Peptide Glossary, every term in plain English

Every abbreviation and bit of jargon you'll encounter on Pepdex. Grouped by what kind of thing it is. Bookmark and come back.

Units

mcg: Microgram. 1 mg = 1,000 mcg.; Microgram, the standard dose unit for most peptides. 1 milligram (mg) equals 1,000 micrograms (mcg). When a protocol says '250 mcg', that's 0.25 mg. Most peptide vials are dosed in mg total, but doses-per-injection are usually in mcg.
mg: Milligram. 1 mg = 1,000 mcg.; Milligram. The unit your peptide vial size is usually labeled in (e.g., 'BPC-157 5mg vial'). Doses are typically a fraction of that, expressed in mcg.

Delivery

Sub-q: Subcutaneous. Inject under the skin into the fat layer.; Subcutaneous injection. The needle goes into the fat layer just under the skin, not into muscle. Most peptides are sub-q. Common sites: lower abdomen (2 inches from the navel), love handles, outer thigh. Use a 29-31 gauge insulin syringe.
IM: Intramuscular. Injection into muscle.; Intramuscular injection. Deeper than sub-q, into the muscle tissue. Less common for peptides, more common for hormones. Uses a longer needle. TB-500 is sometimes given IM.
BAC water: Bacteriostatic water. Sterile water with 0.9% benzyl alcohol.; Bacteriostatic water. Sterile water mixed with 0.9% benzyl alcohol that prevents bacterial growth, letting you reuse the vial across multiple doses without contamination. This is what you mix lyophilized (freeze-dried) peptide powder with to reconstitute it.
Lyophilized: Freeze-dried powder form. Reconstitute before use.; Freeze-dried. The peptide is shipped as a fluffy white powder in a sealed vial, stable for months in the fridge or years in the freezer. To use it, you reconstitute (mix) with bacteriostatic water.
Reconstitution: Mixing dry peptide with BAC water to make it injectable.; The process of mixing a lyophilized peptide vial with bacteriostatic water so it becomes liquid and injectable. Done once per vial. See the Reconstitution guide for the step-by-step.
Insulin syringe: Tiny 29-31g needle; 1mL barrel = 100 'units'.; Standard insulin syringe. 1mL barrel total volume, marked in 100 'units' instead of mL, so 0.5mL = 50 units. The needle is 29-31 gauge, very thin, designed for sub-q injection. The Calculator converts your peptide dose into exact units to draw.

Schedule

EOD: Every other day.; Every other day. A common dosing frequency. Mon/Wed/Fri/Sun/Tue, etc. Used when daily would over-saturate the receptor or when the half-life is long enough to skip days.
Cycle: On-period of dosing followed by an off-period for receptor reset.; A defined on-period of dosing (e.g., 8 weeks) followed by an off-period (e.g., 4 weeks). Why: your receptors downregulate over time when constantly stimulated, so the same dose stops working. The break lets receptors upregulate back to baseline.
Loading phase: Higher / more frequent doses at the start of a cycle.; The opening 1-3 weeks of a cycle where doses are higher or more frequent than maintenance. Used to push tissue concentrations up faster. TB-500 and MT-1/2 are the classic examples.
Maintenance phase: Lower / less frequent doses to hold the effect.; The phase after loading where doses drop to a lower frequency or amount. Goal: hold the effect with less product and fewer side effects.
Titration: Slowly increasing dose over weeks to manage side effects.; Step-up dosing. Common with GLP/GIP class drugs (semaglutide, tirzepatide, retatrutide). You start at the lowest dose, hold it for 4 weeks, then increase if tolerated. Skipping titration spikes side effects badly.

Biology

Half-life: How long until 50% of a dose is cleared from your body.; The time it takes for half of a dose to be metabolized and cleared. Drives how often you dose. A 4-hour half-life peptide needs daily or twice-daily dosing; a 6-day half-life peptide is comfortably weekly. Steady-state (your level stops rising) is reached around 4-5 half-lives in.
GLP-1: Glucagon-like peptide-1 receptor. The main appetite-control receptor.; Glucagon-like peptide-1. A natural hormone your gut releases after eating that signals satiety to your brain. The 'GLP-1 drugs' (semaglutide, liraglutide) mimic and prolong this signal, suppressing appetite and slowing gastric emptying.
GIP: Second appetite-control receptor; Tirzepatide and Reta hit it.; Glucose-dependent insulinotropic polypeptide. A second appetite/insulin-related receptor. Adding GIP activation to GLP-1 (as Tirzepatide does) gives stronger weight-loss effect than GLP-1 alone.
DAC: Drug Affinity Complex. Extends a peptide's half-life dramatically.; Drug Affinity Complex. A modification that binds the peptide to albumin in your blood, dramatically extending half-life (CJC-1295 with DAC = ~8-day half-life vs ~30 min without). The 'no-DAC' version of CJC-1295 is the short-acting one used in peptide stacks.
Receptor downregulation: Receptors get less responsive when stimulated continuously.; When you stimulate a receptor too much for too long, your cells reduce the number of receptors or their sensitivity to compensate. This is why peptides need cycle breaks, your body adapts to the signal.
IGF-1: Insulin-like growth factor 1. Marker for GH-axis activity.; Insulin-like growth factor 1. The downstream hormone produced by your liver when GH hits it. Most GH effects are actually IGF-1 effects. Lab measurement of IGF-1 is the standard way to verify a GH-axis peptide is working.
Agonist: A molecule that switches a receptor ON (most peptides are agonists).; A molecule that binds a receptor and activates it, switching on the downstream effect. Most peptides in this catalog are agonists: a GLP-1 agonist turns on the GLP-1 receptor; a melanocortin agonist turns on pigment and libido receptors. The opposite is an antagonist, which blocks the receptor instead.
Secretagogue: Tells a gland to release more of its own hormone.; A compound that prompts a gland to secrete more of a hormone it already makes, instead of supplying the hormone directly. GH secretagogues (Ipamorelin, MK-677, CJC-1295) nudge your pituitary to release your own growth hormone, which is why they preserve the natural pulse rather than flatlining it the way injected HGH can.
Analog: A tweaked copy of a natural molecule, built to last longer or work better.; A synthetic molecule modeled on a natural one with small changes to improve it, usually a longer half-life or stronger receptor binding. Semaglutide is a GLP-1 analog; Tesamorelin is a GHRH analog. Those tweaks are what let a once-weekly shot replace a hormone your body would otherwise clear in minutes.
GHRH: Growth-hormone-releasing hormone; the upstream 'release GH' signal.; Growth-hormone-releasing hormone, the brain signal that tells your pituitary to release growth hormone. GHRH analogs (Sermorelin, CJC-1295, Tesamorelin) mimic that signal, which is why they're paired with a secretagogue like Ipamorelin for a bigger, cleaner GH pulse.
Dual / triple agonist: One molecule that switches on 2 or 3 receptors at once.; A single peptide built to activate two (dual) or three (triple) receptors at once. Tirzepatide is a dual GLP-1 + GIP agonist; Retatrutide is a triple GLP-1 + GIP + glucagon agonist. Each added receptor tends to add weight-loss effect, which is why the triple agonists post the largest trial numbers so far.
Amylin: A satiety hormone co-released with insulin; a different appetite lever than GLP-1.; A hormone secreted alongside insulin that slows gastric emptying and signals fullness through a separate pathway from GLP-1. Amylin analogs (Cagrilintide, Pramlintide) pair naturally with GLP-1 drugs, which is the whole idea behind CagriSema.
Boxed warning: The FDA's strongest safety warning on a drug label.; Formerly called a 'black-box warning,' the most serious warning the FDA places on a prescription label, reserved for risks that can be serious or life-threatening. Worth knowing which of your compounds carry one and which don't; labels change over time as evidence accumulates (several peptides had warnings removed years after approval).
Contraindication: A condition that makes a compound unsafe for you, a hard no.; A specific situation or pre-existing condition under which you should not use a compound because the risk is too high (for example, a personal or family history of medullary thyroid cancer with GLP-1 drugs). Different from a side effect: a contraindication is a reason not to start at all.

Vendor

Vial: Sealed glass bottle holding the peptide.; The sealed glass bottle that ships from the vendor with peptide powder inside. Has a rubber septum top. You inject BAC water through the septum to reconstitute, then draw doses through the same septum.

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