How do I know if my semaglutide is real?

Test a sample from your batch. Mass spectrometry should find a main component near 4113.58 g/mol average or 4111.12 Da monoisotopic. HPLC then gives the purity, and an amount test reports the milligrams present. In published testing of three online vials, semaglutide made up 7.7% to 14.37% of the powder weight despite a 99% purity claim, so a printed figure alone proves little.

What mass should a semaglutide COA show?

The FDA label lists semaglutide as C187H291N45O59 with a molecular weight of 4113.58 g/mol. PubChem gives a monoisotopic mass of 4111.12 Da. A report should state the expected and measured value and which of the two it compared against. A measured mass far from both suggests a different peptide, a salt mix-up in the calculation, or a modified form.

Is semaglutide sodium or acetate the same as semaglutide?

Not according to the US FDA, which says semaglutide sodium and semaglutide acetate are different active ingredients from the one in the approved drugs, and that it lacks information on whether they behave the same. For testing, the salt matters because it adds weight. A counter-ion test tells you which salt is present and whether the label amount refers to salt or peptide.

Can a semaglutide vial have more than the label and still be bad?

Yes. In a 2024 analysis of three vials bought online, semaglutide content was 28.56% to 38.69% above the label, while semaglutide was only 7.7% to 14.37% of the powder weight. Overfilling with the right molecule does not explain the rest of the powder. Identity, amount and purity are separate results, and a sample needs all three to be judged.

Does light or storage change a semaglutide test result?

It can. A 2026 study found that two compounded semaglutide solutions without light protection lost strength and gained impurities after light exposure, while the originator in its capped pen did not change meaningfully. If a sample sat in light or heat before testing, a lower purity may reflect storage as well as manufacture. Keep vials sealed, dark and cool until sampled, and record their history.

One of the 30 compound testing profiles

Semaglutide

Semaglutide is a 31-amino-acid peptide (C187H291N45O59, average mass 4113.58 g/mol) carrying a C18 fatty diacid on one lysine; a test should confirm that mass, show the salt form, and measure how much semaglutide the vial holds.

Semaglutide is a GLP-1 receptor agonist, an analogue of human GLP-1 with a fatty diacid side chain, and the active ingredient of Ozempic, a prescription medicine approved by the US FDA (Ozempic prescribing information, section 11).

Test your semaglutide

Identity card

Formula: C187H291N45O59
Average mass: 4,113.58 g/mol
Monoisotopic mass: 4,111.12 Da
Length: 31 residues
Modifications: Aib (alpha-aminoisobutyric acid) at position 8 in GLP-1 numbering (position 2 here, shown as X); Lys26 in GLP-1 numbering (position 20 here) carries a hydrophilic spacer and a C18 fatty diacid; Arg at position 34 in GLP-1 numbering (position 28 here) in place of lysine; Free C-terminal acid
Also sold as: Sema
Sequence: HXEGTFTSDVSSYLEGQAAKEFIAWLVRGRG
Note: The FDA label gives the formula and a molecular weight of 4113.58 g/mol; PubChem rounds the average mass to 4114 and gives a monoisotopic mass of 4111.1153770 Da, recorded here as 4111.12. Labels and papers number the residues 7 to 37 after native GLP-1, so position 1 in the sequence string is His7. The sequence string is from KEGG DRUG D10025.

Source: PubChem CID 56843331, Semaglutide, Ozempic (semaglutide) prescribing information, section 11 Description, KEGG DRUG D10025, Semaglutide.

Compare the mass printed on a certificate with the values above. The mass checker explains charge states and which mass a lab usually reports.

What usually goes wrong with semaglutide. Documented failure modes, with sources.

More semaglutide than the label, in a mostly non-semaglutide powder

A 2024 study bought three lyophilised semaglutide vials from online sellers and analysed them by LC-MS. Each held 28.56% to 38.69% more semaglutide than labelled, yet semaglutide made up only 7.7% to 14.37% of the powder weight, against 99% purity claimed by the sellers; one vial held 3.86 mg in 50.1 mg of powder (Ashraf et al., Journal of Medical Internet Research, 2024). No peptide-like impurities were found, and the authors suggested low-mass, weakly ionising material. A 99% HPLC purity and a 7.7% peptide content can describe the same vial, so a report has to say which it measured.

Salt forms that are different active ingredients

The US FDA says some semaglutide is sold as salt forms, including semaglutide sodium and semaglutide acetate, and that these are different active ingredients from the one in the approved drugs (FDA, unapproved GLP-1 drugs). A salt changes what the powder weighs per milligram of peptide, and mass spectrometry of the peptide ion does not show it. Counter-ion analysis shows which salt is present and whether the label amount refers to salt or peptide.

Deletions, insertions, adducts and cross-contamination

LC-MS of compounded and follow-on semaglutide found impurity profiles unlike the originator: amino acid deletions and additions (the authors synthesised examples such as des-Thr11 and endo-Trp31 to study them), covalent dimers and trimers, oxidations and a formaldehyde adduct. One compounded sample contained a component with a mass corresponding to tirzepatide, another one matching insulin glulisine (Kopp et al., Pharmaceutical Research, 2026; the authors are Novo Nordisk staff). Each has a different mass from intact semaglutide, so the report needs the masses of the minor peaks, not only the main one. Deletions and insertions come from incomplete steps in solid-phase synthesis (D'Hondt et al., 2014).

Light exposure before testing

In the same study, originator semaglutide tested in its capped pen showed no change in strength after controlled light exposure, with total impurities moving from 3.4% to 3.5%. Two compounded solutions in packaging with no light protection lost strength, one from 1.02 to 0.86 mg/mL, and their total impurities rose from 1.3% to 5.1% and from 2.5% to 14.2% (Kopp et al., 2026). Those were solutions, not powders, but the point stands: record how a sample was stored and read a low purity result with that history.

Histidine 7 and tryptophan 31

Semaglutide begins with histidine, historically a notoriously racemisation-prone residue during coupling, which yields a diastereomer of identical mass that only chromatography can separate (Behrendt et al., Journal of Peptide Science, 2016). With no methionine, the single tryptophan at position 31 is the main oxidation site (Butré et al., Molecules, 2023).

What we check in Semaglutide. Silent, with captions.

Which tests matter most

LC-MSConfirms the 4113.58 average mass and names the minor peaks: deletions, insertions, adducts and any foreign peptide such as tirzepatide.
HPLC-UVGives the main-peak purity; read it beside a measured peptide content, since the two can differ widely in the same vial.
Counter-ion contentShows whether the vial is a salt form, such as the sodium or acetate salts the FDA has flagged, and lets the amount be stated as peptide.
LAL endotoxinA 2024 study of three online semaglutide vials reported endotoxin in one at 8.9511 EU/mg; identity and purity tests do not measure it.

Reading semaglutide results

Expected mass: 4113.58 g/mol average (FDA label) or 4111.12 Da monoisotopic (PubChem). Some reports number residues 1 to 31, others 7 to 37 after native GLP-1, so the acylated lysine can appear as Lys20 or Lys26 and the tryptophan as Trp25 or Trp31; both describe the same molecule. The mass checker computes expected masses from a sequence.

The approved product is a clear, colourless solution in prefilled pens and syringes, with a peptide backbone produced by yeast fermentation (Ozempic prescribing information). The follow-on semaglutide drug substances in the 2026 study were made by chemical synthesis (Kopp et al., 2026), a different route with a different impurity profile. An amount test against a characterised reference standard gives the milligrams to compare with the label; see amount testing.

Endotoxin is a separate question. In the 2024 online-vial study no viable microorganisms were found; endotoxin measured 8.9511 EU/mg in one vial and was reported as below 2.8658 and 2.1645 EU/mg in the other two (Ashraf et al., 2024). The endotoxin lesson explains the number.

Public Tested.ph results

No public semaglutide records yet

Each public semaglutide record will appear here with its identity, HPLC purity and measured amount against the label, once the first vial is tested.

FIG 13Amount against the label

Questions about testing semaglutide

How do I know if my semaglutide is real?: Test a sample from your batch. Mass spectrometry should find a main component near 4113.58 g/mol average or 4111.12 Da monoisotopic. HPLC then gives the purity, and an amount test reports the milligrams present. In published testing of three online vials, semaglutide made up 7.7% to 14.37% of the powder weight despite a 99% purity claim, so a printed figure alone proves little.
What mass should a semaglutide COA show?: The FDA label lists semaglutide as C187H291N45O59 with a molecular weight of 4113.58 g/mol. PubChem gives a monoisotopic mass of 4111.12 Da. A report should state the expected and measured value and which of the two it compared against. A measured mass far from both suggests a different peptide, a salt mix-up in the calculation, or a modified form.
Is semaglutide sodium or acetate the same as semaglutide?: Not according to the US FDA, which says semaglutide sodium and semaglutide acetate are different active ingredients from the one in the approved drugs, and that it lacks information on whether they behave the same. For testing, the salt matters because it adds weight. A counter-ion test tells you which salt is present and whether the label amount refers to salt or peptide.
Can a semaglutide vial have more than the label and still be bad?: Yes. In a 2024 analysis of three vials bought online, semaglutide content was 28.56% to 38.69% above the label, while semaglutide was only 7.7% to 14.37% of the powder weight. Overfilling with the right molecule does not explain the rest of the powder. Identity, amount and purity are separate results, and a sample needs all three to be judged.
Does light or storage change a semaglutide test result?: It can. A 2026 study found that two compounded semaglutide solutions without light protection lost strength and gained impurities after light exposure, while the originator in its capped pen did not change meaningfully. If a sample sat in light or heat before testing, a lower purity may reflect storage as well as manufacture. Keep vials sealed, dark and cool until sampled, and record their history.

Sources

Ozempic (semaglutide) prescribing information, Novo Nordisk
FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss, US Food and Drug Administration
Ashraf AR et al. Multifactor Quality and Safety Analysis of Semaglutide Products Sold by Online Sellers Without a Prescription. J Med Internet Res. 2024;26:e65440, Journal of Medical Internet Research (PMC)
Kopp KL et al. Impurities and Potential Immunogenicity Associated With Follow-on and Compounded GLP-1 Receptor Agonists. Pharm Res. 2026;43(8):2861-2886, Pharmaceutical Research (PMC)
D'Hondt M et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30, PubMed
Behrendt R, White P, Offer J. Advances in Fmoc solid-phase peptide synthesis. J Pept Sci. 2016;22(1):4-27, Journal of Peptide Science (PMC)
Butré CI et al. Interlaboratory Evaluation of a User-Friendly Benchtop Mass Spectrometer. Molecules. 2023;28(6):2855, Molecules (PMC)
PubChem CID 56843331, Semaglutide, National Library of Medicine

Last reviewed 6 Oct 2026.