Peptide Glossary: 50+ Terms Every Beginner Should Know
Walking into the peptide world without understanding the language is like trying to navigate a foreign country without a phrasebook. You'll encounter abbreviations like GH, GHRH, and GLP-1, dosing terms like mcg/kg and IU, and practical terms like reconstitution, lyophilization, and bacteriostatic water — all of which carry specific, important meanings. Misunderstanding a single term can lead to dosing errors, safety mistakes, or simply wasted money on the wrong product. This glossary gives you the vocabulary foundation you need to read research, evaluate products, follow protocols, and have informed conversations with healthcare providers. We've organized these terms alphabetically and written each definition in plain, accessible language. No PhD required — just curiosity and a desire to learn. For the full interactive experience with cross-linked terms and expanded explanations, visit our online glossary. The building blocks of peptides and proteins. There are 20 standard amino acids that combine in different sequences to form every peptide and protein in the human body. A peptide is simply a short chain of amino acids linked together by peptide bonds. Understanding amino acids helps you grasp why peptide sequence matters — even a single amino acid change can dramatically alter a peptide's function. Referring to the building-up of complex molecules and tissues in the body. Anabolic processes include muscle protein synthesis, bone formation, and tissue repair. Many peptides — particularly GH secretagogues — promote anabolic activity, which is why they're popular in fitness and recovery contexts. Sterile water containing 0.9% benzyl alcohol as a preservative. BAC water is the standard solvent used to reconstitute lyophilized peptides. The benzyl alcohol inhibits bacterial growth, giving reconstituted peptides a longer shelf life (typically 2–4 weeks refrigerated) compared to reconstitution with plain sterile water. Always use BAC water unless specifically directed otherwise. The percentage of an administered substance that reaches systemic circulation in active form. Subcutaneous injection provides near-100% bioavailability, while oral administration typically offers 1–5% for most peptides due to digestive degradation. Bioavailability directly affects how much peptide you need to take for a given effect. Learn more in our oral vs injectable comparison. A synthetic peptide derived from a naturally occurring protein found in human gastric juice. BPC-157 is 15 amino acids long and is widely researched for its tissue-healing, anti-inflammatory, and gut-protective properties. It's one of the few peptides with meaningful evidence for both oral and injectable administration. A protein that binds to a peptide or hormone in the bloodstream, extending its half-life and regulating its availability to tissues. For example, IGF-binding proteins (IGFBPs) carry IGF-1 in the blood, controlling how much free IGF-1 is available to act on cells. A synthetic analog of growth hormone-releasing hormone (GHRH). CJC-1295 comes in two forms: with DAC (Drug Affinity Complex), which extends its half-life to about 8 days, and without DAC (also called Modified GRF 1-29), which has a half-life of about 30 minutes. The non-DAC version is preferred for more natural, pulsatile GH release. The amount of peptide per unit volume of solution after reconstitution. Expressed as mcg per unit (on an insulin syringe) or mg/mL. Calculating concentration correctly is essential for accurate dosing. See our dosage calculator guide for step-by-step math. The body's primary stress hormone, produced by the adrenal glands. Cortisol is catabolic (tissue-breaking) and can counteract the anabolic effects of growth hormone and other peptides. Chronic elevated cortisol impairs peptide protocol results. Managing stress and sleep is essential for peptide effectiveness. The practice of using a peptide for a set period (the "on" cycle), then taking a break (the "off" cycle). Cycling helps prevent receptor desensitization, where the body's receptors become less responsive to a peptide over time. Common cycles are 8–12 weeks on, 2–4 weeks off, though specific protocols vary by peptide. A chemical modification added to CJC-1295 that allows it to bind to albumin in the blood, dramatically extending its half-life from ~30 minutes to ~8 days. CJC-1295 with DAC creates a sustained GH elevation rather than pulsatile release, which is why many practitioners prefer the non-DAC version for more physiological GH patterns. The process by which receptors become less responsive to a substance after prolonged or excessive exposure. If you use a peptide continuously at high doses without breaks, your receptors may downregulate, requiring higher doses for the same effect — or rendering the peptide ineffective. This is why cycling is important. The liquid used to dissolve a lyophilized peptide for injection. The most common diluent is bacteriostatic water (BAC water). Sterile water and sodium chloride solution are alternatives but offer shorter shelf life after reconstitution. The relationship between the amount of a substance administered and the magnitude of its effect. For most peptides, this curve is not linear — doubling the dose does not double the effect. There's typically a "sweet spot" where increasing the dose produces diminishing returns, and beyond a certain point, side effects increase faster than benefits. The network of glands and organs that produce, store, and release hormones into the bloodstream. Includes the pituitary gland, thyroid, adrenals, pancreas, and gonads. Many peptides work by stimulating or modulating endocrine function — for example, GH secretagogues act on the pituitary gland to increase growth hormone output. A polymer coating applied to oral peptide capsules or tablets that resists dissolution in the acidic stomach environment but dissolves in the more neutral pH of the small intestine. This protects acid-sensitive peptides from stomach degradation, improving oral bioavailability. Refraining from food intake before and after peptide administration. Most GH secretagogues require a fasting window because elevated blood sugar and insulin blunt GH release. Typical recommendation: no food for 1–2 hours before and 20–30 minutes after administration. A 191-amino acid protein hormone produced by the anterior pituitary gland. GH regulates growth, body composition, metabolism, and cellular repair. GH declines naturally with age (a process called somatopause). Many peptides — called GH secretagogues — work by stimulating the body's natural GH production rather than replacing it with exogenous hormone. A hormone produced by the hypothalamus that signals the pituitary gland to release growth hormone. CJC-1295 and sermorelin are synthetic analogs of GHRH — they mimic this natural signaling molecule to stimulate GH release. A class of synthetic peptides that stimulate GH release through the ghrelin/GHS receptor pathway (different from the GHRH pathway). Examples include GHRP-2, GHRP-6, and hexarelin. These are often combined with GHRH analogs for synergistic GH release. A naturally occurring incretin hormone produced in the gut that stimulates insulin secretion, suppresses glucagon, slows gastric emptying, and promotes satiety. GLP-1 receptor agonists like semaglutide and tirzepatide mimic and enhance these effects for metabolic health and weight management. The time it takes for the concentration of a peptide in your body to decrease by half. Half-life determines dosing frequency — short half-life peptides (minutes to hours) need more frequent dosing, while long half-life peptides (days) can be dosed less often. For example, CJC-1295 without DAC has a ~30-minute half-life (dose 1–3x daily), while semaglutide has a ~7-day half-life (dose weekly). A blood test measuring average blood sugar over the past 2–3 months. Reported as a percentage. Normal is below 5.7%; prediabetic is 5.7–6.4%; diabetic is 6.5%+. Important to monitor when using GH secretagogues or metabolic peptides. See our blood work guide for complete lab recommendations. The body's tendency to maintain stable internal conditions. When you introduce a peptide that shifts a parameter (like GH levels), your body may activate feedback mechanisms to counteract the change. Understanding homeostasis helps explain why peptide effects can plateau over time and why cycling matters. A hormone primarily produced by the liver in response to growth hormone stimulation. IGF-1 mediates many of GH's effects on tissues, including muscle growth, fat metabolism, and cellular repair. IGF-1 blood levels are the best single marker for assessing GH secretagogue effectiveness, as they reflect integrated GH output over time rather than a single-point measurement.Key Takeaways
Why Learn Peptide Terminology?
A–C
Amino Acid
Anabolic
Bacteriostatic Water (BAC Water)
Bioavailability
BPC-157 (Body Protection Compound-157)
Carrier Protein
CJC-1295
Concentration
Cortisol
Cycling
D–G
DAC (Drug Affinity Complex)
Desensitization (Receptor Downregulation)
Diluent
Dose-Response Curve
Endocrine System
Enteric Coating
Fasting (in peptide context)
GH (Growth Hormone / Somatotropin)
GHRH (Growth Hormone-Releasing Hormone)
GHRP (Growth Hormone-Releasing Peptide)
GLP-1 (Glucagon-Like Peptide-1)
H–L
Half-Life
HbA1c (Glycated Hemoglobin)
Homeostasis
IGF-1 (Insulin-Like Growth Factor 1)
Insulin Syringe
A small syringe with fine gauge needle (29–31 gauge) designed for subcutaneous injection. Marked in "units" where 100 units = 1 mL. The standard tool for peptide injection. The thin needle makes injections virtually painless when proper technique is used.
Intramuscular (IM)
An injection route where the substance is delivered directly into muscle tissue. Provides faster absorption than subcutaneous injection. Less commonly used for peptides but may be appropriate for certain compounds or when faster onset is desired.
Ipamorelin
A selective GH secretagogue that acts on the ghrelin receptor to stimulate growth hormone release. Considered one of the "cleanest" GH-releasing peptides because it doesn't significantly raise cortisol or prolactin at normal doses. Often paired with CJC-1295 (no DAC) for synergistic effect.
IU (International Unit)
A standardized unit of biological activity used for certain hormones and compounds (HGH, HCG, insulin, vitamins). The relationship between IU and weight units (mg, mcg) varies by substance. For example, 1 mg of pharmaceutical HGH equals approximately 3 IU.
Lyophilization (Freeze-Drying)
The process of removing water from a peptide solution by freezing it and then reducing pressure to allow the frozen water to sublimate directly into vapor. This produces a stable, dry powder (lyophilized cake) that can be stored long-term and reconstituted when ready to use. Most injectable peptides arrive in lyophilized form.
M–P
mcg (Microgram)
One-millionth of a gram or one-thousandth of a milligram. The standard unit for peptide dosing. Written as mcg or μg. Critical distinction: 1 mg = 1,000 mcg. Confusing these units is the most common and potentially dangerous dosing error.
MK-677 (Ibutamoren)
A non-peptide, orally active growth hormone secretagogue that mimics ghrelin and stimulates GH release. Despite being commonly grouped with peptides, it's technically a small molecule with excellent oral bioavailability. Known for increasing appetite, GH, and IGF-1. Can impact blood sugar levels.
Modulated GRF 1-29 (Modified GRF)
Another name for CJC-1295 without DAC. It's a modified version of the first 29 amino acids of natural GHRH, with amino acid substitutions at positions 2, 8, 15, and 27 to improve stability and resistance to enzymatic degradation.
Peptide
A short chain of amino acids linked by peptide bonds. Technically, a peptide contains 2–50 amino acids; chains longer than 50 amino acids are generally classified as proteins. Peptides serve as signaling molecules in the body, carrying messages between cells, tissues, and organs. Therapeutic peptides mimic or enhance these natural signaling functions.
Peptide Bond
The chemical bond that links amino acids together in a chain. Formed between the carboxyl group of one amino acid and the amino group of the next, with the release of a water molecule. Peptide bonds are what digestive enzymes target when breaking down dietary proteins — and what makes oral peptide delivery challenging.
Pharmacokinetics (PK)
The study of how a drug or peptide moves through the body over time: absorption, distribution, metabolism, and elimination (ADME). Pharmacokinetic properties determine a peptide's half-life, time to peak effect, duration of action, and optimal dosing schedule.
Pituitary Gland
A pea-sized gland at the base of the brain often called the "master gland" because it controls many other endocrine glands. The anterior pituitary produces and releases growth hormone in response to GHRH signaling. GH secretagogue peptides ultimately work by stimulating the pituitary to produce more GH naturally.
Pulsatile Release
The natural pattern of GH secretion where the hormone is released in discrete bursts (pulses) rather than a constant stream. The largest natural GH pulse occurs during deep sleep. GH secretagogues like CJC-1295 (no DAC) and Ipamorelin are designed to amplify these natural pulses rather than create a flat, continuous GH elevation.
R–S
Receptor
A protein on the surface of (or inside) a cell that binds to a specific molecule (like a peptide), triggering a biological response. Think of it as a lock that only accepts certain keys. Peptides work by binding to their target receptors — for example, GH secretagogues bind to GHRH receptors or ghrelin receptors on pituitary cells.
Receptor Agonist
A substance that binds to a receptor and activates it, mimicking the effect of the natural signaling molecule. Most therapeutic peptides are receptor agonists — they "turn on" the same pathways as the body's own peptides, often with greater potency or longer duration.
Reconstitution
The process of dissolving a lyophilized (freeze-dried) peptide powder with a diluent (typically bacteriostatic water) to create an injectable solution. Proper reconstitution technique is essential for peptide stability and accurate dosing. See our step-by-step reconstitution guide for detailed instructions.
Semaglutide
A GLP-1 receptor agonist used for type 2 diabetes management and weight loss. Available in injectable (Ozempic®, Wegovy®) and oral (Rybelsus®) forms. Semaglutide has a long half-life (~7 days) enabling once-weekly dosing. It works by enhancing insulin secretion, suppressing appetite, and slowing gastric emptying.
Sermorelin
A synthetic analog of the first 29 amino acids of natural GHRH. One of the earliest GH secretagogues developed. It stimulates natural GH production from the pituitary gland. Generally considered milder than newer secretagogues like CJC-1295 but has a well-established safety profile.
Subcutaneous (SubQ)
An injection route where the substance is delivered into the fat layer just beneath the skin. The standard injection method for most peptides. Common injection sites include the abdomen (around the navel), thigh, and upper arm. SubQ injection provides slower, more sustained absorption than intramuscular injection.
Synergy (Peptide Stacking)
The enhanced effect achieved by combining two or more peptides that work through complementary mechanisms. The most common example is stacking a GHRH analog (CJC-1295) with a GHRP (Ipamorelin) — the two peptides stimulate GH release through different receptor pathways, producing a greater combined effect than either alone.
T–Z
TB-500 (Thymosin Beta-4 Fragment)
A synthetic peptide fragment of the naturally occurring protein Thymosin Beta-4. It's 43 amino acids long and is researched for its roles in tissue repair, wound healing, inflammation reduction, and cellular migration. Often used alongside BPC-157 in healing protocols.
Tesamorelin
A synthetic GHRH analog FDA-approved for reducing visceral adipose tissue (belly fat) in HIV-associated lipodystrophy. It stimulates pituitary GH production and has been studied for its effects on body composition, liver fat reduction, and cognitive function.
Tirzepatide
A dual GIP/GLP-1 receptor agonist that combines the mechanisms of two incretin hormones. Marketed as Mounjaro® for diabetes and Zepbound® for weight management. Has shown greater weight loss efficacy than semaglutide in head-to-head clinical trials.
Titration
The process of gradually adjusting a peptide dose — usually starting low and increasing incrementally — to find the optimal balance between effectiveness and side effects. Titration is a fundamental principle of safe peptide use. "Start low, go slow" is the mantra.
Tachyphylaxis
A rapid decrease in response to a drug or peptide after repeated doses over a short period. Similar to desensitization but occurs more quickly. This is why some peptides require cycling or dose holidays to maintain effectiveness.
Vial
The small glass container in which lyophilized peptide powder is stored. Sealed with a rubber stopper and aluminum crimp cap. Peptide vials typically contain 2 mg, 5 mg, or 10 mg of lyophilized peptide. After reconstitution, vials should be stored refrigerated and used within 2–4 weeks.
Visceral Fat
Fat stored deep within the abdominal cavity, surrounding internal organs. Distinguished from subcutaneous fat (under the skin). Visceral fat is metabolically active and associated with increased health risks. Several peptides, including tesamorelin and GLP-1 agonists, specifically target visceral fat reduction.
Putting It All Together
This glossary is your foundation for understanding peptide science and practice. As you read research, follow protocols, and engage with the peptide community, these terms will come up repeatedly. Having a clear understanding of each one helps you make better decisions, avoid mistakes, and communicate effectively with healthcare providers.
For the full interactive glossary experience — with search functionality, cross-linked terms, and expanded explanations — visit peptideplaybook.health/glossary.
Ready to put your knowledge into practice? Explore our guides on peptide dosing, blood work monitoring, and reconstitution technique. Or start from the beginning at Peptide Playbook.
Medical Disclaimer: This article is for informational and educational purposes only. It is not medical advice and should not be used to diagnose, treat, or prevent any disease or condition. Peptides are research compounds and their use may not be approved by regulatory agencies in your jurisdiction. Always consult with a qualified healthcare provider before starting any peptide protocol or making changes to your health regimen. Individual results may vary. Peptide Playbook does not endorse the use of any compound without proper medical supervision.