Glossary

Peptide glossary

Short, sourced definitions of the terms used across the guides and calculators, from A to Z.

Glossary A to Z

Short, sourced definitions of the terms used across the guides and calculators.

A

Absolute weight: The amount of pure peptide in a product. Bachem gives absolute weight = gross weight x purity x assay.
Acetate salt: A peptide supplied with acetic acid as the counter-ion. Bachem states that most peptides contain trifluoroacetic acid or acetic acid as a bound salt and that its API peptides are sold as acetate salts.
Aggregation: Peptide molecules clumping together. FDA cites the potential for aggregation as an immunogenicity concern for GHRP-6 and related peptides.
Agonist: A compound that binds a receptor and activates it. Many peptides on this site are receptor agonists, for example GLP-1 receptor agonists.
Aliquot: A measured portion of a solution. Bachem advises aliquoting peptide solutions and keeping them frozen below -15 C if they must be stored.
Amino acid: The building block of peptides and proteins; each has an amino group, a carboxyl group and a side chain.
Amino acid analysis: A strong acid breaks a peptide into amino acids, which are separated and quantified. Bachem notes it provides no sequence information.
Amylin analog: A compound that mimics amylin, a peptide hormone co-secreted with insulin; cagrilintide is an example.
Antimicrobial peptide: A short peptide of the innate immune system with activity against microbes; LL-37 (cathelicidin) is an example.
Aseptic technique: Practices that keep injection materials free of contamination. CDC advises always using it when preparing and giving injections.
Assay (content value): The lot-specific content value used with purity to find absolute weight; it is taken from each lot's analytical data sheet.

B

BAC water: Common name for Bacteriostatic Water for Injection, USP: sterile water with 0.9% or 1.1% benzyl alcohol, supplied in multiple-dose vials for diluting or dissolving injectable drugs.
Bacteriostatic: Describes an agent that stops bacteria from reproducing without necessarily killing them; surviving bacteria can multiply again if it is removed.
Batch: FDA regulations define a batch as a specific quantity of a drug or other material intended to have uniform character and quality.
Benzyl alcohol: The preservative in bacteriostatic water, at 0.9% (9 mg/mL) or 1.1% (11 mg/mL). The label is headed "WARNING: NOT FOR USE IN NEONATES" and lists use in neonates as a contraindication because of potential toxicity.
Beyond-use date: The date after which an opened or prepared product should be discarded. CDC: an opened multi-dose vial should be dated and discarded within 28 days unless the manufacturer states another date, and never later than the manufacturer expiration date.
Bioavailability: The fraction of an administered compound that reaches the circulation. It depends on the route; intravenous delivery is the reference.
Blend: Two or more compounds co-formulated in one vial, for example a research-material blend of BPC-157, thymosin beta-4, GHK-Cu and KPV, at a ratio fixed at manufacture.

C

CAS Registry Number: A unique numeric identifier assigned by the Chemical Abstracts Service to a chemical substance described in the open scientific literature.
Category 2 (FDA bulk substances): FDA's interim-policy category for bulk drug substances that may present significant safety risks; GHRP-2, GHRP-6 and ipamorelin were added September 29, 2023.
Certificate of analysis (COA): A lab report for one lot listing tests, methods and results. Bachem calls its version a CofA or Analytical Data Sheet.
Compounded drug: A drug made by a pharmacy or outsourcing facility rather than a manufacturer. FDA states compounded drugs are not approved by FDA.
Concentration: Amount of substance per volume of solution, usually mg/mL. Bachem gives the molar form as absolute weight divided by (volume x molecular weight).
Contraindication: A condition in which a label says a product must not be used. Labels list them in a numbered section, for example the EGRIFTA WR label section 4.
Controlled room temperature: A labelled storage range of 20 to 25 C (68 to 77 F), with excursions permitted to 15 to 30 C, as on the EGRIFTA WR label.
Counter-ion: The ion that balances a peptide's charge in a salt. Bachem notes most peptides also contain trifluoroacetic acid or acetic acid.
Cycle: A planned period of use followed by a break, repeated. Mostly a community convention: a 2026 review lists cycle duration and time since last use as exposure history items.
Cyclic peptide: Polypeptide chains that contain a circular sequence of bonds, for example through a connection between the amino and carboxyl ends (cyclosporin) or a disulfide bond between two cysteines (oxytocin).

D

DAC (drug affinity complex): A moiety that extends CJC-1295's half-life; with DAC the estimated half-life was 5.8 to 8.1 days, and the no-DAC form carries no such moiety.
Desensitization: A reduced response after repeated or continuous exposure. In perifused rat pituitary cells, GHRP-6 and GHRH each desensitized discrete pathways; cells perifused 1 min with GHRP-6 needed 60 min in medium alone to regain GHRP-6 responsiveness.
Desiccator: A sealed container with a drying agent. Bachem advises letting a peptide container reach ambient temperature in one before opening.
Diluent: The liquid added to a lyophilised powder. Product labels name it: one label specifies Sterile Water for Injection and another Bacteriostatic Water for Injection.
Disulfide bond: A linkage of two bonded sulfur atoms (R-S-S-R'). Disulfide bridges are a common post-translational modification in proteins.

E

Endotoxin: A collective term for components of the outer membrane of Gram-negative bacteria (lipopolysaccharide), tested for in injectable products.
Endotoxin unit (EU): The unit of endotoxin activity. FDA's archived guide gives 0.25 EU/mL for Water for Injection and 0.5 EU/mL for Bacteriostatic Water for Injection.

G

GH secretagogue: A compound that stimulates growth hormone release through the ghrelin (GHS) receptor, such as GHRP-6 and ipamorelin.
GHRH analog: A compound that mimics growth-hormone-releasing hormone; examples are sermorelin, tesamorelin and CJC-1295.
GIP/GLP-1 dual agonist: A compound that activates both the GIP and GLP-1 receptors; tirzepatide is described this way on its label.
GLP-1 receptor agonist: A compound that activates the glucagon-like peptide-1 receptor; semaglutide is an example with an approved once-weekly label.

H

Half-life: The time for the concentration of a substance, such as a medication, to fall by half. Steady state takes about 4 to 5 half-lives.
Heavy metals (USP 232): A pharmacopeial test for elemental impurities. A COA example reports heavy metals below 0.01 ppm by USP 232.
HPLC: High-performance liquid chromatography: separates a sample into components by pumping it through a packed column. The standard purity method for peptides.
Human equivalent dose (HED): A dose in humans derived from an animal dose by scaling on a mg/m2 basis, as defined in FDA's 2005 guidance on maximum safe starting doses.

I

IGF-1: Insulin-like growth factor 1, a hormone driven by growth hormone and used as a surrogate marker in GH-axis studies.
Immunogenicity: The potential of a product to provoke an immune response. FDA cites immunogenicity risk from aggregation and peptide-related impurities for several peptides.
In vitro: Done in a dish or test tube, outside a living organism, such as a cell-line study.
In vivo: Done in a living organism, such as an animal or human study.
Insulin syringe: A syringe graduated in units to match insulin strength. One common 0.3 mL model has 0.01 mL graduations and a 31 G needle that is 5/16 in (8 mm) long.
Intramuscular injection: An injection into muscle. A diabetes injection consensus advises avoiding it with long-acting insulin because severe hypoglycemia may result.
ISO/IEC 17025: The international standard for the competence, impartiality and consistent operation of testing and calibration laboratories (2017 edition).

K

Karl Fischer titration: A method for water content. Bachem lists it as the test for water content of a peptide.

L

LAL test: The Limulus Amebocyte Lysate test, also called the bacterial endotoxins test, used to measure endotoxin activity.
LC-MS/MS: Liquid chromatography coupled to tandem mass spectrometry; a COA example states identity confirmed via LC-MS/MS.
Lipohypertrophy: Lumpy tissue at overused injection sites. A 2016 consensus says it distorts insulin absorption, injections should not be given into it, and site rotation is advised.
Lot: FDA regulations define a lot as a specific identified portion of a batch having uniform character and quality within specified limits.
Lot number: Per 21 CFR 210.3, any distinctive combination of letters, numbers or symbols from which the complete history of the manufacture, processing, packing, holding and distribution of a batch or lot can be determined.
Lyophilisate: The freeze-dried peptide powder in a sealed vial. Bachem advises storing it tightly closed below -15 C and letting it reach room temperature before opening.
Lyophilisation (freeze drying): A low-temperature dehydration process that freezes the material, lowers the pressure and lets the ice turn directly into vapour. Produces the dry powder in most peptide vials.

M

m/z: Mass-to-charge ratio, the x-axis of a mass spectrum. A multiply charged ion appears at a fraction of the molecular mass.
Maintenance dose: The dose continued after titration. The Zepbound label lists maintenance of 5 mg, 10 mg or 15 mg once weekly after escalation in 2.5 mg steps.
MALDI-MS: Matrix-assisted laser desorption/ionisation mass spectrometry; another COA example lists identity by MALDI-MS.
Mass spectrometry (MS): Measures the mass-to-charge ratio of ions to confirm a molecule's mass. Used for peptide identity testing.
Melanocortin receptor agonist: A compound that activates melanocortin receptors; bremelanotide (PT-141) and melanotan II are examples.
Molecular weight: The mass of one mole of a compound in g/mol. Bachem states the molecular weight of a peptide is determined by mass spectrometry.
Multi-dose vial: A vial from which more than one withdrawal is intended. CDC guidance, citing USP General Chapter 797, is that it be dated and discarded within 28 days of opening.

N

Needle gauge: A number giving needle diameter; a higher gauge means a thinner needle. Needles in common medical use run from 7 gauge (largest) to 34 (smallest).
Net peptide content: The percentage of peptide relative to non-peptide material, mostly counter-ions and moisture. It is not the same as HPLC purity.
NOAEL: No observed adverse effect level: the highest tested animal dose without adverse effects, converted to a human equivalent dose in FDA's 2005 guidance.

O

Off-label use: Unapproved use of an approved drug. FDA notes it has not determined such uses to be safe and effective.
Oxidation: A chemical degradation pathway. Bachem notes peptides containing Trp, Met or Cys need special care to avoid it.

P

Pen needle: A short needle for a pen injector. A 2016 consensus on diabetes injections recommends 4 mm pen needles and 6 mm syringe needles as first choice for all patients.
Peptide: A short chain of amino acids linked by peptide bonds. Larger chains are called proteins.
Peptide hormone: A hormone made of amino acids, such as insulin or growth-hormone-releasing hormone. Several compounds on this site are analogues of peptide hormones.
Peptide-related impurities: Unwanted peptides formed during synthesis, such as truncated or modified sequences. FDA cites them as a risk for several unapproved peptides.
Pharmacokinetics: What the body does to a compound: absorption, distribution, metabolism and elimination. Half-life is a pharmacokinetic measure.
Placebo: An inactive comparator given instead of the study compound, used so effects can be attributed to the compound.
Purity: The percentage of the target peptide among UV-absorbing species in one HPLC run, typically read at 210 to 220 nm as main-peak area over total peak area.

R

Randomized controlled trial: A study that randomly assigns participants to groups, one of which gets a comparator, to compare outcomes. Grade B on this site.
Reconstitution: Adding a diluent to a lyophilised powder to make a solution. Labels state the diluent, its volume and the mixing method (for example swirl, do not shake).
Research use only (RUO): Labelling that material is supplied for controlled laboratory research only, not for human or veterinary use.
Retention time: How long a component takes to come off an HPLC column, in minutes. Reported with the chromatogram to show which peak is which.
RP-HPLC: Reversed-phase HPLC, which separates by hydrophobicity using a non-polar column. A COA example states purity by RP-HPLC at 214 nm.

S

Section 503A: The part of the FD&C Act covering pharmacy compounding exempt from certain requirements, including a condition of valid patient-specific prescriptions.
Section 503B: The part of the FD&C Act creating outsourcing facilities, which are subject to CGMP requirements and may supply drugs on a provider's order.
Sharps container: An FDA-cleared puncture-resistant container for used needles and syringes. Labels say not to recap needles and not to put loose sharps in household trash.
Single-dose vial: Per CDC, a single-dose vial of liquid medication intended for injection or infusion is approved for use in a single patient, for a single case, procedure or injection. CDC says not to pool or store leftover contents for later use.
Site rotation: Changing the injection site each time. Labels advise it to decrease injection site reactions; one label says not to inject into scar tissue, bruises or the navel.
Stack: Using two or more separate compounds in the same period. A 2026 review notes clinicians should ask whether agents were combined (stacked).
Sterile Water for Injection: Sterile diluent supplied with the 2 mg tesamorelin kit, whose label says to administer immediately after reconstitution and throw away unused solution and diluent.
Sterility test (USP 71): A pharmacopeial test for viable microorganisms. A COA example reports sterility as not detected by USP 71.
Subcutaneous injection: An injection into the fat layer under the skin, not into muscle or a vein. Common sites are the abdomen, thigh and buttock.
Syringe unit: A volume marking on a U-100 syringe: 1 unit equals 0.01 mL (1 mL / 100 units). It is not a unit of mass; mass follows only from the solution's concentration.

T

Third-party testing: Testing by a laboratory independent of the seller. One service describes tests of identity, purity and quantity, with a standard test that adds endotoxins.
Thymosin: Thymosin beta-4 is a 44-amino-acid protein in humans, encoded by the TMSB4X gene. TB-500 is described in a doping-control paper as a synthetic version of an active region of thymosin beta-4.
Titration: Stepwise adjustment of a dose. The Ozempic label increases the dosage only after at least 4 weeks on the prior dosage.
Trifluoroacetic acid (TFA): An acid that, per Bachem, most peptides contain bound as a salt (with acetic acid) in the finished powder; it is not measured by the methods that determine purity.
Tuberculin syringe: A syringe read in mL. A review of concentrated insulin shows the risk of confusing it with a U-100 syringe: 5 units of U-500 reads 1 unit on U-100 but 0.01 mL on a tuberculin syringe.

U

U-100: Standard insulin strength of 100 units per 1 mL. Insulin syringes for U-100 are marked in units, and each small notch on a 1 mL insulin syringe is 1 unit.
USP: The United States Pharmacopeia, which publishes quality standards and general chapters for drug substances and products.

W

Washout: The period after stopping a compound for its effect to clear. A 2026 review advises retesting after washout before pursuing imaging when hormone tests are abnormal.