What are peptides?

What a peptide is

A peptide is a chain of amino acids. The usual dividing line is length: a peptide typically has 2 to 50 amino acids joined by chemical links called peptide bonds, and a chain of 51 or more is called a polypeptide.¹ The proteins made inside cells are built from one or more polypeptides.¹

US drug rules draw the line in a different place. Under FDA's biologics regulations, a protein is any alpha amino acid polymer with a specific, defined sequence of more than 40 amino acids.⁴ So a 45-amino-acid chain can be a peptide in a biology glossary and a protein in FDA's rules.¹⁴

Therapeutic peptides usually have molecular weights of about 500 to 5,000 daltons, according to a 2022 review.² Six compounds profiled on this site show the spread, from 3 amino acids to 43:

Two related rows sit on opposite sides of FDA's 40-amino-acid line. TB-500 is a 7-amino-acid fragment (positions 17 to 23) of thymosin beta-4, while the full 43-amino-acid thymosin beta-4 is long enough to count as a protein under FDA's definition.⁴¹¹ The difference is more than a label: FDA states that TB-500 and thymosin beta-4 are not the same substance.¹¹

Why most peptide drugs are injected

Peptides are easily broken down by digestive enzymes in the stomach and intestine, so most peptide drugs are given by injection, according to a 2022 narrative review.² Once in the body, peptides also tend to have a short half-life and fast elimination.² BPC-157 shows how short that window can be: after one intravenous dose, its half-life was 5.3 minutes in dogs and 15.2 minutes in rats.¹³

CJC-1295 with DAC shows how a change to the molecule can lengthen that window. It carries an added linker that binds albumin in the blood, and researchers suggested this binding accounts for its stability.¹⁰ In two placebo-controlled 2006 trials in healthy adults, its estimated half-life was 5.8 to 8.1 days.¹⁴ The version without DAC lacks that linker, and FDA says the DAC version's pharmacology cannot be extrapolated to it.¹⁰

Some peptide medicines can be swallowed: semaglutide is approved as daily tablets, sold as Rybelsus and Ozempic tablets, for adults with type 2 diabetes, and the label states the two tablet brands are not substitutable milligram for milligram.¹⁵

Peptide medicines, from insulin to the GLP-1 class

A 2022 review dates the first therapeutic peptide, insulin, to 1921.² The same review counted more than 80 peptide drugs approved worldwide and more than 170 peptides in active clinical development.² A 2021 review in Nature Reviews Drug Discovery also counted more than 80 peptide drugs on the market.¹⁶

GLP-1 receptor agonists, the class that includes semaglutide and tirzepatide medicines, work by mimicking GLP-1, a hormone in the intestines.¹⁷ FDA describes three effects: they stimulate insulin release, they lower blood sugar after a meal, and they slow food moving through the digestive tract, which can help a person feel full longer.¹⁷ FDA approved the first one, exenatide (Byetta), in 2005.¹⁷ Tirzepatide, the active ingredient in Mounjaro, acts on two receptors: its label calls it a GIP receptor and GLP-1 receptor agonist.¹⁸

The GLP-1 hub covers the class, and brand-name products such as Ozempic have profiles built from their FDA labels.

Approved, compounded and research-labeled

The same peptide can reach people through different channels, and each channel has passed different checks.

Approved medicines go through FDA's review for safety, effectiveness and quality before they are marketed.²¹ Unapproved versions skip that review, FDA says, and compounded drugs are not FDA approved.²⁰ A prescriber sets the dose of an approved medicine within its label. The Mounjaro label, for example, starts at 2.5 mg once weekly and, if more blood sugar control is needed, raises the dose in 2.5 mg steps after at least 4 weeks on the current dose.¹⁸

Compounded drugs are made by state-licensed pharmacies under section 503A or by outsourcing facilities under section 503B, and FDA reminds compounders of legal restrictions on making copies of FDA-approved drugs.²¹

Research-labeled peptides carry wording such as "for research use only." In FDA regulations, that statement is the required label for in vitro diagnostic products (lab tests) in the laboratory research phase.²² The label does not settle how FDA treats a product: in an August 2026 warning letter, FDA said evidence from a seller's website showed that products labeled "for research use only" were intended as drugs for human use.²⁹ The research peptides page explains what the label does and does not mean, how FDA judges a product's intended use, and what a certificate of analysis should show.

How strong the evidence is

Health sentences on these pages name their evidence level: an FDA label, a human trial (with its size), an observational study, an animal study (with the species) or a cell study. The gap between levels can be wide. When FDA reviewed reports of suicidal thoughts with GLP-1 drugs, it pooled 91 placebo-controlled trials with 107,910 patients and found no increased risk.¹⁹ For BPC-157, a 2025 systematic review found 36 eligible studies, and 35 of them were preclinical, with no clinical safety data.²³ A 2026 systematic review of the human evidence found 3 small studies with 30 people in total, all of poor methodological quality, and rated the certainty of evidence very low.²⁴

Reviews of unapproved peptides as a group reach a similar conclusion. A 2026 review in Sports Medicine found that peptides such as BPC-157 and TB-500 show tissue-repair benefits in animal models, but that rigorous human safety data are scarce and they carry potential for serious harm to patients.³

How this section is organized

Peptides A to Z

Peptides A to Z lists every profile on the site, 51 in all. A profile starts by saying what the compound is, then gives its reported findings, side effects, dosing information, storage and legal status, and many link to deeper pages on one of those topics.

Peptides by type

Four hubs each cover one group. GLP-1 compounds covers semaglutide, tirzepatide and retatrutide. Research peptides explains research labeling, testing and certificates. Blends and stacks shows what is inside each multi-peptide vial and what research exists on each combination. Peptide supplies covers the diluent, syringes, needles and swabs used in lab handling.

Peptides by research goal

Goal pages map what people search for with a goal in mind, compound by compound, and name the evidence level of each finding. They make no promises. The goals are weight loss, muscle, skin and hair, sleep, tanning, healing and recovery, focus and memory, testosterone, men's health, blood sugar and women's health.

Comparisons

The comparisons page sets GLP-1 brands and their active ingredients side by side, for example Ozempic vs Wegovy and Mounjaro vs Ozempic, with cited label facts in each table.

Guides, calculators and reference

The peptide calculator works through reconstitution and syringe arithmetic, and the reconstitution calculator and units converter handle single steps. Measuring errors are a documented hazard: FDA has received reports of patients giving themselves 5 to 20 times the intended dose of compounded semaglutide, and says confusion between milliliters, milligrams and "units" may have contributed.²⁵ Step-by-step guides cover reconstituting a vial, storage and safety basics. The glossary defines the terms used across the site, and the questions and answers page collects common questions by topic.

The store

Compounds sold by D2C Peptides are listed in the peptide shop, and each product page links back to its profile.

Where the facts come from

Health and science facts on these pages come from regulators such as FDA and Health Canada, drug labels on DailyMed, NIH databases such as PubMed and PubChem, peer-reviewed journals, trial registries and anti-doping bodies. Supplier pages are used only for what a vial contains, never for a health claim. Each page lists its sources at the end, and the page on how we source and review our content explains the process.