PEPTIDE GLOSSARY: EVERY TERM EXPLAINED
A comprehensive reference for every term you will encounter reading about peptides — from chemistry fundamentals to regulatory labels to protocol jargon. Organized by category for efficient lookup.
1.Dosing and measurement terms
mcg (microgram): one millionth of a gram (0.000001 g). The primary unit for research peptide dosing. 1,000 mcg = 1 mg. 1,000,000 mcg = 1 g. Most research peptides are dosed in hundreds of mcg (e.g., 250-500 mcg BPC-157). GLP-1 agonists are typically dosed in mg (e.g., 0.5 mg semaglutide = 500 mcg).
Reconstitution: the process of dissolving lyophilized (freeze-dried) peptide powder in a liquid (typically BAC water) to create a solution for injection. Lyophilization: freeze-drying under vacuum — a process that removes water from the peptide to create a stable powder that is significantly more stable than the aqueous solution form. BAC water: bacteriostatic water, a sterile water solution containing 0.9% benzyl alcohol as a preservative against bacterial contamination in multi-dose vials.
Concentration (mcg/mL or mcg/unit): the amount of peptide per unit volume of solution. A 5 mg vial reconstituted in 1 mL BAC water = 5,000 mcg/mL = 50 mcg per unit on a U-100 syringe. A 5 mg vial in 2 mL = 2,500 mcg/mL = 25 mcg per unit. U-100: the syringe calibration standard where 100 units = 1 mL, used universally for insulin syringes in the US. Units on a U-100 syringe: 1 unit = 0.01 mL. To convert: mcg per unit = concentration (mcg/mL) / 100.
SubQ (subcutaneous): injection into the adipose (fat) layer beneath the skin. IM (intramuscular): injection into muscle tissue. IV (intravenous): injection directly into a vein — not used for research peptides. Intranasal: administration via nasal spray for CNS-targeted peptides (Semax, Selank). Biological half-life: the time required for the drug's pharmacological effect to reduce by half — often longer than plasma half-life. Plasma half-life: the time required for the drug's plasma concentration to fall by 50%.
Dead space: the residual volume of solution remaining in a needle and hub after the plunger is fully depressed. Fixed-needle syringes have minimal dead space (important for accurate small-volume dosing). Gauge (G): needle diameter. Higher gauge = thinner needle. 29G-31G insulin syringe needles are thin enough that most users barely feel SubQ injection. 23G needles are appropriate for IM injection in most applications.
2.Chemistry and mechanism terms
Peptide: a chain of 2-50 amino acids linked by peptide bonds. Below 2 amino acids = single amino acid. Above 50 amino acids begins the territory of polypeptides and small proteins. Amino acids: the 20 standard molecules that are the building blocks of peptides and proteins. Peptide bond: the covalent chemical bond that links amino acids in a chain, formed between the carboxyl group of one amino acid and the amino group of the next.
Agonist: a molecule that binds to a receptor and activates it, producing a biological response. Partial agonist: binds and activates but with lower maximum effect than a full agonist. Antagonist: binds to a receptor without activating it, blocking other molecules from activating it. Most therapeutic peptides are agonists at their target receptors. Receptor: a protein (usually on the cell surface or inside the cell) that binds to specific molecules and translates binding into cellular responses.
GHRH: Growth Hormone Releasing Hormone — the hypothalamic peptide that stimulates the pituitary to release GH. CJC-1295 no DAC is a GHRH analog. GHRP: Growth Hormone Releasing Peptide — synthetic peptides that trigger GH release through the ghrelin receptor (GHSR1a). Ipamorelin, GHRP-2, and GHRP-6 are GHRPs. Ghrelin: the natural peptide hormone that GHRPs mimic — released from the stomach to signal hunger and stimulate GH release.
GLP-1: Glucagon-Like Peptide-1 — an incretin hormone secreted from intestinal L-cells after eating, regulating insulin release, gastric emptying, and satiety. GIP: Glucose-Dependent Insulinotropic Polypeptide — the second incretin hormone, synergistic with GLP-1 for insulin secretion and appetite regulation. Incretin: gut-derived hormones that enhance insulin secretion in response to eating. IGF-1: Insulin-Like Growth Factor 1 — produced in the liver in response to GH, mediating most of GH's tissue-building effects.
VEGF: Vascular Endothelial Growth Factor — a signaling protein that drives angiogenesis (new blood vessel formation). BPC-157 upregulates VEGF. BDNF: Brain-Derived Neurotrophic Factor — the primary growth factor for neuron survival and synaptic plasticity. Semax and Selank upregulate BDNF. Actin: a structural protein critical for cell movement and shape. TB-500 promotes actin polymerization. Collagen: the most abundant structural protein in the body, providing tensile strength to skin, tendons, and connective tissue. GHK-Cu stimulates collagen synthesis.
3.Protocol and cycle terminology
Cycle: a defined period of compound use followed by a planned break. On-cycle: the period of active use. Off-cycle: the break period that allows receptor resensitization and physiological reset. Frontloading: starting a cycle with a higher-than-maintenance dose for the first 1-2 weeks to achieve therapeutic concentrations faster, then reducing to maintenance dose. Used with TB-500 in acute injury protocols.
PCT (Post-Cycle Therapy): compounds taken after a hormone-suppressive cycle (steroids, SARMs) to restore natural production. Not applicable to most peptide protocols — peptides generally do not suppress the HPTA. Understanding this is one reason peptides are often preferred to steroids for long-term wellness protocols.
Pulsatile dosing: timing injections to coincide with natural biological rhythms — particularly important with GH peptides timed to natural GH secretion pulses (pre-sleep, pre-workout). Continuous dosing: maintaining a steady plasma concentration, typically using compounds with longer half-lives (CJC-1295 DAC, semaglutide). Receptor desensitization (tachyphylaxis): reduced receptor response from prolonged continuous stimulation — the primary reason GH peptide cycles require off-periods.
Taper: gradually reducing dose at the end of a cycle rather than stopping abruptly. Relevant for GLP-1 users wanting to minimize weight regain. Induction dose: the higher initial dose used to establish therapeutic effect. Maintenance dose: the lower dose that maintains established effects after the induction phase. Titration: systematically adjusting dose based on response and tolerability — the standard approach for GLP-1 escalation protocols.
Stacking: combining two or more compounds in the same protocol. Synergistic stack: compounds that enhance each other's effects through complementary mechanisms (BPC-157 + TB-500, CJC-1295 + ipamorelin). Additive stack: compounds with effects that add but do not multiply (any two compounds working independently). PRN (pro re nata): as needed — dosing only when required. Continuous protocol: ongoing use without planned cycling breaks, typically for compounds without receptor desensitization concerns.
4.Regulatory and quality terms
FDA (Food and Drug Administration): the US federal agency responsible for approving drugs for human use and regulating food safety. A compound without FDA approval cannot be legally marketed as a drug for human use in the US. Research use only: the legal label that allows sale of unapproved compounds for laboratory research purposes without triggering FDA pharmaceutical regulations. Does not mean illegal to possess — means the vendor cannot make human-use marketing claims.
Scheduled substance: a compound classified under the Controlled Substances Act. Schedule I (high abuse potential, no accepted medical use), Schedule II through V with decreasing restriction. Most peptides are not scheduled. Anabolic steroids are Schedule III. Some peptide hormones (EPO, certain growth factors) are explicitly scheduled under the Federal Analogue Act provisions of FDCA rather than the CSA.
WADA (World Anti-Doping Agency): the international body maintaining the prohibited list for athletes in sanctioned sports. WADA banning a compound does not make it illegal — it means athletes in WADA-signatory sports cannot use it without facing sporting sanctions. GLP-1 agonists are currently unlisted but under monitoring review. All GH secretagogues are on the prohibited list.
CoA (Certificate of Analysis): a lab document certifying compound identity and purity for a specific batch. HPLC (High-Performance Liquid Chromatography): the primary method for purity quantification in peptide testing. MS (Mass Spectrometry): molecular identification through fragmentation pattern — confirms compound identity beyond just purity. Endotoxin testing (LAL test): testing for bacterial lipopolysaccharides that can cause fever and inflammation when injected. GMP (Good Manufacturing Practice): the manufacturing standards required for pharmaceutical-grade compound production.
Compounding pharmacy: a licensed pharmacy that produces customized medications including peptides, requiring a physician prescription under 503A (individual patient) or 503B (healthcare facility) provisions of FDCA. The legal pathway for pharmaceutical-grade peptide access in the US. HPTA (Hypothalamic-Pituitary-Testicular Axis): the hormonal feedback loop controlling testosterone production. Peptides generally do not suppress the HPTA, unlike anabolic steroids and SARMs — a key safety distinction for long-term use.
Sources & Studies
Lau JL, Dunn MK., Bioorg Med Chem, 2018
IUPAC nomenclature of peptides and proteins
IUPAC-IUB Joint Commission, J Biol Chem, 1984