What are peptides?
What peptides are, how they work, why most are injected, and which compounds on this site aren’t peptides at all.
Short chains of amino acids
Peptides are chains of amino acids, the same building blocks that make up proteins, joined by peptide bonds. The difference is length. Under FDA’s definition, a chain of more than 40 amino acids is a protein (21 CFR 600.3); shorter chains are peptides. Glutathione has 3 amino acids, BPC-157 has 15 and semaglutide has 31.
The line matters in the US: since March 2020, products over it, such as hCG and tesamorelin, are regulated as biologics rather than ordinary drugs.
How they work
Most peptides here act as signals. They fit a receptor on a cell, like a key in a lock, and switch a process on or off: GLP-1 drugs act on receptors that slow the stomach and curb appetite, and growth hormone secretagogues act on receptors in the pituitary. Many are modified copies of the body’s own hormones, changed to last longer. Semaglutide carries a fatty acid that lets it bind blood albumin, stretching a natural half-life of minutes to about a week.
Why most are injected
Swallowed peptides are broken down by stomach acid and digestive enzymes, like the protein in food, and they’re too large to cross the gut wall easily. Even oral semaglutide (Rybelsus), made with an absorption enhancer and taken on an empty stomach, gets only about 0.4 to 2% of each tablet into the blood, which is why the tablet dose is far higher than the injected one.
Size of each compound
Amino acids in each chain. The dashed line is FDA’s 40-amino-acid line between peptide and protein.
Follistatin-344 (315), hCG (237), IGF-1 LR3 (83) and IGF-1 DES (67) run past the end of the scale. Thymosin Beta-4 (43), Tesamorelin (44) and FOXO4-DRI (46) are just over the line.
Is it a peptide?
Several compounds sold alongside peptides aren’t peptides. That changes little about how they’re mixed and dosed, but it explains why some come as tablets.
| Compound | What it is |
|---|---|
| 5-Amino-1MQ | Not a peptide (small molecule)159.2 g/mol |
| SLU-PP-332 | Not a peptide (small molecule)290.3 g/mol |
| L-Carnitine | Not a peptide (small molecule)161.2 g/mol |
| Orforglipron | Not a peptide (small molecule)883 g/mol |
| Tesofensine | Not a peptide (small molecule)328.3 g/mol |
| NAD+ | Not a peptide (small molecule)663.4 g/mol |
| MK-677 | Not a peptide (small molecule)528.7 g/mol |
| Cerebrolysin | Mixture of peptides |
| Thymalin | Mixture of peptides |
| Thymosin Beta-4 | Usually called a peptide; a protein under FDA’s 40-amino-acid rule, 43 amino acids4,963.5 g/mol |
| Tesamorelin | Usually called a peptide; a protein under FDA’s 40-amino-acid rule, 44 amino acids5,135.9 g/mol |
| FOXO4-DRI | Usually called a peptide; a protein under FDA’s 40-amino-acid rule, 46 amino acids5,358.1 g/mol |
| IGF-1 DES | Protein, 67 amino acids |
| IGF-1 LR3 | Protein, 83 amino acids |
| hCG | Protein, 237 amino acids |
| Follistatin-344 | Protein, 315 amino acids |
Every other compound in the library is a peptide of 3 to 39 amino acids, and the blends are mixes of them.
Approved, compounded or research-only
The same molecule can be an approved drug, a compounded preparation or a research chemical, and only the first has been checked for identity, purity and dose by a regulator. See legal and regulatory status for where each compound stands.
Sources: 21 CFR 600.3(h)(6) (current as of October 1, 2026); FDA, List of approved NDAs for biological products that were deemed to be BLAs on March 23, 2020; Rybelsus prescribing information, section 12.3 (revised October 2025); Drucker, Nat Rev Drug Discov 2020, PMID 31848464. Amino acid counts are from each compound’s datasheet.