Plain-language definitions for the terms you'll encounter in peptide research.
Any unwanted medical occurrence in a person using a treatment, whether or not it was caused by the treatment itself. Clinical trials track adverse events to build a safety profile. Not all adverse events are side effects — some may be coincidental.
Also known as: AE
See also: Side effect, Contraindication
A molecule that binds to a receptor and activates it, producing a biological response. Many therapeutic peptides are receptor agonists — for example, semaglutide is a GLP-1 receptor agonist that mimics the natural hormone GLP-1.
See also: Antagonist, Receptor, Binding affinity
Read more →The building blocks of peptides and proteins. There are 20 standard amino acids encoded by human DNA, each with a unique chemical structure. Peptides are short chains of amino acids linked together by peptide bonds.
See also: Peptide bond, Sequence
Read more →A synthetic version of a natural molecule that has been chemically modified to improve its properties — such as lasting longer in the body, being more potent, or resisting breakdown. Semaglutide is an analog of the natural hormone GLP-1.
Also known as: analogue
A molecule that binds to a receptor but blocks it instead of activating it, preventing the natural signal from getting through. Antagonists are used therapeutically to dampen overactive biological pathways.
When the immune system recognizes a peptide as foreign and produces antibodies against it. This can reduce the peptide's effectiveness over time or, rarely, cause allergic reactions. Also called immunogenicity.
Also known as: anti-drug antibodies, ADA
See also: Immunogenicity, Tachyphylaxis
Sterile water that contains 0.9% benzyl alcohol as a preservative. Used to reconstitute lyophilized peptides. Unlike plain sterile water, bacteriostatic water inhibits bacterial growth, allowing a reconstituted vial to be used over multiple days.
Also known as: BAC water
See also: Reconstitution, Sterile water, Lyophilized
Read more →How strongly a molecule attaches to its target receptor. Higher binding affinity means the molecule binds more tightly and is more likely to produce an effect at lower concentrations. Measured in laboratory assays and reported as a dissociation constant (Kd).
The percentage of a dose that actually reaches your bloodstream and is available to work. Injected peptides typically have near-100% bioavailability, while oral peptides lose most of their dose to digestion — which is why most peptides are injected rather than swallowed.
See also: Oral bioavailability, Subcutaneous injection
A measurable biological indicator — like a blood test result, hormone level, or imaging finding — used to track health status or treatment response. Examples include HbA1c for blood sugar control and IGF-1 levels for growth hormone activity.
See also: Endpoint, Primary outcome
The raw active ingredient used by compounding pharmacies to prepare custom medications. The FDA maintains a list of bulk drug substances that may be used in compounding. Whether a peptide is on this list determines whether pharmacies can legally compound it.
See also: Compounding pharmacy, 503A pharmacy, 503B outsourcing facility
A detailed medical account of a single patient's experience with a treatment. Case reports can flag interesting observations but cannot prove a treatment works — they lack the controls needed to rule out coincidence.
See also: Randomized controlled trial, Cohort study
A document from a laboratory that reports the test results for a specific batch of a product — typically including purity percentage, identity confirmation, and contaminant screening. A credible COA should come from an independent, third-party lab.
Also known as: COA
See also: HPLC, Good manufacturing practice, Purity
Read more →A study that follows a group of people over time to see how a treatment or exposure affects outcomes. Stronger than case reports because it includes more people, but weaker than randomized trials because participants aren't randomly assigned.
See also: Randomized controlled trial, Case report
A pharmacy that custom-prepares medications to meet individual patient needs, rather than dispensing mass-manufactured drugs. Compounding pharmacies are the primary legal channel for many peptides. They operate under either 503A or 503B federal designations.
Also known as: compounder
See also: 503A pharmacy, 503B outsourcing facility
Read more →A specific situation in which a treatment should not be used because it could be harmful. For example, GLP-1 agonists like semaglutide are contraindicated in people with a personal or family history of medullary thyroid carcinoma.
See also: Adverse event, Drug interaction
A small signaling protein released by cells to coordinate immune and inflammatory responses. Some peptides, like thymosin alpha-1, work by influencing cytokine production — either boosting immune activity or calming excessive inflammation.
See also: Neuropeptide, Peptide hormone
Read more →The relationship between how much of a treatment you take and how strong the effect is. A clear dose-response relationship — where higher doses produce stronger effects — is one sign that a treatment is genuinely working rather than producing a placebo effect.
See also: Loading dose, Maintenance dose
How often a treatment is taken — daily, weekly, twice weekly, etc. Dosing frequency depends on how quickly the body breaks down the peptide (its half-life). Longer-acting peptides like semaglutide need only weekly dosing.
See also: Half-life, Loading dose, Maintenance dose
A study design where neither the participants nor the researchers know who is receiving the real treatment versus a placebo. This prevents bias from influencing the results and is considered the gold standard for clinical research.
See also: Randomized controlled trial, Placebo, Open-label
When the body reduces the number or sensitivity of receptors in response to sustained stimulation. This can make a treatment less effective over time, which is why some peptide protocols include breaks or cycling periods.
See also: Upregulation, Tachyphylaxis, Receptor
When one medication affects how another medication works — making it stronger, weaker, or causing unexpected side effects. GLP-1 agonists, for example, slow stomach emptying, which can change how quickly oral medications are absorbed.
See also: Contraindication, Adverse event
The Dietary Supplement Health and Education Act of 1994. This U.S. law defines what counts as a dietary supplement and limits the FDA's ability to regulate supplements before they reach the market. Peptides generally do not qualify as dietary supplements under DSHEA.
Also known as: Dietary Supplement Health and Education Act
See also: FDA, Off-label use
A number that tells you how large a treatment's impact actually is, separate from whether the result is statistically significant. A treatment can be statistically significant but have a tiny effect size — meaning it works, but barely.
See also: Statistical significance, P-value
The European Medicines Agency — the European Union's equivalent of the FDA. The EMA evaluates and approves medications for use across EU member states. A peptide approved by the EMA may or may not also be approved by the FDA, and vice versa.
Also known as: European Medicines Agency
See also: FDA
Produced naturally within the body. GLP-1, growth hormone, and BPC-157 are all endogenous peptides — your body already makes them. Therapeutic peptides are often synthetic versions of endogenous molecules, sometimes modified to last longer.
See also: Analog, Peptide hormone
The specific outcome a clinical trial is designed to measure. The primary endpoint is the main question the study is trying to answer (e.g., 'does this peptide reduce body weight?'). Secondary endpoints are additional measurements tracked alongside it.
Also known as: primary endpoint, secondary endpoint
See also: Primary outcome, Randomized controlled trial
An inactive ingredient added to a drug formulation for stability, preservation, or delivery — not for therapeutic effect. Mannitol is a common excipient in lyophilized peptides, used to create the powder cake and protect the peptide during freeze-drying.
See also: Mannitol, Lyophilized
The U.S. Food and Drug Administration — the federal agency that approves medications, regulates compounding pharmacies, and enforces drug safety standards. FDA approval means a drug has been rigorously tested in clinical trials and found safe and effective for a specific use.
Also known as: Food and Drug Administration
See also: New drug application, Investigational new drug, EMA
A measurement of needle thickness. Higher gauge numbers mean thinner needles. Subcutaneous peptide injections typically use 27-31 gauge needles — thin enough to minimize discomfort. Insulin syringes commonly come in 29 or 31 gauge.
See also: Insulin syringe, Subcutaneous injection
Growth hormone — a peptide hormone produced by the pituitary gland that regulates growth, metabolism, and body composition. Several therapeutic peptides (sermorelin, CJC-1295, ipamorelin) work by stimulating your body to produce more of its own growth hormone rather than injecting GH directly.
Also known as: growth hormone, human growth hormone, HGH
Read more →Growth hormone-releasing hormone — the natural signal from the hypothalamus that tells the pituitary gland to release growth hormone. Peptides like sermorelin and CJC-1295 are GHRH analogs that mimic this signal.
Also known as: growth hormone-releasing hormone
Read more →Glucagon-like peptide-1 — a hormone produced in the gut after eating that helps regulate blood sugar and appetite. GLP-1 receptor agonists like semaglutide and tirzepatide are among the most widely prescribed peptide-based medications, primarily for type 2 diabetes and weight management.
Also known as: glucagon-like peptide-1
Read more →A set of quality standards for producing pharmaceuticals that ensure products are consistently made, tested, and meet purity specifications. GMP (or cGMP, where the 'c' stands for 'current') compliance is required for 503B outsourcing facilities and is a key quality signal when evaluating peptide sources.
Also known as: GMP, cGMP
See also: 503B outsourcing facility, Certificate of analysis, USP
The time it takes for half of a substance to be cleared from the body. A short half-life (minutes) means the peptide is broken down quickly and may need frequent dosing. Chemical modifications like fatty acid chains can extend a peptide's half-life from minutes to days.
See also: Bioavailability, Dosing frequency
High-performance liquid chromatography — a laboratory technique used to separate, identify, and quantify the components in a mixture. HPLC is the standard method for measuring peptide purity. A certificate of analysis should report an HPLC purity percentage, typically 95% or higher for research-grade peptides.
Also known as: high-performance liquid chromatography
See also: Purity, Mass spectrometry, Certificate of analysis
Insulin-like growth factor 1 — a hormone produced mainly by the liver in response to growth hormone. IGF-1 mediates many of growth hormone's effects on tissue growth and repair. Blood levels of IGF-1 are commonly used as a biomarker for growth hormone activity.
Also known as: insulin-like growth factor 1
The tendency of a substance to trigger an immune response. When a therapeutic peptide is immunogenic, the body may produce antibodies against it, potentially reducing its effectiveness or causing allergic reactions over time.
See also: Antibody formation, Adverse event
A small syringe originally designed for insulin but widely used for peptide injections. Comes in standard sizes (0.3 mL, 0.5 mL, 1 mL) with fine-gauge needles (29-31G). The markings are in 'units' — 100 units equals 1 mL.
See also: Gauge, Subcutaneous injection, Units
An injection delivered into muscle tissue, typically the deltoid (upper arm) or vastus lateralis (thigh). Less common for peptides than subcutaneous injection, but used for some formulations that require deeper tissue delivery.
Also known as: IM injection
See also: Subcutaneous injection, Intravenous
Delivered through the nose, typically as a spray. Some peptides like selank are available in intranasal formulations, which can provide a non-injection route of administration. Absorption through nasal membranes allows peptides to bypass the digestive system.
See also: Subcutaneous injection, Oral bioavailability
Read more →Delivered directly into a vein, providing immediate and complete absorption. IV administration is common in clinical settings but rarely used for at-home peptide protocols. NAD+ infusions are a well-known example of IV peptide-adjacent therapy.
Also known as: IV
See also: Subcutaneous injection, Intramuscular injection
An FDA designation that allows a new drug to be used in clinical trials on human subjects. An IND application must be filed before human testing can begin. Many peptides of interest are at the IND stage — being studied in humans but not yet approved.
Also known as: IND
See also: New drug application, FDA
A neuropeptide that plays a central role in triggering puberty and regulating reproductive hormones. Kisspeptin signals the brain to release GnRH (gonadotropin-releasing hormone), which in turn stimulates the production of sex hormones like testosterone and estrogen.
See also: Neuropeptide, Peptide hormone
Read more →The dose of a substance that is lethal to 50% of test animals. LD50 is a standard (if blunt) measure of acute toxicity used in preclinical safety research. A very high LD50 relative to therapeutic doses suggests a wide margin of safety.
Also known as: lethal dose 50
See also: Therapeutic index, Toxicology
A higher initial dose given at the start of treatment to quickly build up the peptide's concentration in the body. After the loading phase, the dose is typically reduced to a lower maintenance dose. Not all peptide protocols use loading doses.
See also: Maintenance dose, Dosing frequency
Freeze-dried. Most research peptides are sold as a lyophilized powder — a dry, fluffy cake in a sealed vial. Freeze-drying removes water while preserving the peptide's structure, making it stable for storage. The powder must be reconstituted with bacteriostatic water before use.
Also known as: freeze-dried, lyophilization
See also: Reconstitution, Bacteriostatic water, Excipient
Read more →The ongoing dose used after an initial loading phase, designed to sustain therapeutic levels without overexposure. The maintenance dose is typically lower than the loading dose.
See also: Loading dose, Dosing frequency
A sugar alcohol commonly used as an excipient in lyophilized peptide formulations. Mannitol helps form the visible powder cake in a vial and protects the peptide's structure during the freeze-drying process. It has no therapeutic effect.
See also: Excipient, Lyophilized
A laboratory technique that identifies molecules by measuring their mass. Used alongside HPLC to confirm that a peptide has the correct molecular weight and structure. A certificate of analysis that includes mass spectrometry data provides stronger identity confirmation.
Also known as: MS, mass spec
See also: HPLC, Purity, Certificate of analysis
The specific biological process by which a treatment produces its effects. Understanding a peptide's mechanism of action — which receptors it binds, which pathways it activates — helps predict both its benefits and potential side effects.
Also known as: MOA
A statistical method that combines data from multiple studies on the same topic to arrive at a more reliable overall conclusion. Meta-analyses sit near the top of the evidence hierarchy because they pool data from many trials, reducing the risk that any single study's flaws skew the picture.
See also: Systematic review, Randomized controlled trial
One millionth of a gram (0.000001 g). Abbreviated as mcg or µg. Many peptides are dosed in micrograms because they are potent at very small amounts. For context, 1 milligram equals 1,000 micrograms.
Also known as: mcg, µg
One thousandth of a gram (0.001 g). Abbreviated as mg. Some peptides are dosed in milligrams (e.g., semaglutide at 0.25-2.4 mg weekly), while others use micrograms. Peptide vials are typically labeled by total milligrams of peptide content.
Also known as: mg
The total mass of all atoms in a molecule, measured in daltons (Da). Peptide molecular weights typically range from 500 to 5,000 Da. Molecular weight affects how a peptide is absorbed, distributed, and eliminated from the body.
Also known as: MW
See also: Amino acid, Sequence
A peptide that acts as a chemical messenger in the nervous system, influencing mood, pain, stress, and other brain functions. Selank is an example of a synthetic neuropeptide studied for its anti-anxiety and cognitive effects.
See also: Peptide hormone, Cytokine
Read more →The formal request a drug company submits to the FDA to get approval to sell a new medication. An NDA includes all clinical trial data, manufacturing details, and proposed labeling. FDA-approved peptides like semaglutide and tirzepatide went through this process.
Also known as: NDA
See also: FDA, Investigational new drug
Using an FDA-approved medication for a purpose, population, or dose that wasn't part of its official approval. Off-label prescribing is legal and common — doctors can prescribe approved drugs for any use they deem appropriate. Many peptide uses are off-label.
See also: FDA, Research use only
A study design where both researchers and participants know who is receiving the treatment. Open-label studies are useful for tracking safety and tolerability but are more prone to bias than double-blind studies because expectations can influence reported outcomes.
See also: Double-blind, Placebo
The percentage of an orally taken dose that survives digestion and reaches the bloodstream. Most peptides have near-zero oral bioavailability because stomach acid and enzymes destroy them. Oral semaglutide uses a special absorption enhancer (SNAC) to achieve about 1% bioavailability — enough to be therapeutically useful at higher doses.
See also: Bioavailability, Subcutaneous injection
A number that tells you how likely a study's result would be if the treatment had no real effect. A p-value below 0.05 is conventionally considered 'statistically significant' — meaning there's less than a 5% chance the result is due to random chance alone. But a low p-value doesn't tell you how large or clinically meaningful the effect is.
See also: Statistical significance, Effect size
The chemical link that connects one amino acid to the next in a peptide chain. Peptide bonds form through a condensation reaction and are broken by digestive enzymes — which is why most peptides can't be taken orally.
See also: Amino acid, Sequence
Read more →The FDA's classification system for peptides in the context of compounding. Category 1 peptides can be compounded by pharmacies. Category 2 peptides are under evaluation. Category 3 peptides cannot be compounded due to safety or other concerns. Reclassification between categories is an ongoing process that directly affects patient access.
Also known as: FDA peptide categories, Category 1, Category 2, Category 3
See also: FDA, Compounding pharmacy, Bulk drug substance
Read more →A hormone made of amino acid chains that carries signals through the bloodstream to distant organs. Insulin, growth hormone, and GLP-1 are peptide hormones. Unlike steroid hormones, peptide hormones cannot cross cell membranes and must bind to receptors on the cell surface.
See also: Receptor, Endogenous, Neuropeptide
Read more →What a drug does to the body — its biological effects, mechanism of action, and the relationship between concentration and response. In simpler terms: pharmacokinetics asks 'how does the body handle this drug?' while pharmacodynamics asks 'how does this drug affect the body?'
Also known as: PD
See also: Pharmacokinetics, Mechanism of action, Dose-response
How the body absorbs, distributes, metabolizes, and eliminates a drug over time. Pharmacokinetic studies tell us how quickly a peptide reaches peak levels, how long it stays active, and how the body clears it — all of which determine dosing schedules.
Also known as: PK
See also: Pharmacodynamics, Half-life, Bioavailability
The first stage of testing a new drug in humans, focused primarily on safety. Phase I trials typically enroll 20-80 healthy volunteers to determine safe dosage ranges, identify side effects, and study how the body processes the drug.
See also: Phase II trial, Phase III trial, Investigational new drug
The second stage of human testing, focused on whether a drug actually works for its intended condition. Phase II trials typically enroll 100-300 patients and test different doses to find the most effective one. Many promising drugs fail at this stage.
See also: Phase I trial, Phase III trial
Large-scale trials (typically 1,000-5,000+ patients) designed to confirm a drug's effectiveness and monitor side effects in a broader population. Phase III trials are the final step before seeking FDA approval. Semaglutide's STEP and SUSTAIN programs were Phase III trials.
See also: Phase II trial, New drug application, Randomized controlled trial
An inactive treatment — like a saline injection or sugar pill — given to a control group in a clinical trial. Comparing outcomes between the treatment group and the placebo group reveals how much of the observed effect is actually caused by the drug, versus the patient's belief that they are receiving treatment.
See also: Double-blind, Randomized controlled trial
The single most important result a clinical trial is designed to measure. The entire study is powered (sized) to detect a meaningful difference in this outcome. Results for the primary outcome carry the most weight when evaluating whether a treatment works.
See also: Endpoint, Statistical significance
The percentage of a peptide product that is the intended molecule, as opposed to contaminants, degradation products, or synthesis byproducts. Measured by HPLC. Research-grade peptides typically have 95-99%+ purity. Higher purity reduces the risk of unwanted reactions.
See also: HPLC, Certificate of analysis
A study where participants are randomly assigned to receive either the treatment being tested or a comparison (often a placebo). Randomization helps ensure the groups are comparable, making RCTs the strongest type of evidence for whether a treatment works.
Also known as: RCT
See also: Double-blind, Placebo, Meta-analysis
A protein on or inside a cell that acts like a lock — when the right molecule (the 'key') binds to it, it triggers a specific biological response. Peptide hormones and therapeutic peptides work by binding to receptors. Each receptor type responds to specific molecules.
See also: Agonist, Antagonist, Binding affinity
Read more →The process of adding sterile liquid (usually bacteriostatic water) to a vial of lyophilized peptide powder to create an injectable solution. Proper reconstitution technique — including gentle mixing (no shaking) and correct volume — is essential for accurate dosing.
See also: Lyophilized, Bacteriostatic water, Sterile water
Read more →A label indicating that a product is sold exclusively for laboratory research and not for human consumption. 'Research use only' (RUO) peptides exist in a legal gray area — they are often purchased by individuals for personal use, but this is not their intended or legally sanctioned purpose.
Also known as: RUO
See also: Off-label use, FDA
The specific order of amino acids in a peptide chain, written from the N-terminus (start) to the C-terminus (end). The sequence determines the peptide's three-dimensional shape and biological function. Even a single amino acid change can dramatically alter how a peptide works.
See also: Amino acid, Peptide bond
An unwanted effect caused by a treatment, distinct from the intended therapeutic effect. Unlike an adverse event (which may be coincidental), a side effect has a known or plausible causal link to the treatment. Nausea with GLP-1 agonists is a well-documented side effect.
See also: Adverse event, Contraindication
The substance being dissolved in a solution. When you reconstitute a peptide, the lyophilized powder is the solute and the bacteriostatic water is the solvent. The concentration of your solution depends on how much solute you dissolve in how much solvent.
See also: Solvent, Reconstitution
The liquid used to dissolve another substance. In peptide preparation, bacteriostatic water or sterile water serves as the solvent. The volume of solvent you add to a vial determines the concentration of your reconstituted peptide solution.
See also: Solute, Bacteriostatic water, Reconstitution
A result is statistically significant when the data suggest the observed effect is unlikely to be due to chance alone (conventionally, p < 0.05). Statistical significance tells you a result is real, but not necessarily meaningful — a statistically significant weight loss of 0.5 pounds isn't clinically useful.
See also: P-value, Effect size
Water that has been purified and sterilized but contains no preservative. Unlike bacteriostatic water, sterile water does not inhibit bacterial growth, so a vial reconstituted with sterile water should ideally be used within 24 hours.
See also: Bacteriostatic water, Reconstitution
An injection into the fat layer just beneath the skin, typically in the abdomen, thigh, or upper arm. This is the most common method of administering peptides at home. Subcutaneous injections are generally less painful than intramuscular injections and allow for slow, steady absorption.
Also known as: subQ, SC injection
See also: Intramuscular injection, Insulin syringe, Gauge
The laboratory process of building a peptide chain one amino acid at a time. Solid-phase peptide synthesis (SPPS) is the standard method, where amino acids are attached sequentially to a resin support. The quality of synthesis directly affects purity.
See also: Purity, Sequence, Amino acid
A rigorous, structured summary of all available research on a specific question. Unlike a simple literature review, a systematic review follows a predefined protocol to find, evaluate, and synthesize all relevant studies. When combined with statistical pooling, it becomes a meta-analysis.
See also: Meta-analysis, Randomized controlled trial
A rapid decrease in response to a treatment after repeated doses. The body essentially 'gets used to' the stimulus. Tachyphylaxis is why some peptide protocols include cycling — periods on and off treatment — to maintain effectiveness.
See also: Downregulation, Dosing frequency
The ratio between a drug's toxic dose and its therapeutic dose. A wide therapeutic index means there's a large safety margin between the dose that helps and the dose that harms. Peptides generally have favorable therapeutic indices compared to many small-molecule drugs.
See also: LD50, Dose-response
Applied to the skin surface — as a cream, gel, serum, or patch. GHK-Cu is a well-known topical peptide used in skincare. Topical peptides avoid the challenges of injection but are limited by how well they penetrate the skin barrier.
See also: Transdermal, Subcutaneous injection
Read more →The study of how substances can harm living organisms. Toxicology studies are a required step in drug development, conducted in animals before human trials begin. They establish safe dosage ranges and identify which organs might be affected at high doses.
See also: LD50, Therapeutic index, Phase I trial
Delivered through the skin and into the bloodstream, typically via a patch or specialized formulation. Transdermal delivery provides sustained, steady absorption and avoids both injection and digestive breakdown. Few peptides have effective transdermal formulations due to their molecular size.
See also: Topical, Subcutaneous injection
A standardized measure of biological activity, commonly marked on insulin syringes. On a U-100 insulin syringe, 100 units equals 1 mL. When reconstituting peptides, you calculate your dose in micrograms or milligrams and convert to the equivalent volume in units based on your reconstitution concentration.
Also known as: IU, international units
See also: Insulin syringe, Microgram, Milligram
When the body increases the number or sensitivity of receptors in response to a stimulus — the opposite of downregulation. Some therapeutic strategies aim to upregulate beneficial pathways. For example, BPC-157 is thought to upregulate growth factor receptors involved in tissue repair.
See also: Downregulation, Receptor
Read more →The United States Pharmacopeia — an independent organization that sets quality standards for medications, supplements, and their ingredients. USP-grade ingredients meet specific purity and quality benchmarks. Compounding pharmacies following USP standards (particularly USP <797> and <800>) are held to higher preparation quality.
Also known as: United States Pharmacopeia
See also: Good manufacturing practice, Purity
A small sealed glass container used to store lyophilized peptides or reconstituted solutions. Peptide vials typically have a rubber stopper that you pierce with a needle to add reconstitution liquid or withdraw doses. Vials should be stored according to manufacturer instructions — usually refrigerated after reconstitution.
See also: Lyophilized, Reconstitution
A break between treatment phases in a clinical trial, long enough for the previous treatment to be fully cleared from the body. Washout periods prevent one treatment from carrying over and affecting the results of the next. The length depends on the drug's half-life.
See also: Half-life, Randomized controlled trial