What are peptides?

Peptide vs protein: the 40-amino-acid line

Amino acids join end to end into chains. Short chains are called peptides and long ones proteins, and in everyday science the boundary is fuzzy. U.S. drug law makes it exact. Since March 23, 2020, a "protein" is "any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size".^[1]

The line has practical effects:

• 40 amino acids or fewer: regulated as a drug, approved through a new drug application.
• More than 40: regulated as a biological product. Approved drugs on the protein side were moved to biologics licenses on March 23, 2020.^[2]
• Biologics cannot be compounded. The FDA says licensed biological products "are not eligible for the exemptions for compounded drugs".^[3]

How big are the peptides on this site?

The peptides on this site range from 3 to 44 amino acids and from about 340 to 5,136 g/mol.

Sources: PubChem records and FDA documents.^{[4][5][6][7][8]} The weights matter for arithmetic: 100 mcg of ipamorelin is about seven times as many molecules as 100 mcg of tesamorelin. Each peptide page converts micrograms to nanomoles for that molecule.

Tesamorelin, at 44 amino acids, sits just over the line. Its approved product, Egrifta, was transferred to a biologics license in 2020.^[2][9] Full-length thymosin beta-4, at 43, is also over the line, while TB-500, a 7-amino-acid fragment of it, is well under.

What do peptides do, and what are they used for?

Most peptide drugs act as signals: each binds specific receptors and switches on a specific response, which is why one peptide treats abdominal fat and another low sexual desire. The table shows what the peptides on this site act on and what they are used or studied for.

Outside medicine, many peptides are used in research, and some appear in cosmetics. Being sold for one of those purposes does not make a peptide tested for injection; the safety page explains what the FDA says about that.

Lyophilized peptides: why they come as powder

Many peptides are supplied freeze-dried (lyophilized) in a vial and dissolved before use, which is where the arithmetic on this site comes in. Approved products specify exactly how: the Egrifta SV label names the diluent, the volume and how soon to use the solution.^[9] Others need no mixing at all; bremelanotide comes as a prefilled autoinjector.^[10] The reconstitution guide explains the math.

What makes a peptide a medicine

Being a peptide says nothing about being safe or effective. A peptide becomes an approved medicine only when a company tests a specific product for a specific use and the FDA approves it. Of the molecules in the table above, only three are in FDA-approved products sold today: tesamorelin, bremelanotide and elamipretide (SS-31). The others range from "approved once, then discontinued" (sermorelin) to "no human trial at all" (TB-500, MOTS-c).

Peptide terms explained

These are the words that come up around peptides and reconstitution, each defined in a sentence or two, with a source where one is needed.

503A

The section of the federal Food, Drug, and Cosmetic Act that sets the conditions for compounding by state-licensed pharmacies, including which bulk substances may be used.^[15]

Amino acid

One of the building blocks of peptides and proteins. A peptide is a short chain of amino acids joined end to end.

Bacteriostatic water

Sterile water with 0.9% benzyl alcohol added as a preservative, which allows more than one withdrawal from the same container. Its label warns against use in newborns.^[14]

Category 2 (503A)

The FDA's list of nominated bulk substances that it says raise significant safety risks when compounded. Several peptides on this site are on it. See the status table.^[15]

Compounding

A licensed pharmacist or physician combining, mixing or altering ingredients to make a drug for an individual patient. Compounded drugs are not FDA-approved.

Concentration

How much substance is in each millilitre of solution, written mg/mL. After reconstitution it equals the mg in the vial divided by the mL of liquid added.

Diluent

The liquid used for reconstitution, such as sterile water or bacteriostatic water. Approved medicines name the diluent on their label.

Doses per vial

The vial amount divided by the amount measured each time. In practice a little is lost in the syringe and needle, so the real number is slightly lower.

Half-life

The time it takes for the amount of a substance in the body to fall by half.

IU (international unit)

A measure of biological activity defined separately for each substance. It has no fixed conversion to mg and is unrelated to syringe units.

Lyophilized

Freeze-dried. Many peptides are supplied as a dry powder or "cake" in a vial because they are more stable that way than in solution.

mcg (microgram)

One millionth of a gram, or one thousandth of a milligram. Also written µg. 1 mg = 1,000 mcg. See the mg to mcg chart.

mcg/kg

Micrograms per kilogram of body weight. Studies often report amounts this way. It is a rate, not a total.

mg (milligram)

One thousandth of a gram. Vial strengths are usually given in mg.

Peptide

A short chain of amino acids. In U.S. drug law, a chain of 40 amino acids or fewer is a peptide; a longer one is a protein and is regulated as a biological product.^[1]

Reconstitution

Dissolving a lyophilized powder in a measured volume of liquid. The volume sets the concentration. See the reconstitution guide.

Research use only (RUO)

A label placed on chemicals meant for laboratory work. It is not an approval for any use in people. See the safety page.

Subcutaneous

Under the skin, into the fat layer. Abbreviated SC or SQ.

U-100 syringe

An insulin syringe marked so that 100 units equal 1 mL.^[13] One unit is 0.01 mL. The sizes are 0.3 mL (30 units), 0.5 mL (50 units) and 1 mL (100 units).

Unit (syringe)

A volume mark on an insulin syringe. On a U-100 syringe one unit is 0.01 mL. It says nothing about how much drug the volume contains.

WADA Prohibited List

The World Anti-Doping Agency's annual list of substances and methods banned in sport. Several peptides are named on it.^[16]

Where to go next

• Peptide status tableApproval, compounding, sport and evidence for each
• Peptide calculatorTurn mg, mcg and mL into syringe units
• How to store peptidesWhat approved labels say about storage