The Meto Peptide Glossary: 60 Terms Every Patient Should Know Before Starting Therapy

If you are researching peptide therapy glossary terms, this reference gives you every definition you need — without the jargon overload. Peptide therapy is one of the fastest-growing fields in metabolic and hormonal medicine. But the terminology can stop curious adults cold before they even book a consultation.

This glossary covers 60 terms across six categories: foundational science, delivery and pharmacokinetics, the major therapeutic peptide classes, hormonal pathways, regulatory status, and clinical monitoring. Read it once. Use it as a reference. Then talk to your clinician with confidence.

Why Peptide Terminology Matters Before You Start

Knowing the language is not academic. It changes outcomes. A patient who understands what a GH secretagogue does is better positioned to ask the right questions, spot an underdosed protocol, and recognise side effects early.

Every term in this glossary has clinical relevance. None are included for decoration.

Part 1 — Foundational Science: What Peptides Actually Are

Understanding this section is non-negotiable. It tells you what you are actually putting in your body.

1. Peptide A short chain of amino acids — typically 2 to 50 — linked by peptide bonds. Peptides are smaller than proteins and are naturally produced throughout the body. They act as signalling molecules, instructing cells to perform specific biological functions. (Bhambhani et al., 2021, Journal of Pharmaceutical Sciences)

2. Amino Acid The individual building blocks that form peptides and proteins. There are 20 standard amino acids. The specific sequence of amino acids determines what a peptide does.

3. Peptide Bond The chemical link between two amino acids. It forms when the carboxyl group of one amino acid reacts with the amino group of another, releasing water. This bond is what gives peptides their structural integrity.

4. Endogenous Peptide A peptide the body produces on its own. Insulin, glucagon, and growth hormone-releasing hormone (GHRH) are all endogenous peptides. Many therapeutic peptides are synthetic analogues of these natural compounds.

5. Exogenous Peptide A peptide introduced from outside the body — via injection, nasal spray, oral capsule, or topical application. Therapeutic peptides are exogenous by definition.

6. Receptor Agonist A compound that binds to a receptor and activates it, mimicking the effect of the body's own signalling molecule. Most therapeutic peptides work as receptor agonists.

7. Receptor Antagonist A compound that binds to a receptor but blocks it, preventing activation. Less common in peptide therapy but relevant in some hormonal applications.

8. Ligand Any molecule that binds to a receptor. Therapeutic peptides are ligands for their target receptors — for example, GLP-1 receptor agonists are ligands for the GLP-1 receptor.

9. Bioavailability The proportion of a drug that reaches systemic circulation in an active form. Injectable peptides typically have near-100% bioavailability. Oral and nasal routes are significantly lower due to enzymatic degradation in the gut and mucous membranes.

10. Pharmacokinetics The study of how the body absorbs, distributes, metabolises, and excretes a substance. Understanding pharmacokinetics tells you when a peptide is active and for how long.

Part 2 — Delivery, Dosing, and Pharmacokinetics: Peptide Half-Life Explained

This is where many patients get confused. These terms directly affect how your protocol is structured.

11. Half-Life The time it takes for half of a substance to be eliminated from the body. Peptide half-life explained simply: if a peptide has a 30-minute half-life, half of the administered dose is gone within 30 minutes. Half-life determines dosing frequency. (Vlieghe et al., 2010, Drug Discovery Today)

12. Subcutaneous Injection (SQ or SC) Injection into the fatty tissue just below the skin, not into muscle. This is the most common delivery method for peptide therapy. It produces slower, steadier absorption than intramuscular injection. The abdomen and thigh are typical injection sites.

13. Intramuscular Injection (IM) Injection directly into muscle tissue. Faster absorption than subcutaneous. Less commonly used for peptide protocols but applicable in certain clinical contexts.

14. Intranasal Delivery Administration via the nasal mucosa. Used for peptides like PT-141 and some cognitive peptides (Semax, Selank). Bioavailability is variable — typically 10–40% depending on the molecule and formulation.

15. Oral Bioavailability The fraction of an orally administered peptide that reaches circulation intact. Most short-chain peptides are degraded by stomach acid and proteolytic enzymes before absorption. This is why most therapeutic peptides require injection.

16. Lyophilised Powder A freeze-dried form of a peptide, used to extend shelf life and stability. Most injectable peptides are supplied as lyophilised powder in vials and must be reconstituted with bacteriostatic water before use.

17. Bacteriostatic Water (BW) Sterile water containing 0.9% benzyl alcohol, used to reconstitute lyophilised peptides. The benzyl alcohol inhibits microbial growth, extending the usable life of the reconstituted solution to approximately 28 days when refrigerated.

18. Reconstitution The process of dissolving lyophilised peptide powder in bacteriostatic water. Done slowly, with the water directed at the side of the vial, not the powder directly. Swirl — never shake — to avoid denaturing the peptide.

19. Titration Gradually increasing a dose over time to assess tolerance and optimise response while minimising side effects. Most peptide protocols begin at a low dose and titrate upward based on clinical response and lab markers.

20. Pulsatile Release The natural, episodic pattern in which growth hormone and other hormones are secreted. Growth hormone secretagogues are designed to preserve this pattern, unlike synthetic HGH, which creates a continuous (non-physiological) elevation.

Part 3 — Key Peptide Classes: Peptide Terminology Guide

This section covers the major therapeutic peptide families. Understanding these categories is the backbone of any peptide terminology guide.

Growth Hormone Axis Peptides

21. GH Secretagogue Any compound that stimulates the body to secrete its own growth hormone. GH secretagogue meaning: it does not introduce exogenous GH — it triggers endogenous release. This is a critical distinction from synthetic HGH therapy. Examples include sermorelin, ipamorelin, and CJC-1295. (Giustina & Veldhuis, 1998, Endocrine Reviews)

22. GHRH (Growth Hormone-Releasing Hormone) The endogenous hypothalamic hormone that stimulates the pituitary gland to release growth hormone. GHRH analogues — like sermorelin, CJC-1295, and tesamorelin — mimic this action at the pituitary level.

23. GHRP (Growth Hormone-Releasing Peptide) A class of synthetic peptides that stimulate GH release via the ghrelin receptor. Examples include GHRP-2, GHRP-6, and ipamorelin. GHRPs and GHRH analogues are often stacked together to produce synergistic GH release.

24. Sermorelin A 29-amino acid analogue of GHRH. One of the most clinically validated GH secretagogues. It stimulates pituitary GH release in a physiological, pulsatile manner. Used for adult GH deficiency, body composition, and recovery. (Walker et al., 1990, Journal of Clinical Endocrinology & Metabolism)

25. Ipamorelin A highly selective GHRP that stimulates GH release with minimal effect on cortisol or prolactin. Preferred over older GHRPs (GHRP-2, GHRP-6) due to its cleaner hormonal profile. Commonly stacked with CJC-1295.

26. CJC-1295 A GHRH analogue engineered with Drug Affinity Complex (DAC) technology, which extends its half-life from minutes to approximately 6–8 days. This extended duration allows once or twice-weekly dosing. Often combined with ipamorelin for synergistic GH release.

27. Tesamorelin A GHRH analogue with FDA approval (as Egrifta®) for HIV-associated lipodystrophy. Has strong evidence for visceral fat reduction and is being studied for its metabolic and cardiovascular benefits. (Falutz et al., 2010, Lancet)

28. IGF-1 (Insulin-Like Growth Factor 1) The primary downstream mediator of growth hormone's anabolic effects. Produced mainly by the liver in response to GH stimulation. IGF-1 levels are used as a primary biomarker to assess GH status and response to secretagogue therapy.

29. AOD-9604 A synthetic fragment of the growth hormone molecule (amino acids 176–191) designed specifically to stimulate lipolysis (fat breakdown) without the growth-promoting or insulin-desensitising effects of full HGH. Targets visceral and stubborn subcutaneous fat. (Heffernan et al., 2001, Endocrinology)

30. Somatostatin An endogenous inhibitory hormone that suppresses GH release. Understanding somatostatin is important because some advanced peptide protocols account for somatostatin tone when timing injections to maximise GH pulse amplitude.

GLP-1 and Metabolic Peptides

31. GLP-1 (Glucagon-Like Peptide-1) An incretin hormone produced in the gut after eating. It stimulates insulin secretion, suppresses glucagon, slows gastric emptying, and reduces appetite. The biological basis for semaglutide and tirzepatide.

32. GLP-1 Receptor Agonist A synthetic peptide that binds and activates the GLP-1 receptor. Includes semaglutide (Ozempic®, Wegovy®) and liraglutide. Produces potent appetite suppression, blood sugar regulation, and weight loss. (Drucker, 2006, Cell Metabolism)

33. GIP (Glucose-Dependent Insulinotropic Polypeptide) A second incretin hormone. Tirzepatide is a dual GLP-1/GIP receptor agonist, which accounts for its superior weight loss outcomes compared to GLP-1 monotherapy. Meto clinicians work with both GLP-1 and dual-agonist therapies — see Meto's Weight Loss Program.

34. Incretin Effect The amplified insulin response to oral glucose compared to intravenous glucose. Incretins like GLP-1 and GIP account for up to 70% of the post-meal insulin response. Blunted in type 2 diabetes.

35. Tirzepatide A dual GLP-1/GIP receptor agonist with clinical data showing superior weight reduction versus semaglutide in head-to-head trials. Available as Mounjaro® and Zepbound®. (Jastreboff et al., 2022, NEJM)

Tissue Repair and Recovery Peptides

36. BPC-157 (Body Protection Compound 157) A synthetic peptide derived from a protective gastric protein. Has demonstrated regenerative effects on tendons, ligaments, gut mucosa, and muscle tissue in preclinical models. Evidence in humans is emerging. Mechanism involves upregulation of VEGF (vascular endothelial growth factor) and growth hormone receptors.

37. TB-500 (Thymosin Beta-4 Fragment) A synthetic analogue of Thymosin Beta-4. Promotes actin polymerisation, accelerates tissue repair, reduces inflammation, and supports angiogenesis. Used for injury recovery and musculoskeletal health.

38. PT-141 (Bremelanotide) A melanocortin receptor agonist developed from the tanning peptide Melanotan II. Acts centrally on the hypothalamus to stimulate sexual desire in both men and women. FDA-approved as Vyleesi® for hypoactive sexual desire disorder (HSDD) in premenopausal women. (Clayton et al., 2016, NEJM)

Cognitive and Neuroprotective Peptides

39. Semax A synthetic analogue of ACTH (adrenocorticotropic hormone) fragments. Promotes BDNF (brain-derived neurotrophic factor) production. Used in Russia as an approved drug for cognitive enhancement, stroke recovery, and neuroprotection. Currently legal for compounding in the US following 2026 regulatory updates.

40. Selank A synthetic analogue of the endogenous neuropeptide tuftsin. Produces anxiolytic and nootropic effects without sedation or dependence. Modulates GABAergic and serotonergic systems. Legal for compounding in the US post-2026. (Zozulya et al., 2001, Bulletin of Experimental Biology and Medicine)

Mitochondrial and Longevity Peptides

41. MOTS-c A mitochondria-derived peptide encoded within the mitochondrial genome. Activates AMPK, improving insulin sensitivity, glucose uptake, and cellular energy metabolism. Levels decline with age. Strong preclinical evidence for metabolic restoration. (Lee et al., 2015, Cell Metabolism)

42. Humanin The first mitochondrial-derived peptide identified. Exerts cytoprotective effects on neurons and pancreatic beta cells. Correlates inversely with metabolic disease and cardiovascular risk. Levels decline significantly after age 40. (Muzumdar et al., 2009, FASEB Journal)

43. SHLP-2 (Small Humanin-Like Peptide 2) A humanin-related mitochondrial peptide. Suppresses appetite via hypothalamic POMC neurons and promotes thermogenesis. Associated with reduced obesity and improved insulin sensitivity in early research.

44. DSIP (Delta Sleep-Inducing Peptide) A neuropeptide that promotes delta-wave sleep and modulates cortisol and GH secretion. Research suggests DSIP may support metabolic recovery through improved sleep quality and reduced hypothalamic-pituitary-adrenal (HPA) axis hyperactivity.

Part 4 — Hormonal Pathways: The Axis That Governs It All

Peptide therapy does not operate in isolation. It interacts with established hormonal feedback loops.

45. HPG Axis (Hypothalamic-Pituitary-Gonadal Axis) The hormonal cascade from hypothalamus → pituitary → gonads (testes or ovaries) governing sex hormone production. Kisspeptin — an endogenous neuropeptide — is the master regulator of this axis.

46. HPA Axis (Hypothalamic-Pituitary-Adrenal Axis) The stress response axis governing cortisol, ACTH, and CRH. Chronic stress-induced HPA activation suppresses GH secretion and impairs metabolic function. Some peptides (including Selank and DSIP) modulate HPA tone.

47. Kisspeptin An endogenous peptide produced in the hypothalamus that directly stimulates GnRH (gonadotropin-releasing hormone) release, triggering LH and FSH production, and downstream testosterone or estrogen synthesis. A key target for male hormonal restoration research. See Meto's article on kisspeptin and male hormone optimisation.

48. LH (Luteinising Hormone) A pituitary hormone that triggers testosterone production in men (via Leydig cells) and ovulation in women. Peptides that stimulate GnRH — including kisspeptin — indirectly raise LH.

49. FSH (Follicle-Stimulating Hormone) A pituitary hormone driving spermatogenesis in men and follicle development in women. Tracked alongside LH in hormonal baseline labs.

50. SHBG (Sex Hormone-Binding Globulin) A protein that binds sex hormones (testosterone, estradiol, DHT) in circulation, rendering them biologically inactive. High SHBG means low free (active) hormone even when total levels look normal. Critical to assess in hormonal panels.

51. Free Testosterone The fraction of testosterone not bound to SHBG or albumin. This is the biologically active form. Many men with "normal" total testosterone have low free testosterone due to elevated SHBG. Relevant to anyone on Meto's andropause or hormonal restoration protocol.

52. Cortisol The primary stress hormone, produced by the adrenal cortex. Chronically elevated cortisol suppresses GH pulsatility, promotes visceral fat accumulation, and impairs insulin sensitivity. Managed indirectly by some peptide protocols targeting HPA regulation.

Part 5 — Regulatory and Legal Framework

Regulatory status shapes what you can access, how, and under what oversight.

53. Compounded Peptide A peptide prepared by a licensed compounding pharmacy, typically for an individual patient based on a clinician's prescription. Not FDA-approved as a finished drug but legal when prepared by a 503A or 503B compounding facility. The regulatory landscape was significantly updated in 2026.

54. 503A Compounding Pharmacy A state-licensed pharmacy that compounds medications for individual patients based on a valid prescription. Most peptide compounding for individual patients occurs under 503A regulation.

55. 503B Outsourcing Facility A federally registered compounding facility that can produce larger batches of compounded drugs without patient-specific prescriptions. Subject to stricter FDA oversight than 503A pharmacies.

56. Research-Grade vs. Pharmaceutical-Grade Research-grade peptides are not manufactured to pharmaceutical standards and are not intended for human use. Pharmaceutical-grade (or compounded clinical-grade) peptides meet sterility, potency, and purity standards required for human administration. Never use research-grade peptides therapeutically.

57. PCAC (Pharmacy Compounding Advisory Committee) The FDA advisory body that evaluates whether specific compounded substances — including peptides — meet the criteria for compounding under DQSA regulations. PCAC decisions directly affect which peptides can be legally compounded for patients.

58. Off-Label Use Prescribing a medication for a purpose, population, or dose not specified in its FDA-approved labelling. Clinically legitimate and common. Most peptide prescriptions outside FDA-approved indications (e.g., tesamorelin outside HIV lipodystrophy) constitute off-label use.

Part 6 — Clinical Monitoring: What Your Lab Work Actually Means

59. IGF-1 Baseline and Monitoring IGF-1 is the primary marker for growth hormone axis activity. A pre-therapy baseline is essential. Monitoring during GH secretagogue therapy ensures the protocol is working and IGF-1 does not exceed age-appropriate reference ranges, which would signal over-stimulation.

60. Metabolic Panel (CMP/BMP) A standard set of blood tests measuring kidney function (creatinine, BUN), liver enzymes (ALT, AST), electrolytes, blood glucose, and other metabolic markers. Used as a safety baseline before starting peptide therapy and for ongoing monitoring. Meto's Comprehensive Metabolic Panel ($199) includes these markers plus HbA1c, fasting insulin, lipids, and inflammation markers — giving your clinician a complete metabolic picture before prescribing.

Peptide Therapy Glossary Terms at a Glance

How Meto Applies This in Clinical Practice

Every term in this glossary connects to something your Meto clinician evaluates. Before prescribing any peptide protocol, your provider will:

1. Review your symptom history and metabolic baseline — via intake assessment
2. Order targeted labs — IGF-1, fasting glucose and insulin, full hormonal panel, liver and kidney markers
3. Design a protocol specific to your biology — selecting peptide class, dose, delivery method, and timing
4. Monitor and adjust — tracking IGF-1 response, metabolic markers, and subjective outcomes across visits

Meto works with semaglutide, tirzepatide, sermorelin, and NAD+, among other therapeutic agents, within a clinician-supervised framework. Your protocol is built on data, not guesswork.

If you are managing insulin resistance, hormonal decline, or body composition concerns — see how Meto approaches prediabetes, PMOS management, perimenopause, and andropause as root-cause conditions.

Conclusion

This peptide therapy glossary gives you 60 precise definitions — across science, delivery, therapeutic classes, hormonal physiology, regulation, and monitoring. That is the full map. Knowing this vocabulary does not make you a clinician. But it makes you a better patient: one who can hold an informed conversation, understand a protocol rationale, and recognise when something is off.

Peptide therapy is not simple. It deserves clinical oversight, proper labs, and an evidence-based approach. That is what Meto provides.

Start your peptide journey fully informed — connect with a Meto clinician today. Get started here →

Frequently Asked Questions

What is the difference between a peptide and a protein?

Peptides are short amino acid chains — typically fewer than 50 amino acids. Proteins are longer and more structurally complex. The distinction is not always rigid, but in therapeutic contexts, peptides are smaller, more targeted, and more easily administered than protein-based drugs. Many therapeutic peptides are fragments of or analogues to larger proteins the body already produces.

How is peptide half-life different from duration of effect?

Half-life measures how long a peptide stays in circulation before being eliminated by 50%. Duration of effect refers to how long the biological action lasts — and these are not the same. GH secretagogues with short half-lives (minutes) can trigger a GH pulse that has downstream effects over hours. CJC-1295 has a very long half-life (days) but still works by stimulating episodic GH release rather than maintaining constant GH elevation.

Are all compounded peptides the same quality?

No. Quality depends entirely on the compounding pharmacy. A 503A pharmacy compounding for an individual patient is held to state-level standards; a 503B outsourcing facility is subject to FDA oversight and GMP requirements. Patients should always confirm their peptide is sourced from a licensed, inspectable facility. Your Meto clinician prescribes only through vetted, compliant compounding pharmacies.

Do I need to understand all 60 terms before starting therapy?

No. The terms most critical before your first consultation are: GH secretagogue, half-life, subcutaneous injection, IGF-1, titration, and compounded peptide. The rest will become relevant as your protocol progresses. This glossary is a reference — return to it as your therapy evolves.

What labs should I have before starting peptide therapy?

At minimum: IGF-1, fasting glucose and insulin, HbA1c, a full lipid panel, liver enzymes (ALT, AST), kidney markers (creatinine, eGFR), and a complete hormonal panel (total and free testosterone, LH, FSH, SHBG, estradiol, and thyroid markers). Meto's Comprehensive Metabolic Panel covers the metabolic side comprehensively. Your clinician will advise on additional hormonal markers based on your symptoms.

Is off-label peptide use safe?

Off-label prescribing is legal, common, and standard in many specialties — it is not a red flag. What matters is whether the prescription is supported by evidence, the prescribing clinician has reviewed your full clinical picture, monitoring is in place, and the compound itself meets pharmaceutical-grade standards. All Meto protocols meet these criteria.

This article is for informational and educational purposes only. It does not constitute medical advice. Consult a licensed clinician before beginning any therapeutic protocol. Meto clinicians are available to review your full clinical picture and guide your care.