Peptide Glossary: Units, mcg, Reconstitution and COA Terms Explained

Plain-language definitions of peptide terms: bacteriostatic water, reconstitution, mcg vs mg vs IU, syringe units, half-life, titration, COA, HPLC and more.

Bacteriostatic water: Sterile water with 0.9% benzyl alcohol, which stops bacteria growing. It is used to dissolve freeze-dried peptides so the vial can be used for several doses.
Sterile water: Water with no preservative. Once opened it should be used once, so it is a poor choice for multi-dose vials.
Lyophilized: Freeze-dried. Most peptides ship as a white powder or "cake" under vacuum, which keeps far longer than a liquid.
Reconstitution: Adding water to a freeze-dried peptide to dissolve it. The amount of water sets the concentration and therefore how many units each dose is.
Concentration: How much peptide is in each mL of liquid, for example 5 mg/mL. It equals the vial size divided by the water added.
mg (milligram): One thousandth of a gram. Vial sizes are given in mg.
mcg (microgram): One thousandth of a milligram: 1 mg = 1,000 mcg. Many peptide doses are given in mcg, so mixing the two up is a 1,000× error.
IU (international unit): A unit based on biological activity, used for hormones such as HGH and hCG. It is not the same as the "units" marked on an insulin syringe.
Units (syringe): The tick marks on a U-100 insulin syringe. 100 units = 1 mL, so 10 units = 0.1 mL. A number of units means nothing without the concentration.
U-100 insulin syringe: The standard insulin syringe, marked in units where 100 units = 1 mL. Common sizes hold 0.3 mL (30 units), 0.5 mL (50 units) or 1 mL (100 units).
Gauge: Needle thickness. A higher number is thinner: 31g is finer than 27g.
Subcutaneous (SubQ): Injected into the fat layer under the skin, usually the abdomen or thigh. Most peptides are given this way.
Intramuscular (IM): Injected into a muscle such as the deltoid or glute. Oil-based steroids are usually given this way.
Half-life: The time it takes for half of a dose to be cleared from the body. It drives how often a compound is dosed.
Titration: Starting at a low dose and raising it in steps, so the body can adjust and side effects stay manageable. GLP-1 drugs are titrated every 4 weeks or more.
Cycle: A planned period on a compound, often followed by a break ("off" period).
Stack: Two or more compounds used in the same period.
Blend: Two or more peptides sold together in one vial, such as CJC-1295 with ipamorelin. Each peptide's dose depends on its share of the vial.
Beyond-use date: The date after which a mixed vial should be thrown away. For reconstituted peptides in a fridge this is often about 28 days, but it varies by compound.
COA (certificate of analysis): A lab report for a batch, usually showing purity (HPLC) and identity (mass spectrometry). It is only useful if it names the lab and matches the batch you hold.
HPLC: High-performance liquid chromatography: a lab test that separates a sample to measure purity, reported as a percentage.
Mass spectrometry (MS): A lab test that measures molecular weight to confirm the compound is what the label says.
Endotoxin: Toxic fragments of bacteria that survive sterilisation and can cause fever and inflammation if injected. Measured with the LAL test.
Research use only (RUO): A label meaning a product is sold for laboratory research and is not approved for human use. FDA has said this label does not protect sellers whose websites show the product is meant for people.
Compounding: A pharmacy preparing a medicine for a specific patient from bulk ingredients. US rules (sections 503A and 503B) limit which ingredients can be used.
Category 2 (FDA): FDA's list of bulk substances it says raise significant safety concerns, which compounding pharmacies should not use while they are under review.
GLP-1 receptor agonist: A drug that mimics the gut hormone GLP-1, slowing stomach emptying and reducing appetite. Semaglutide is one; tirzepatide also acts on GIP.
Growth hormone secretagogue (GHS): A compound that makes the pituitary gland release more of its own growth hormone, such as ipamorelin or CJC-1295.
Desensitization: A weaker response to a compound after repeated use, which is one reason some protocols include breaks.
Injection site rotation: Moving each injection to a different spot so the skin and fat can recover, reducing lumps, bruising and irritation.

Crestigen is an educational research reference. Nothing here is medical advice; many of these compounds are not approved for human use. Talk to a licensed clinician before using any substance.