, can be used to locate publications but should not replace independent sources or be treated as clinical guidance.

References

Muttenthaler M, King GF, Adams DJ, Alewood PF. Trends in peptide drug discovery. Nature Reviews Drug Discovery, 2021.

Vasireddi N, Hahamyan H, Salata M, et al. Emerging Use of BPC-157 in Orthopaedic Sports Medicine: A Systematic Review. Sports Health, 2025.

Pickart L, Margolina A. Regenerative and Protective Actions of the GHK-Cu Peptide in the Light of the New Gene Data. International Journal of Molecular Sciences, 2018.