This glossary defines 30 regulatory, research and evidence terms used across PeptideWikis, from 503A compounding and WADA’s S0 class to half-life and randomized controlled trials. Each definition comes first, facts link to the primary source, and each entry points to the PeptideWikis article where the term matters most.
0–9 · A · B · C · E · G · H · I · N · P · R · W
0–9
503A
503A is the section of the US Food, Drug, and Cosmetic Act that lets a licensed pharmacist or physician compound a drug for an identified individual patient with a valid prescription. Drugs compounded under it are exempt from FDA approval, current good manufacturing practice and certain labeling requirements only if every condition in the section is met (21 U.S.C. 353a). Read: 503A vs 503B peptides
503B
503B is the section of the Food, Drug, and Cosmetic Act for outsourcing facilities: sites that compound sterile drugs, register with FDA as outsourcing facilities and need not be licensed pharmacies. Their drugs are exempt from FDA approval and certain labeling requirements, but not from current good manufacturing practice (21 U.S.C. 353b). Read: 503A vs 503B peptides
A
Adverse event
An adverse event is any harmful or unwanted medical occurrence in a person using a drug, whether or not the drug caused it. FDA’s rule for investigational drugs defines it as “any untoward medical occurrence associated with the use of a drug in humans, whether or not considered drug related” (21 CFR 312.32). An adverse event alone does not prove the drug was the cause. Read: Ipamorelin
Agonist
An agonist is a molecule that binds to a receptor and switches it on, producing the response the body’s own signal would. An antagonist binds without switching the receptor on and blocks it. Many peptide drugs are agonists; semaglutide’s label, for example, describes it as a GLP-1 receptor agonist (Wegovy label). Read: Retatrutide
Amino acid
Amino acids are the small building blocks that link into chains to form peptides and proteins. The body uses 20 standard amino acids, and the order in which they are linked, the amino acid sequence, gives each peptide its identity. Research peptides often include modified or unnatural amino acids that the body does not make. Read: What are peptides?
Analogue
An analogue is a molecule deliberately changed from a natural one, usually by swapping, adding or removing a few amino acids, so that it lasts longer or acts more strongly or selectively. CJC-1295, for example, is a long-acting analogue of growth hormone-releasing hormone (Teichman 2006). An analogue is a new substance with its own safety record, not the natural hormone. Read: CJC-1295
Approved label
An approved label is the prescribing information a regulator approves with a medicine, stating its indications, warnings, side effects and trial results. On PeptideWikis “Approved label” is also the highest evidence level, used when a claim comes from that document. Tesamorelin, for example, is approved as Egrifta to reduce excess abdominal fat in people with HIV and lipodystrophy (FDA approval letter, 2010). Read: Tesamorelin
B
Bulk drug substance
A bulk drug substance is the raw active ingredient a pharmacy or outsourcing facility uses to compound a drug. FDA’s rule says the term, as used in sections 503A and 503B, means the same as “active pharmaceutical ingredient” (21 CFR 207.3). Peptides such as BPC-157 reach compounding only as bulk drug substances, which is why FDA’s bulks lists matter for them. Read: FDA PCAC peptide vote (July 2026)
C
Category 1 and Category 2
Category 1 and Category 2 are FDA’s interim groupings of bulk drug substances nominated for 503A compounding. FDA does not intend to act against compounders using Category 1 substances if conditions are met; Category 2 substances raise significant safety risks, so that policy does not apply to them (FDA). Neither category is an approval or an evidence grade. Read: Category 1 vs Category 2 peptides
Compounding pharmacy
A compounding pharmacy mixes or alters drug ingredients to make a medicine for a particular patient, under section 503A, or in bulk at an outsourcing facility under 503B. Compounded drugs are not FDA-approved, which means FDA does not verify their safety, effectiveness or quality before they are marketed (FDA). Read: 503A vs 503B peptides
E
Evidence level
An evidence level is the label PeptideWikis puts on every cited claim to show what kind of study supports it. From weakest to strongest: Lab (cells or tissue), Animal, Human (case series or observational, with n), Human RCT (randomized controlled trial, with n) and Approved label. A peptide’s strongest level appears in each entry’s status box; animal or lab findings are never presented as proven effects in people. Read: BPC-157
G
Ghrelin mimetic (GH secretagogue)
A ghrelin mimetic, or growth hormone secretagogue, is a compound that triggers growth hormone release by activating the ghrelin receptor, a different route from GHRH. Examples include GHRP-2, GHRP-6, ipamorelin and the oral compound MK-677. In rat pituitary cells, rats and pigs, ipamorelin released growth hormone without raising ACTH or cortisol (Lab and Animal) (Raun 1998). Read: Ipamorelin
GHRH
GHRH, growth hormone-releasing hormone, is the hypothalamic peptide that tells the pituitary gland to release growth hormone. Synthetic GHRH analogues include sermorelin, CJC-1295 and tesamorelin. WADA prohibits GHRH and its analogues at all times under class S2 (WADA 2026 List). Read: CJC-1295
GIP
GIP, glucose-dependent insulinotropic polypeptide, is a gut hormone released after eating that boosts insulin secretion. Tirzepatide activates GIP and GLP-1 receptors, and the investigational drug retatrutide activates GIP, GLP-1 and glucagon receptors (Human RCT, n=338) (Jastreboff 2023). Retatrutide is not FDA-approved. Read: Retatrutide
GLP-1 receptor agonist
A GLP-1 receptor agonist is a drug that activates the receptor for glucagon-like peptide-1, a gut hormone that raises insulin after meals, slows stomach emptying and reduces appetite. Semaglutide is one: its label calls Wegovy “a glucagon-like peptide-1 (GLP-1) receptor agonist” (Wegovy label). Gastrointestinal side effects are the most common. Read: CagriSema
H
Half-life
Half-life is the time it takes for the amount of a substance in the blood to fall by half. Natural peptides are often broken down within minutes; analogues are modified to last longer. In healthy adults, the estimated half-life of CJC-1295 with DAC was 5.8 to 8.1 days (Human RCT) (Teichman 2006). Read: CJC-1295
I
IGF-1
IGF-1, insulin-like growth factor 1, is a hormone made mainly by the liver in response to growth hormone, and it carries out many of growth hormone’s effects. Studies of growth hormone-releasing peptides usually measure blood IGF-1 to show the effect. WADA prohibits IGF-1 and its analogues at all times under class S2 (WADA 2026 List). Read: CJC-1295
Immunogenicity
Immunogenicity is the ability of a substance to provoke an immune response, such as antibodies or allergic reactions. Peptides can clump together or carry impurities that raise this risk, and FDA cites it repeatedly, for example warning that compounded GHRP-2 “may pose risk for immunogenicity due to the potential for aggregation and peptide-related impurities” (FDA). Read: Category 1 vs Category 2 peptides
In vitro and in vivo
In vitro means a study done outside a living body, in cells, tissue or a test tube; in vivo means a study in a living organism, usually an animal or a person. On PeptideWikis in vitro results are graded Lab and animal results Animal. Neither shows that a peptide works or is safe in people. Read: BPC-157
N
NDA and BLA
An NDA (New Drug Application) and a BLA (Biologics License Application) are the two applications a company files to have a medicine approved in the US. Peptides of 40 or fewer amino acids usually go through an NDA; FDA’s rule defines a protein, a biological product, as a chain “greater than 40 amino acids in size” (21 CFR 600.3). Read: Retatrutide FDA approval timeline
P
PCAC
PCAC is FDA’s Pharmacy Compounding Advisory Committee, an outside expert panel that advises FDA on scientific, technical and medical questions about compounding under sections 503A and 503B (FDA), including which bulk drug substances may be used. Its votes are recommendations; FDA makes the decisions. Read: FDA PCAC peptide vote (July 2026)
Peptide
A peptide is a short chain of amino acids joined by peptide bonds; longer chains fold into proteins. Peptides include hormones such as oxytocin and GLP-1 and many drugs and research compounds. US regulation draws the line at 40: FDA’s rule defines a protein as an amino acid chain “greater than 40 amino acids in size” (21 CFR 600.3). Read: What are peptides?
Phase 1, 2 and 3 trials
Phase 1, 2 and 3 are the stages of human testing before approval. FDA’s rule describes phase 1 as the first use in humans, studying metabolism and side effects; phase 2 as controlled studies of effectiveness and short-term risks in usually no more than several hundred patients; and phase 3 as expanded trials that gather the safety and effectiveness data for approval (21 CFR 312.21). Read: Retatrutide FDA approval timeline
Placebo
A placebo is an inactive dummy treatment, such as a saline injection or sugar pill, made to look like the real one. Comparing a drug with placebo shows how much of a change comes from the drug itself rather than from expectation, natural recovery or chance. Studies with no placebo or other control group cannot show that a peptide caused an improvement. Read: Retatrutide
R
Randomized controlled trial
A randomized controlled trial (RCT) assigns participants by chance to the treatment or to a comparison, often placebo, so the groups differ only in what they receive. It is the strongest test of whether a treatment works. PeptideWikis grades such evidence Human RCT and gives the number of participants (n), because a small RCT proves less than a large one. Read: Retatrutide
Research use only (RUO)
“Research use only” (RUO) is a label stating that a product is meant for laboratory research and not for use in people. In FDA rules the phrase comes from the labeling of laboratory-stage diagnostic products, which must say “For Research Use Only. Not for use in diagnostic procedures” (21 CFR 809.10). The label is not an approval, and RUO products are not for human use. Read: Research use only peptides
W
WADA Prohibited List
The WADA Prohibited List is the World Anti-Doping Agency’s annual list of substances and methods banned in sport, a mandatory International Standard of the World Anti-Doping Program. A new list takes effect each 1 January; the 2026 List came into effect on 1 January 2026 (WADA 2026 List). Classes name examples “including, but not limited to” listed substances, so an unnamed peptide is not automatically permitted. Read: WADA banned peptides 2027
WADA S0
S0 is the WADA Prohibited List class for non-approved substances: any pharmacological substance not covered by another class and with “no current approval by any governmental regulatory health authority for human therapeutic use” is prohibited at all times (WADA 2026 List). BPC-157 is the class’s named peptide example; unnamed research peptides with no such approval can fall under it as well. Read: WADA banned peptides 2027
WADA S2
S2 is the WADA Prohibited List class for peptide hormones, growth factors, related substances and mimetics, prohibited in and out of competition. It names, among others, GHRH analogues such as CJC-1295 and tesamorelin, growth hormone secretagogues such as ipamorelin, IGF-1, and thymosin beta-4 derivatives such as TB-500, and it also covers substances with similar structure or effect (WADA 2026 List). Read: WADA banned peptides 2027
Warning letter
An FDA warning letter is a formal notice that FDA believes a company has significantly violated federal law, for example through poor manufacturing, unproven claims or wrong directions for use. It gives the company a chance to correct the problem before enforcement action (FDA). Read: Research use only peptides
Change log
2026-10-04: first published after review by the editorial team.