Peptide Research Glossary: Key Terms, Definitions & Quick Reference
Peptide research terminology quick reference: clear definitions for half-life, reconstitution, bioavailability, lyophilization, GHRP, agonist, BAC water, and 30+ more terms.
TL;DR — Key Takeaways
- 30+ peptide research terms defined in plain language — pharmacology, chemistry, administration, and calculator concepts.
- Half-life determines dosing frequency; bioavailability determines route selection; receptor affinity determines dose range.
- BAC water, lyophilization, and reconstitution are the three core prep concepts every researcher needs to understand before handling any peptide.
- Use the Peptide Performance Calculator alongside this glossary to apply definitions directly to your vial and dose math.
- Bookmark this page as a reference — definitions link out to compound-specific guides throughout the database.
Research Disclaimer: All compounds referenced in this glossary are research compounds not approved by the FDA for human use unless otherwise stated. All information is for educational purposes only and does not constitute medical advice.
A quick-reference guide to peptide research terminology across pharmacology, chemistry, administration, compound types, research methodology, and calculator concepts. Definitions are kept to two sentences for fast lookup. For compound-specific detail, see the peptide compound database.
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Pharmacology Terms
| Term | Definition | See Also |
|---|---|---|
| Half-Life (t½/₂) | Time required for plasma concentration of a compound to decrease by 50% after administration. Determines dosing frequency: short half-life compounds (GHRPs at ~30 min) need multiple daily doses; long half-life compounds (CJC-1295 DAC at ~8 days) sustain levels from once-weekly dosing. | Half-Life Calculator |
| Bioavailability | The fraction of an administered dose that reaches systemic circulation in active form. IV administration is 100% bioavailable; subcutaneous bioavailability for most peptides is 70–90%; oral bioavailability is very low because digestive enzymes cleave peptide bonds before absorption. | — |
| Pharmacokinetics (PK) | What the body does to a drug: absorption, distribution, metabolism, and elimination (ADME). The most practically relevant PK parameters for peptide researchers are half-life, time to peak concentration (Tmax), and area under the curve (AUC). | Half-Life Calculator |
| Pharmacodynamics (PD) | What a drug does to the body — the relationship between drug concentration and biological effect. In peptide research, PD outcomes include GH pulse amplitude in response to a GHRP, T-cell count changes following Thymosin Alpha-1, or wound closure rate in BPC-157 tissue repair models. | — |
| Receptor Agonist | A compound that binds to a receptor and activates it, mimicking the effect of the receptor's endogenous ligand. Most research peptides are agonists: Ipamorelin (GHS-R1a), CJC-1295 (GHRH receptor), Semaglutide (GLP-1 receptor). Agonist activity is typically quantified by EC50 — concentration producing half-maximal activation. | — |
| Receptor Antagonist | Binds to a receptor and blocks it without activating it, preventing the endogenous ligand or agonist from producing its effect. While most research peptides are agonists, antagonists are used in research as pathway controls. Pegvisomant is a GH receptor antagonist used experimentally to block GH/IGF-1 signaling. | — |
| Selectivity | A compound's preference for one specific receptor or receptor subtype over others. High selectivity is desirable in research because it allows effects to be attributed to a specific pathway. Ipamorelin is more selective than GHRP-6 — it produces GH release with minimal off-target stimulation of cortisol and prolactin. | — |
| Desensitization | The decrease in receptor responsiveness with repeated or continuous agonist exposure, involving receptor internalization or transcriptional downregulation. For GHRPs, desensitization is most relevant for Hexarelin, which desensitizes fastest among major GHRPs — protocols cycle 4–6 weeks on, 4 weeks off. | Hexarelin Guide |
| Saturation Dose | The dose above which increasing the amount produces no further increase in effect because all available receptors are occupied. For most GHRPs, receptor saturation for acute GH release occurs around 100–300 mcg per injection depending on compound. Doses above saturation add exposure without additional benefit. | — |
Peptide Chemistry Terms
| Term | Definition | See Also |
|---|---|---|
| Amino Acid | The molecular building blocks of peptides and proteins, each consisting of an amine group, carboxyl group, and a variable side chain (R group). The amino acid sequence — primary structure — determines three-dimensional conformation, receptor binding properties, and biological activity. Synthetic analogs often incorporate D-configuration amino acids for improved stability. | — |
| Peptide Bond | The covalent bond formed between the carboxyl group of one amino acid and the amine group of the next, releasing water (condensation reaction). Peptide bonds are susceptible to enzymatic cleavage by proteases, which is why most peptides have short half-lives; D-amino acid substitutions improve metabolic stability. | — |
| Lyophilization | The process of removing water from a solution by freezing it then applying vacuum to convert ice directly to vapor (sublimation). The resulting dry, stable powder can be stored without significant degradation; most research peptides are supplied lyophilized and must be reconstituted before use. | Reconstitution Calculator |
| Reconstitution | The process of dissolving a lyophilized peptide powder in a liquid solvent to prepare it for injection. The most common solvents are bacteriostatic water (BAC water) for most peptides and dilute acetic acid for IGF-1 variants; the volume of solvent added determines the working concentration. | Reconstitution Calculator |
| Bacteriostatic Water (BAC Water) | Sterile water containing 0.9% benzyl alcohol as a preservative that inhibits bacterial and fungal growth. It allows the reconstituted vial to be accessed multiple times over 28–30 days without contamination risk, making it the standard reconstitution solvent for most multi-dose research peptide vials. | Reconstitution Calculator |
| Sterile Water for Injection | Water that has been purified and sterilized but contains no additives or preservatives. It is appropriate for single-use reconstitution when the entire reconstituted volume will be used in one session; solutions in sterile water have no preservative and must be used within hours. | — |
| Acetic Acid (for Reconstitution) | Dilute acetic acid (typically 0.6% solution) used as the primary reconstitution solvent for IGF-1 variants (IGF-1 DES, IGF-1 LR3) that are unstable at neutral pH. The acetic acid solution is typically diluted with sterile saline to raise pH to a more physiologically compatible level before injection. | — |
| Peptide Purity | The percentage of the supplied material that is the intended peptide compound, determined by HPLC and mass spectrometry. Research-grade peptides are typically listed at ≥98% purity; lower purity means doses calculated from labeled weight are less accurate. | — |
| Molecular Weight (Daltons) | The total mass of a peptide molecule in Daltons (Da) or kiloDaltons (kDa), relevant to receptor access, IGFBP binding, and renal filtration rate. For reference: BPC-157 is ~1,419 Da; IGF-1 LR3 is ~9,111 Da; native hGH is ~22,000 Da. | — |
Administration Terms
| Term | Definition | See Also |
|---|---|---|
| Subcutaneous Injection (SubQ) | Deposits compound into the adipose tissue layer beneath the dermis, above the muscle fascia. The hypodermis is well-vascularized for reliable absorption; SubQ injection is the standard route for the vast majority of research peptides, using a 28–31G insulin syringe at 45–90 degree angle. | Peptide Injection Guide |
| Intramuscular Injection (IM) | Deposits compound directly into muscle tissue, which is more vascular than subcutaneous fat and produces faster absorption. IM is used for compounds like IGF-1 DES where local tissue targeting is desired; needle length (5/8"–1") and angle (90°) differ from SubQ. | — |
| Intranasal Administration | Applying a peptide solution to the nasal mucosa using a nasal spray or dropper. A subset of peptides — including Semax and Selank — are administered intranasally because the olfactory route enables CNS delivery that bypasses systemic circulation. | Nootropics Section |
| Bolus Injection | Administration of the entire dose as a single delivery, creating a concentration peak followed by exponential decay. All subcutaneous and intramuscular peptide injections are bolus administrations; the decay curve makes half-life the primary parameter for determining dosing frequency. | Half-Life Calculator |
| Insulin Syringe (U-100) | A syringe calibrated for insulin at 100 units per milliliter, standard for subcutaneous peptide injection. Fine-gauge needles (28–31G), small volume range (0.3–1.0 mL), and unit-based scale make them the practical tool of choice; on a U-100 syringe, 1 unit = 0.01 mL. | Dosage Calculator |
| Transdermal Administration | Delivery of a compound through the skin via passive diffusion. Most peptides are too large and hydrophilic for effective transdermal delivery; however, certain small, lipophilic peptide analogs (including some melanocortin compounds) have demonstrated transdermal activity. | — |
Compound Type Terms
| Term | Definition | See Also |
|---|---|---|
| GHRH Analog | Synthetic peptides that mimic endogenous GHRH, stimulating the GHRH receptor on pituitary somatotrophs to promote GH synthesis and release. CJC-1295, Sermorelin, and Tesamorelin are the principal GHRH analogs; they act synergistically with GHRPs by increasing GH pulse amplitude. | CJC-1295 + Ipamorelin Guide |
| GHRP (Growth Hormone-Releasing Peptide) | Synthetic peptides that stimulate GH secretion via the GHS-R1a (ghrelin) receptor. GHRPs trigger release and suppress somatostatin tone, complementing GHRH analogs; major GHRPs include Ipamorelin, GHRP-2, GHRP-6, and Hexarelin — each differing in potency, side effects, and desensitization rate. | GHRP-2 vs GHRP-6 Guide |
| GLP-1 Agonist | Peptide compounds that activate the GLP-1 receptor, increasing glucose-dependent insulin secretion, suppressing glucagon, delaying gastric emptying, and producing satiety signaling. Semaglutide, Liraglutide, and Tirzepatide are the major GLP-1 agonists in current clinical and research use. | GLP-1 Comparison Guide |
| IGF (Insulin-Like Growth Factor) | A family of peptide hormones structurally similar to proinsulin; IGF-1 is the primary mediator of GH's anabolic effects, produced in the liver in response to GH signaling. Research variants include IGF-1 LR3 (systemic, long-acting) and IGF-1 DES (local, short-acting, highly potent). | IGF-1 DES vs LR3 Guide |
| Melanocortin Peptide | Compounds that activate melanocortin receptors (MC1R–MC5R) with functions including skin pigmentation (MC1R), energy homeostasis (MC3R/MC4R), and sexual function (MC4R). Melanotan II and PT-141 (Bremelanotide/Vyleesi) are the principal melanocortin research peptides. | PT-141 Guide |
| Thymic Peptide | Compounds derived from or based on thymus gland secretions responsible for T-lymphocyte maturation. Thymosin Alpha-1 is the most clinically advanced thymic peptide with approval in 35+ countries; Thymosin Beta-4 (TB-500 is its synthetic analog) and Thymulin are related compounds. | — |
| Nootropic Peptide | Research compounds studied for effects on cognitive function, neuroprotection, or brain plasticity. Includes ACTH-derived peptides (Semax), anxiolytic peptides (Selank), and diverse compounds with CNS activity. | Nootropics Section |
Research Methodology Terms
| Term | Definition | See Also |
|---|---|---|
| Phase I / II / III Clinical Trial | Structured human research studies in ascending phases. Phase I assesses safety in healthy volunteers; Phase II evaluates preliminary efficacy and dosing; Phase III tests large randomized controlled trials for regulatory approval. Thymosin Alpha-1 has completed Phase III trials; most research peptides remain at preclinical or Phase I stage. | — |
| Randomized Controlled Trial (RCT) | A study design in which participants are randomly assigned to experimental treatment or control condition to minimize selection bias. RCTs provide the strongest evidence for efficacy, and their availability for a peptide compound is a strong signal of evidence quality. | — |
| In Vitro | Research conducted outside a living organism — in cell cultures, tissue samples, or isolated biochemical systems. In vitro studies allow precise experimental control but must be interpreted cautiously as they do not replicate living biological complexity; most initial peptide mechanistic research begins in vitro. | — |
| In Vivo | Research conducted in living organisms, capturing full pharmacokinetic and pharmacodynamic complexity including metabolism, immune responses, and multi-organ interactions. More expensive and ethically constrained than in vitro but more translationally relevant. | — |
| Research Compound | A substance synthesized or identified for scientific study but without regulatory approval for clinical human use. Research compounds may have varying evidence levels — from early in vitro data to completed Phase III trials — but the designation indicates therapeutic use falls outside approved indications. | — |
| FDA Approval Status | Indicates whether a compound has received regulatory clearance for clinical use in the United States for a specific indication. FDA-approved peptide drugs include Semaglutide (Ozempic/Wegovy), Tirzepatide (Mounjaro/Zepbound), and Tesamorelin (Egrifta). Approval requires controlled clinical trials demonstrating safety and efficacy. | — |
| Off-Label Use | Use of an FDA-approved drug for a purpose, population, or dosage not specified in its approved labeling. Physicians may legally prescribe approved drugs off-label; research compounds that are not FDA-approved are categorically distinct from off-label use. | — |
Calculator Terms
| Term | Definition | See Also |
|---|---|---|
| Reconstitution Concentration (mcg/mL) | The mass of peptide per unit volume after dissolving a lyophilized peptide in solvent, calculated by dividing total peptide mass (mcg) by solvent volume (mL). For example, a 5 mg (5,000 mcg) vial dissolved in 2 mL BAC water yields 2,500 mcg/mL. Concentration is the essential input for calculating syringe volume for a target dose. | Reconstitution Calculator |
| Syringe Units | The markings on a U-100 insulin syringe where each unit = 0.01 mL. To calculate units: divide dose (mcg) by concentration (mcg/mL) to get mL, then multiply by 100. Example: 200 mcg ÷ 2,500 mcg/mL = 0.08 mL × 100 = 8 units. Syringe unit calculation is automated by the dosage calculator. | Dosage Calculator |
Frequently Asked Questions
Q: What does lyophilized mean for peptides? A: Lyophilized means freeze-dried. The peptide solution is frozen and placed under vacuum, causing ice to sublimate and leave a dry, stable powder. Lyophilized peptides are significantly more stable than liquid solutions, allowing refrigerated or frozen storage for months to years. Before use, they must be reconstituted — dissolved in an appropriate solvent — to create a solution suitable for injection.
Q: What is BAC water and why is it used? A: BAC water (bacteriostatic water) is sterile water containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol prevents bacterial and fungal contamination after the rubber septum has been punctured, allowing safe refrigerator storage and multiple-use access over 28–30 days. It is preferred over plain sterile water for all multi-dose research vials.
Q: What's the difference between mcg and mg in peptide dosing? A: 1 mg = 1,000 mcg. Most GH axis peptides (GHRPs, GHRH analogs) are dosed in micrograms per injection (100–500 mcg). Some peptides are expressed in milligrams (Thymosin Alpha-1 at 1.6 mg, TB-500 at 2–5 mg). Always confirm the unit before calculating syringe volumes — a factor-of-1,000 error is the most consequential mistake in peptide dosing.
Q: How do I calculate peptide concentration after reconstitution? A: Concentration (mcg/mL) = total peptide mass in vial (mcg) ÷ volume of solvent added (mL). For a 5 mg vial, convert first: 5 mg × 1,000 = 5,000 mcg. If you add 2 mL BAC water: 5,000 ÷ 2 = 2,500 mcg/mL. The peptide reconstitution calculator performs this automatically and outputs the exact syringe units for your target dose.
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All content is for educational and research purposes only. Not medical advice.
Disclaimer: For educational and research purposes only. Nothing in this article constitutes medical advice, diagnosis, or treatment recommendation. All compounds discussed are research chemicals or investigational compounds unless explicitly noted otherwise. Consult a qualified healthcare professional before making any health-related decisions. Researchers must comply with all applicable laws and regulations in their jurisdiction.
Written by the Peptide Performance Calculator Research Team
Our team compiles research guides based on published literature for educational purposes. All content is for research use only — not medical advice. Read our disclaimer.
Frequently Asked Questions
What is lyophilization?
Lyophilization is freeze-drying: water is removed from a peptide solution under vacuum at low temperature, producing a stable powder. Lyophilized peptides have much longer shelf lives than reconstituted solutions.
What is the difference between a peptide agonist and antagonist?
An agonist activates a receptor (e.g. BPC-157 at growth hormone receptor). An antagonist blocks a receptor without activating it. Peptide research compounds are nearly all agonists.
What does bioavailability mean for peptides?
Bioavailability is the fraction of a compound that reaches systemic circulation. Subcutaneous injection has ~80–100% bioavailability for most peptides. Oral bioavailability for peptides is typically near zero due to digestive breakdown.
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