- Protocol
- A structured, individualized plan — in a medical context, one that should be built around a specific patient, not copied from a forum post.
- Bioavailability
- The proportion of a substance that actually enters circulation and is able to have an active effect, which varies significantly by administration method.
- Half-life
- The time it takes for half of a substance to be cleared from the body — a key factor in how often a compound needs to be administered.
- Agonist / antagonist
- An agonist activates a biological receptor to produce an effect; an antagonist blocks a receptor, preventing an effect from occurring.
- Off-label use
- Prescribing an FDA-approved drug for a use, dose, or population other than what it was originally approved for — legal when directed by a physician, but not the same as an approved indication.
- Clinical trial phases
- The staged human testing process (Phase I, II, III) a drug candidate moves through on the path toward possible FDA approval, testing safety, then efficacy signals, then large-scale confirmation.
- Endotoxin testing
- Testing for bacterial toxins that can remain in a product after manufacturing — a key purity and safety check for injectable compounds.
- GLP-1 (glucagon-like peptide-1)
- A naturally occurring gut hormone released after eating that affects insulin release, appetite, and digestion speed. Several metabolic peptide medications, including semaglutide, are built to mimic it.
- GIP (glucose-dependent insulinotropic polypeptide)
- A second incretin hormone, related to GLP-1, involved in insulin regulation. Tirzepatide is designed to activate both the GLP-1 and GIP receptors at once, which is why it's called a "dual agonist."
- Incretin
- A general term for gut hormones, including GLP-1 and GIP, that are released after eating and help regulate insulin secretion. "Incretin-based therapy" is a common way the semaglutide/tirzepatide drug class is described in the medical literature.
- Dual or multi-receptor agonist
- A drug designed to activate more than one type of receptor at once, rather than just one. Tirzepatide is a dual GLP-1/GIP agonist; it's an example of how newer metabolic peptides are engineered for broader effects than earlier single-target drugs.
- Mechanism of action
- The specific biological process by which a substance produces its effect — for example, "activates the GLP-1 receptor" or "binds the growth hormone secretagogue receptor." Knowing the mechanism doesn't by itself tell you the substance is safe or effective in humans; those are separate questions.
- Peptide vs. small-molecule drug
- Peptides are chains of amino acids, generally larger and more fragile than "small-molecule" drugs (like aspirin or metformin), which is why most peptides are injected rather than taken as a pill — they'd typically be broken down by digestion first.
- Lyophilized
- Freeze-dried into a stable powder form. Many peptides are shipped and stored lyophilized because they're more stable that way than as a liquid, and must be reconstituted before any clinical use.
- Reconstitution
- The process of turning a lyophilized (freeze-dried) peptide back into a usable liquid form, typically by a pharmacist or trained provider under sterile conditions — not a step to be approximated at home.
- Immunogenicity
- The tendency of a substance to trigger an unwanted immune response in the body. It's a specific safety concern the FDA has cited for some research peptides, including certain growth hormone-releasing peptides, due to impurities or unnatural amino acid sequences.
- Hormone-axis suppression
- When introducing an outside hormone or hormone-like signal causes the body to reduce its own natural production of that hormone, because internal feedback loops sense there's already enough. A key reason hormone-adjacent therapies need individualized medical oversight rather than casual use.
- Pharmacokinetics
- How a substance moves through the body over time — absorption, distribution, metabolism, and elimination. Half-life (see below) is one piece of a compound's overall pharmacokinetic profile.
- Receptor selectivity
- How narrowly or broadly a substance binds to its target receptor(s) versus related ones. Low selectivity can mean more off-target effects; it's one reason closely related peptides in the same family can have different safety profiles.
- Pentadecapeptide
- A peptide made of exactly 15 amino acids. BPC-157 is a commonly discussed example of a synthetic pentadecapeptide.
- Tetrapeptide
- A peptide made of exactly 4 amino acids — a very short chain. Epitalon is an example of a research compound in this category.
- Tripeptide
- A peptide made of exactly 3 amino acids. GHK-Cu and KPV are both examples of tripeptides discussed on this site.
- Peptide analog
- A modified version of a naturally occurring peptide, engineered to change how long it lasts in the body, how selectively it binds a receptor, or other properties. Tesamorelin and sermorelin are both analogs of the natural hormone GHRH.
- Peptide fragment
- A shorter piece cut from a larger, naturally occurring peptide or protein, rather than the whole molecule. TB-500 is described as a fragment of the naturally occurring protein thymosin beta-4.
- Bioregulator
- A term (more common in some non-U.S. research traditions) for a short peptide believed to influence gene expression or cell signaling in a regulatory way, rather than acting like a classic hormone or drug. Epitalon is often described this way in the literature it originates from.
- Nootropic
- A substance studied or marketed for potential cognitive effects, such as memory, focus, or mood. Selank and Semax are peptides commonly discussed in nootropic contexts.
- Investigational
- A status meaning a substance is being studied but has not completed the regulatory review needed for approval — it is not yet a legally marketed drug for the use being studied.
- Investigational New Drug (IND) application
- The formal application a sponsor files with the FDA to begin human clinical trials of a new drug. A compound circulating as "research use only" online has, in most cases, not gone through this process for the uses it's being sold for.
- Randomized controlled trial (RCT)
- A study design where participants are randomly assigned to receive a treatment or a comparison (often a placebo), which is the standard human-evidence structure that FDA drug approvals are built on. See our guide to evaluating peptide research for how to spot one.
- Preprint
- A study posted publicly before, or without ever, undergoing peer review. Useful for tracking emerging research, but not yet independently vetted the way a peer-reviewed publication has been.
- Peer review
- The process by which independent experts evaluate a study's methods and conclusions before a journal publishes it. It's an imperfect but real quality filter that vendor pages and preprints don't pass through.
- Case report
- A detailed account of what happened with a single patient. Useful for flagging possible rare effects or generating hypotheses, but without a comparison group it can't establish what a treatment typically does.
- Systematic review / meta-analysis
- A structured analysis combining results across multiple studies (often RCTs) on the same question. When available and well-conducted, it's generally the strongest single source of evidence on a topic.
- ClinicalTrials.gov
- A free, public U.S. government registry of clinical studies, searchable by compound name, where you can check a trial's phase, status, and posted results yourself.
- PubMed
- The National Library of Medicine's free search engine for biomedical research literature — the standard place to look up peer-reviewed studies on a specific compound.
- PMID (PubMed ID)
- A unique identifying number assigned to each article indexed in PubMed. A specific, checkable citation should include one (or a DOI); a claim with neither is much harder to verify.
- Boxed warning
- The FDA's most serious type of prescription drug warning, printed directly on the label. Semaglutide and tirzepatide both carry boxed warnings related to thyroid C-cell tumors observed in rodent studies.
- Carcinogenicity signal
- A finding — often from animal studies — suggesting a substance may be associated with tumor growth, which triggers further scrutiny even when human relevance isn't yet established. This is part of why some GLP-1/GIP drugs carry a thyroid-tumor boxed warning.
- Gray market
- Products sold outside authorized, regulated distribution channels — not necessarily illegal to possess in every case, but without the oversight, testing, or accountability of an FDA-approved or pharmacy-compounded supply chain.
- Banned substance (sport)
- A substance prohibited in athletic competition by an anti-doping body such as WADA (World Anti-Doping Agency), independent of whether it's legal to buy or possess generally. BPC-157 and TB-500 are both examples of peptides banned in competitive sport.
- Orphan drug designation
- A special FDA status granted to encourage development of treatments for rare diseases, which can come with development incentives. It signals a drug is being formally developed for a specific rare condition, not that it's broadly approved.
- Systemic vs. topical administration
- Systemic means a substance enters the bloodstream and can affect the whole body; topical means it's applied locally (like to skin) with much more limited absorption. GHK-Cu's well-established use is topical; its systemic/injectable use is a different, far less established question.
- FDA Category 2 bulk substance
- A specific FDA list of bulk drug substances that "may present significant safety risks" for use in pharmacy compounding. Ipamorelin acetate is currently on this active list; several other peptides were nominated and later withdrawn from review, which is not the same as being cleared.
