What Are Peptides? A Science-Based Introduction

Peptides are short chains of amino acids — typically between 2 and 50 — linked together by peptide bonds. They are the building blocks of proteins but are distinguished from proteins by their shorter length. While the boundary between peptides and proteins is not rigidly defined, molecules with fewer than 50 amino acids are generally classified as peptides.

The human body produces hundreds of endogenous peptides that regulate critical biological functions. Insulin, a 51-amino-acid peptide, controls blood sugar. Oxytocin, a 9-amino-acid peptide, plays roles in social bonding and childbirth. Endorphins modulate pain perception. Growth hormone-releasing hormones signal the pituitary gland. These natural peptides act as hormones, neurotransmitters, growth factors, and antimicrobial agents.

What distinguishes peptides from other molecules is their specificity. Each peptide has a unique amino acid sequence that determines its three-dimensional shape, which in turn determines which receptors it can bind to and what biological effects it triggers. This specificity makes peptides attractive to researchers — they can potentially target specific pathways with fewer off-target effects than traditional small-molecule drugs.

Peptides exert their effects by binding to specific receptors on cell surfaces or, in some cases, by entering cells directly. When a peptide binds to its receptor, it triggers a cascade of intracellular signals — like turning a key in a lock that activates machinery inside the cell.

Receptor binding and signal transduction. Most therapeutic and research peptides work by mimicking or modulating endogenous peptide signaling. For example, semaglutide (Ozempic/Wegovy) mimics the natural GLP-1 peptide hormone, binding to GLP-1 receptors in the pancreas and brain to regulate insulin secretion and appetite. Bremelanotide (PT-141) activates melanocortin receptors involved in sexual arousal pathways.

Bioavailability challenges. A key limitation of peptides is that they are rapidly broken down by enzymes in the digestive system, which is why most peptide drugs are administered by injection rather than taken orally. The stomach and intestines contain proteases and peptidases that cleave peptide bonds, destroying the molecule before it can reach the bloodstream. Some researchers have developed modified peptides with improved oral bioavailability — oral semaglutide (Rybelsus) uses a permeation enhancer to protect the peptide through the GI tract.

Half-life and duration of action. Natural peptides are typically degraded within minutes in the bloodstream. Synthetic modifications can extend half-life dramatically — semaglutide has a half-life of approximately one week due to fatty acid modifications that allow it to bind to albumin, while natural GLP-1 has a half-life of just 2-3 minutes.

The peptide landscape spans a broad spectrum from FDA-approved drugs with extensive clinical evidence to research compounds studied only in animal models. Understanding this spectrum is essential for evaluating any peptide claim.

FDA-approved peptide drugs. Several peptides have completed the full regulatory process and are available as prescription medications. These include semaglutide (type 2 diabetes, weight management), tirzepatide (type 2 diabetes, weight management), bremelanotide/PT-141 (hypoactive sexual desire disorder), tesamorelin (HIV-associated lipodystrophy), and others. These compounds have robust clinical trial evidence from thousands of participants.

Peptides in clinical trials. Some peptides are actively being studied in human clinical trials but are not yet approved. Retatrutide, a triple agonist targeting GLP-1, GIP, and glucagon receptors, is in Phase 3 trials for obesity and has shown promising weight loss results.

Research peptides (preclinical). Many peptides that generate public interest — BPC-157, TB-500, CJC-1295, Ipamorelin, Epithalon — have only been studied in animal models or cell cultures. While some show promising preclinical results, the gap between animal evidence and proven human benefit is substantial. Many compounds that work in rodent models fail to translate to humans.

Peptide-adjacent compounds. Some compounds commonly discussed alongside peptides are not technically peptides. MK-677 (Ibutamoren) is a non-peptide growth hormone secretagogue — a small molecule that mimics peptide signaling. NAD+ is a coenzyme, not a peptide. These are sometimes grouped with peptides because of overlapping research communities and use cases.

Research peptides are generally organized by their primary biological activity:

Healing and recovery peptides. BPC-157, TB-500 (Thymosin Beta-4), and GHK-Cu are the most-discussed compounds in this category. Research in animal models has explored their effects on tendon, muscle, bone, and skin repair. BPC-157 has been studied in over 100 animal studies for tissue healing. TB-500 research focuses on actin regulation and cell migration. GHK-Cu has been studied for wound healing and skin regeneration. None are FDA-approved for human use.

Weight loss peptides (GLP-1 receptor agonists). This is the category with the strongest clinical evidence. Semaglutide and tirzepatide are FDA-approved, widely prescribed, and backed by large randomized controlled trials. AOD-9604, a fragment of growth hormone studied for fat reduction, has much weaker evidence.

Growth hormone secretagogues. CJC-1295, Ipamorelin, Sermorelin, Tesamorelin, and MK-677 all stimulate growth hormone release through various mechanisms. Sermorelin and Tesamorelin are FDA-approved for specific indications. Others are research-only.

Nootropic and cognitive peptides. Selank and Semax, developed in Russia, have been studied for anxiolytic and cognitive-enhancing properties. Both have regulatory approval in Russia but not in the US or EU. Published research is mixed, with some studies in small human cohorts.

Sexual health peptides. PT-141 (Bremelanotide), sold as Vyleesi, is FDA-approved for hypoactive sexual desire disorder in premenopausal women. Melanotan II, studied for tanning and sexual function, is not approved and carries significant safety concerns.

Longevity and anti-aging peptides. Epithalon (Epitalon), MOTS-c, and NAD+ are studied in the context of aging biology. Evidence ranges from preliminary animal studies (Epithalon) to emerging human research (NAD+).

Not all peptide evidence is created equal. At PeptideMark, we use a four-tier evidence rating system:

Strong evidence means multiple human randomized controlled trials (RCTs) with consistent results. Semaglutide and tirzepatide fall in this category — their efficacy and safety have been demonstrated in trials involving thousands of participants across multiple independent studies.

Moderate evidence means limited human data (pilot studies, small trials, or case reports) supported by strong animal evidence. Compounds like Selank fall here — some human studies exist, but they are small or lack rigorous controls.

Preliminary evidence means the compound has been studied primarily in animal models, with no meaningful human clinical data. BPC-157, TB-500, and many other popular research peptides fall in this category. Even when preclinical results are extensive and consistent, the jump from animal models to proven human benefit is uncertain.

Insufficient evidence means minimal published research of any type. Some newer or less-studied compounds fall here.

The critical takeaway: a compound having "over 100 studies" does not mean it works in humans. The type and quality of evidence matters far more than the quantity. A single large, well-designed human RCT provides stronger evidence than dozens of rat studies.

Several converging trends have pushed peptides into the mainstream:

The GLP-1 revolution. The clinical success of semaglutide (Ozempic/Wegovy) and tirzepatide (Mounjaro/Zepbound) for weight management has brought the word "peptide" into everyday vocabulary. These drugs have demonstrated effects that were previously thought impossible — sustained 15-25% body weight reduction — and have become among the most prescribed medications globally.

Growing interest in research peptides. The success of approved peptide drugs has driven curiosity about unapproved research peptides. Search volume for compounds like BPC-157, TB-500, and CJC-1295 has grown substantially since 2024, fueled by discussion on podcasts, social media, and health forums.

Regulatory controversy. The FDA's 2023 decision to ban compounding of several research peptides (including BPC-157 and TB-500) created significant controversy. Physicians, compounding pharmacies, and patients have pushed back, arguing these compounds filled legitimate medical needs. The debate continues through legal challenges and policy discussions.

The information gap. Despite massive public interest, reliable, comprehensive peptide information is surprisingly scarce. Most available content is either vendor-driven (biased toward selling), dismissive (ignoring legitimate research), or overly technical (inaccessible to consumers). This gap between demand for information and supply of trustworthy, balanced content is exactly what PeptideMark exists to address.