Home / Peptide Glossary: Plain-English Definitions
Peptide Glossary: Plain-English Definitions
What a peptide is, and what FDA-approved, compounded, investigational and other terms mean, with definitions and sources for each regulatory claim.
On this page
Peptide marketing borrows regulatory and research words without explaining them. These definitions say what each term means in our guides, and they avoid claims about any specific peptide, because a peptide’s status can change.
Basics
Peptide. A short chain of amino acids (typically 2 to 50) linked by chemical bonds, called peptide bonds. A longer chain of 51 or more is a polypeptide (NHGRI). Being a peptide says nothing about whether something is safe, effective or approved.
Label (prescribing information). The approved labeling that tells healthcare providers how to use an FDA-approved drug safely and effectively (FDA). Compounded and investigational products don’t have an FDA-approved label for that product.
Regulatory terms
FDA-approved. FDA approved a specific product for the uses on its label. Before approval, a company must submit clinical data and show the drug is safe and effective for its intended uses. “Safe” does not mean the drug has no side effects; it means FDA determined the benefits of using it for a particular use outweigh the potential risks. Approval does not extend to compounded versions or to other uses.
FDA-approved for [use] / FDA-approved for other uses. Our wording next to a goal. An approved drug is approved for specific uses, so we name the use on the label, or say “other uses” when the goal isn’t on the label.
Off-label use. Using an FDA-approved drug for an unapproved use (FDA). FDA’s evaluation of benefits and risks covers the approved use.
Investigational. Being studied in people, and not FDA-approved for that use. Before beginning clinical research, drug developers must submit an Investigational New Drug (IND) application to FDA (FDA).
Clinical trial phases. According to FDA: Phase 1 studies involve roughly 20 to 100 healthy volunteers or people with the condition and focus on safety and dosage. Phase 2 studies involve up to several hundred people with the condition, look at efficacy and side effects, and typically aren’t large enough to show whether a drug is beneficial. Phase 3 studies involve 300 to 3,000 people, aim to show whether a product offers a treatment benefit to a specific population, and provide most of the safety data. Phase 4 trials happen after approval.
Compounded drug. A drug made when a licensed pharmacist or physician, or a person supervised by a licensed pharmacist at an outsourcing facility, combines, mixes or alters ingredients. Compounded drugs are not FDA-approved, which means FDA does not review them to evaluate their safety, effectiveness or quality before they reach patients. A compounded version of an approved drug is a different product from the approved drug.
Section 503A and 503B. Two parts of U.S. law that govern compounding (FDA). Section 503A describes the conditions under which compounded products are exempt from FDA approval before marketing, from current good manufacturing practice (CGMP) requirements, and from certain labeling requirements. One condition is that drugs must be compounded based on a valid patient-specific prescription. Section 503B created a voluntary category of compounders called outsourcing facilities, which are subject to CGMP requirements and may distribute compounded drugs either under a patient-specific prescription or in response to an order from a health care provider.
Pharmacy Compounding Advisory Committee (PCAC). A committee that provides FDA with advice as it implements parts of sections 503A and 503B (FDA). Its votes are advice, not decisions.
Enforcement discretion. A statement that a regulator does not intend to take enforcement action under stated conditions. It is not approval. For example, a policy HHS published on Oct. 30, 2026 says it does not establish any rights for any person and is not binding on HHS or the public, and it was published as an interim policy (Federal Register). Whether such a policy is still in force can change, so check the date on any page that mentions one.
Dietary supplement. A product intended for ingestion that, among other requirements, contains a “dietary ingredient” intended to supplement the diet (FDA). FDA does not have authority to approve dietary supplements before they are sold, and the company generally does not have to give FDA its safety evidence. “Not FDA-approved” describes how the whole category works.
Cosmetic. A product such as a cream or serum. The law does not require FDA approval before cosmetics are sold, except for color additives. The line with a drug depends on intended use: FDA says a product intended to “remove wrinkles or increase the skin’s production of collagen” is a drug or a device, not a cosmetic.
Research use only. A label some sellers put on products described as not intended for human use. HHS describes research-use-only peptides supplied by foreign manufacturers as a safety risk. We don’t link to or recommend these sellers.
Adverse event report. A report of a harmful or unexpected event after using a product, such as those collected by FDA’s MedWatch program. FDA says of its reporting database that “reporting is voluntary”, and few reports don’t show that something is safe.
WADA Prohibited List. The World Anti-Doping Agency’s list of substances and methods banned in sport. It sorts them into classes, such as S0 (non-approved substances). Athletes should check their own anti-doping organization before taking anything.
Evidence terms
Human trials. Controlled trials in people, published in a journal, that tested a clinical outcome and showed a benefit. The strongest kind of evidence we report.
Limited human data. Small, short or inconclusive human studies, including observational studies, uncontrolled trials and controlled trials that did not show a clinical benefit.
Animal studies. Results in animals often don’t carry over to people.
Lab studies. Cells or tissue in a dish. An early signal, not an answer.
Animal studies only. We found no published human studies as of the date we checked.
Controlled trial. A trial in which at least one group gets the treatment being tested while another group, at the same time, gets a comparison (NIA glossary). In a randomized controlled trial, people are divided into groups by chance, so the groups are similar and the treatments can be compared more fairly.
Placebo. An inactive look-alike pill, liquid or other intervention with no treatment value, used for comparison (NIH glossary; NIA glossary).
Blinded (masked) and double-blind. In a blinded study, participants don’t know which treatment they are getting, so they can describe what happens without bias. In a double-blind study, the research team doesn’t know either (NIH glossary).
Open-label. A trial in which investigators and participants know which intervention is being given (NIA glossary).
Observational study. Researchers observe people or measure outcomes without trying to affect them. It may compare groups, but the researchers don’t assign the treatment. In our guides, we treat these studies as showing an association, not cause.
Case series and uncontrolled trial. A case series is a group of case reports on patients given similar treatment. In an uncontrolled trial, everyone gets the treatment and there is no comparison group.
Meta-analysis and systematic review. A meta-analysis analyzes data from different studies on the same subject. A systematic review uses explicit methods to find, appraise and synthesize the evidence on a research question. Conclusions depend on the studies included: in one 2025 analysis, results differed when it looked only at higher-quality studies or those without industry funding.
Marker versus clinical outcome. In our guides, a marker is a lab or imaging measurement, such as a hormone level, and a clinical outcome is how a person feels, functions or survives. A change in a marker does not by itself show a benefit to the person. An imaging result can count when it is the basis of an FDA-approved use, and we say so next to it.
Estimand. FDA guidance describes it as “a precise description of the treatment effect reflecting the clinical question posed by a given clinical trial objective.” The same trial can report different ones. For example, Lilly’s release on a retatrutide trial reports an “efficacy estimand,” the effect had all randomized participants remained on treatment, and a “treatment-regimen estimand,” the average effect regardless of adherence, which the journal used.
Rank. In a guide, the order of peptides by the strength of the evidence that they help with the goal: the type of study first, then its quality and its result. A rank isn’t a recommendation. See how we rate peptides.
Frequently asked questions
Is a peptide a protein?
Not exactly. A peptide is a short chain of amino acids, typically 2 to 50; a chain of 51 or more is a polypeptide (NHGRI).
Does “FDA-approved” mean a peptide is approved for my goal?
Not necessarily. Approval covers specific uses on the label, and we show those uses next to each goal.
Does “not FDA-approved” mean illegal?
Not FDA-approved doesn’t answer that by itself, and we don’t give legal conclusions. It means FDA has not approved the product. The rules differ for a supplement, a cosmetic, a compounded drug and an investigational drug, and FDA has sent warning letters to some sellers of unapproved peptides. We don’t call a product illegal unless a source says so.
Does “enforcement discretion” mean approved?
No. It is a regulator’s statement about enforcement, not a finding that a product is safe or effective.
Sources we checked
- NHGRI Talking Glossary: peptideOther. Accessed .
- FDA: compounding laws and policiesFDA. Accessed .
- FDA: understanding unapproved use of approved drugs ("off label")FDA. Accessed .
- FDA: step 3, clinical research (phases of clinical trials)FDA. Accessed .
- FDA: investigational new drug application (IND)FDA. Accessed .
- FDA: questions and answers on dietary supplementsFDA. Accessed .
- FDA authority over cosmetics: how cosmetics are not FDA-approved, but are FDA-regulatedFDA. Accessed .
- FDA: wrinkle treatments and other anti-aging productsFDA. Accessed .
- FDA MedWatch: safety information and adverse event reporting programFDA. Accessed .
- HHS: Enforcement Discretion Policy, with Appendices A and BOther. Accessed .
- Federal Register: HHS enforcement discretion policy on 503A peptide compounding (Oct. 30, 2026)Other. Accessed .
- WADA 2026 Prohibited ListOther. Accessed .
- NIH glossary of common clinical research termsOther. Accessed .
- NIA glossary of clinical research termsOther. Accessed .
- Lilly trial release, May 21, 2026 (example of efficacy and treatment-regimen estimands)Press release. Accessed .
- NCI Dictionary of Cancer Terms: observational studyOther. Accessed .
- NCI Dictionary of Cancer Terms: case seriesOther. Accessed .
- NCI Dictionary of Cancer Terms: randomized clinical trialOther. Accessed .
- NCI Dictionary of Cancer Terms: meta-analysisOther. Accessed .
- NLM MeSH: systematic reviewOther. Accessed .
- FDA guidance E9(R1): statistical principles for clinical trials, estimands addendumFDA. Accessed .
- Collagen supplementation and skin aging, meta-analysis of 23 RCTs, Am J Med 2025 (PubMed)PubMed. Accessed .
- FDA warning letter to Royal Peptides, Aug. 24, 2026FDA. Accessed .
- FDA briefing document on TB-500FDA. Accessed .
Written and maintained by the Best Peptides For Editorial Team. How we rate evidence.