Glossary
Terms you'll see.
Skim before diving in. Worth bookmarking.
- Agonist
- A molecule that binds to a receptor and activates it — the receptor responds as if its natural ligand bound.
- Antagonist
- A molecule that binds to a receptor without activating it, blocking the natural ligand from binding.
- Bacteriostatic Water (BAC)
- Sterile water with 0.9% benzyl alcohol added as a preservative. Used to reconstitute lyophilized peptides; safe to use the same vial for ~28 days.
- Bioavailability
- The fraction of a dose that actually reaches systemic circulation. Subcutaneous injection ≈ 50–100%; oral peptides usually <2%.
- C-max
- Peak concentration in blood after a dose.
- Clinical trial phases
- Phase 1 = safety in small healthy group. Phase 2 = efficacy + dose finding. Phase 3 = large-scale efficacy vs standard. Phase 4 = post-approval surveillance.
- Compounding pharmacy
- A pharmacy that prepares custom medications. In the US, 503A compounding is for individual prescriptions; 503B is bulk for clinics.
- GLP-1 agonist
- A compound that activates the GLP-1 receptor — increases insulin, slows gastric emptying, suppresses appetite. Examples: semaglutide, tirzepatide, retatrutide.
- GIP
- Glucose-dependent insulinotropic polypeptide. An incretin hormone like GLP-1; tirzepatide is a dual GLP-1/GIP agonist.
- Glucagon receptor
- Receptor whose activation increases hepatic glucose output and energy expenditure. Retatrutide hits this in addition to GLP-1/GIP.
- Half-life
- Time for blood concentration to drop by half. Determines dose frequency. Sema/reta ≈ 1 week; BPC-157 ≈ minutes (in vitro).
- IGF-1
- Insulin-like growth factor 1. Liver makes it in response to growth hormone; mediates most of GH's anabolic effects.
- Lyophilized
- Freeze-dried. How most peptides ship — stable as a powder, must be reconstituted with sterile water before injection.
- PK / PD
- Pharmacokinetics (what the body does to the drug — absorption, distribution, metabolism, excretion) and pharmacodynamics (what the drug does to the body).
- Receptor
- A protein on or in a cell that responds to a specific molecule. Peptide drugs work by binding receptors and changing cell behavior.
- Reconstitution
- Adding sterile/bacteriostatic water to a lyophilized peptide vial to make it injectable.
- Subcutaneous
- Injection into fat under the skin — abdomen, thigh, upper arm. Most peptides are dosed this way; absorption is slower than IM.
- Titrate
- Gradually increase a dose to a target level, usually to manage side effects. Standard practice for GLP-1s — start low, step up every 4 weeks.
- Tachyphylaxis
- Diminishing response to repeated doses. Some peptides (like growth hormone secretagogues) are dosed cyclically to avoid it.