Purity vs content explained

Two different numbers

Peptide sellers love to advertise purity. But purity and content answer different questions.

• Purity asks: of the peptide material the test can see, how much is the right peptide? It is usually measured by HPLC, a test that separates molecules and shows them as peaks. Purity is the main peak's share of all the peaks.^[1]
• Content asks: how much of the powder's total weight is actually the peptide? This is a separate measurement.^[1][2]

In drug quality standards, the amount of active ingredient and the level of impurities are listed as separate tests for exactly this reason.^[2]

What else is in the powder

A freeze dried peptide is rarely 100 percent peptide by weight. The rest can include:

• Counter ions. These are small charged molecules that cling to the peptide. Trifluoroacetic acid, or TFA, is used to make and purify many peptides and often stays behind as a salt. Acetate and chloride are other common ones.^[3]
• Water and leftover solvents.^[1]
• Inorganic salts and other residue.^[1]

None of these show up in an HPLC purity number.^[1] They can be large. One study measured TFA at up to 35 percent of the total weight of some peptide salts.^[3] The authors note that this can cause weighing errors in research.^[3]

How a 99 percent pure vial can hold less peptide than the label

Here is a made up example to show the math. Say a vial holds 100 milligrams of powder, and the HPLC purity is 99 percent. Now say counter ions, water and salts make up a quarter of the powder's weight. The actual peptide is then only about 74 milligrams, even though the vial is "99 percent pure."

A vial label may not say whether its milligram number means total powder or actual peptide, and most online CoAs report only purity. FDA found that pattern when it reviewed online CoAs for BPC-157.^[4]

Label milligrams versus what is really there

Even the label itself may not be right. In a 2024 study of semaglutide bought online without a prescription, the vials held 28 to 39 percent more semaglutide than the label said. At the same time, measured purity was only about 8 to 14 percent, compared with the 99 percent the paperwork claimed.^[5] More drug than expected and a powder that is mostly something else are both problems.

Salt forms add another layer. FDA says forms such as semaglutide sodium and semaglutide acetate are different active ingredients from the one in approved drugs, and it has no information showing they act the same.^[6]

The units versus milliliters mistake

Three measurements get mixed up all the time:

• Milligrams (mg) are an amount of drug.
• Milliliters (mL) are a volume of liquid.
• "Units" on an insulin syringe are volume markings. On a U-100 syringe, each unit mark is one hundredth of a milliliter. How much drug sits in one unit depends entirely on how concentrated that particular vial is.^[7]

In July 2024, FDA warned about dosing errors with compounded injectable semaglutide. Compounded products come in different strengths, and some were dosed in "units," which confused patients and even prescribers. Some patients were given syringes far larger than needed. Patients measuring their own doses sometimes took 5 to 20 times the intended amount, and some provider math errors led to 5 to 10 times the intended amount.^[7]

Reported effects included nausea, vomiting, stomach pain, fainting, dehydration, pancreatitis and gallstones. Some people were hospitalized. Because semaglutide lasts about a week in the body, problems may need long monitoring.^[7]

If the true content of a vial is unknown, this kind of error becomes even harder to avoid. FDA's advice is to ask your prescriber or pharmacist exactly how to measure the intended dose.^[7] This site does not give dosing or mixing instructions.

Bottom line

A high purity number does not tell you how much peptide is in a vial. Without a separate content test, the milligrams on the label are a guess.

Sources

1. McCarthy D, Han Y, Carrick K, et al. Reference Standards to Support Quality of Synthetic Peptide Therapeutics. Pharm Res. 2023;40:1317-1328. doi:10.1007/s11095-023-03493-1
2. U.S. Food and Drug Administration. Q6A Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products: Chemical Substances. December 2000. FDA guidance page
3. Erckes V, Streuli A, Chamera Rendueles L, Krämer SD, Steuer C. Towards a Consensus for the Analysis and Exchange of TFA as a Counterion in Synthetic Peptides and Its Influence on Membrane Permeation. Pharmaceuticals. 2025;18(8):1163. doi:10.3390/ph18081163
4. U.S. Food and Drug Administration. FDA Evaluation of BPC-157 Related Bulk Drug Substances. Briefing document, Pharmacy Compounding Advisory Committee meeting, July 23-24, 2026. FDA briefing document (PDF)
5. Ashraf AR, Mackey TK, Vida RG, et al. Multifactor Quality and Safety Analysis of Semaglutide Products Sold by Online Sellers Without a Prescription: Market Surveillance, Content Analysis, and Product Purchase Evaluation Study. J Med Internet Res. 2024;26:e65440. doi:10.2196/65440
6. U.S. Food and Drug Administration. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. Updated October 1, 2026. FDA safety page
7. U.S. Food and Drug Administration. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. July 26, 2024. FDA alert