What Are Peptides? The Complete Research Guide

Peptides work as signalling molecules. They bind to specific cell surface receptors — G protein-coupled receptors (GPCRs), receptor tyrosine kinases, or nuclear receptors — and trigger intracellular signalling cascades that change gene expression, protein synthesis, or cellular behaviour.

The key concept is receptor specificity. Each peptide has a three-dimensional shape that fits its target receptor the way a key fits a lock. A peptide that binds GHS-R1a (the ghrelin receptor) triggers GH release from pituitary somatotrophs. The same peptide has no direct effect on adipocytes, liver cells, or neurons that don't express that receptor. This specificity is why peptides can have targeted effects with relatively limited off-target interactions compared to small-molecule drugs that may interact with dozens of different enzymes and transporters.

Most research peptides are administered via subcutaneous (SC) injection because peptides are broken down by digestive enzymes in the gastrointestinal tract before reaching systemic circulation. Oral bioavailability for standard peptides is typically less than 1–5% without special formulation strategies. SC injection delivers the compound to the subcutaneous fat layer, where it is slowly absorbed into the bloodstream via the capillary network, typically achieving peak plasma levels in 15–60 minutes depending on the peptide's molecular weight and lipophilicity.

Nasal spray formulations work for peptides that can cross the nasal epithelium. Semax and Selank — both Russian neuropeptides — are commonly available and studied in intranasal form because their small size and amphiphilic character allow absorption through the nasal mucosa directly into the bloodstream (and possibly via olfactory pathways into the CNS). Nasal bioavailability for these compounds is estimated at 80–90% in animal models.

Oral formulations for peptides are an active area of pharmaceutical development. Semaglutide — the GLP-1 receptor agonist — became the first peptide drug with proven oral bioavailability after Novo Nordisk developed a co-formulation with the absorption enhancer SNAC (sodium N-[8-(2-hydroxybenzoyl)amino]caprylate). The oral form (Rybelsus) achieves approximately 1% bioavailability — sufficient for a therapeutically active dose because the total dose is scaled accordingly. Research-grade peptides are not typically sold in oral formulations with equivalent enhancers.

The safety profile of peptides is generally favourable compared to many traditional pharmaceutical compounds, for several structural reasons. First, peptides are metabolised by ubiquitous peptidases throughout the body — not just by hepatic enzymes — reducing the likelihood of liver toxicity that can occur with small-molecule drugs. Second, because peptides mimic endogenous signalling molecules, they typically work within physiologic receptor systems that evolved to handle them, rather than forcing entirely novel biochemical pathways.

That said, no compound is without risk. The risks with research peptides fall into several categories: dose-dependent on-target effects (e.g., too much GH from secretagogues causing carpal tunnel or insulin resistance), formulation risks (contamination or incorrect reconstitution), and unknown long-term effects from chronic use of compounds with limited longitudinal human data. The most significant practical risk for most research users is not the peptide itself but poor sourcing — bacterial contamination, misdosing due to concentration errors, or receiving a different compound than labelled.

Best practices for research safety include: sourcing from reputable vendors who provide certificate of analysis (CoA) from third-party testing, using sterile bacteriostatic water for reconstitution, using appropriate needle gauges and sterile technique for injection, starting with conservative doses and titrating up, and monitoring relevant biomarkers via bloodwork (IGF-1 for GH peptides, CBC/CMP as baseline, fasting glucose for metabolic peptides).