Underdosed peptides: which compounds miss the label most (and which are overfilled)

"Underdosed" is one of the most-used words in this market, and one of the least precisely used. People say it about a vial that felt weak, one that a friend said was weak, or one where the label and the lab report simply disagree. Only the last of those is something a lab report can actually establish.

This piece sticks to that last kind: what independent labs measured against what the label claimed, across 1,441 research peptide vials in our database with a quantity result. It's not about what any dose does or should do — that's outside what a lab report can tell you, and outside what we cover, for compounds sold for research use only. It's about a narrower and more answerable question: does the vial hold what the label says it holds?

What "underdosed" means on a lab report

A lab measuring quantity — sometimes called content, net content, or assay — is comparing the actual amount of peptide detected in a vial against the amount printed on the label. That's a different question from purity. Purity asks what share of the material in the vial is the target peptide; quantity asks how much of that peptide is actually there in absolute terms.

A vial can be 99.5% pure and still be badly underdosed, if the peptide making up that 99.5% simply isn't enough. And a vial can measure well over its label while still being highly pure — overfill and purity aren't opposites, they're unrelated axes. Both numbers matter, and reading only one tells you very little about the other.

On VendorVerify, a vial counts as a scoring failure if it's more than 20% under its labeled amount, more than 50% over it, empty, or under 95% purity. That threshold is about scoring consistency, not a claim that 19% under is fine and 21% under isn't; the underlying number is what matters, and we show it either way.

How quantity is actually measured

Labs use a few different methods to get a quantity number, and the certificate doesn't always spell out which one. The most common is comparing the sample's HPLC peak area against a reference standard of known concentration — essentially the same run that produces the purity figure, read a second way. Some labs use UV-Vis spectrophotometry, measuring how much light the dissolved sample absorbs at a specific wavelength and calculating concentration from that. Amino acid analysis, which breaks the peptide down into its building blocks and measures those directly, is another approach, less often seen on reports in this market.

None of these methods is inherently more trustworthy than the others when done correctly, but they can produce slightly different numbers on the same vial, and a report rarely says enough about its method for a reader to judge precision. That's one more reason a single quantity result is a data point, not a verdict — and why we treat a vendor's pattern across many reports as more informative than any one certificate.

What 1,441 measured vials show

Here's how quantity results compared with their labels across every vial in our database with a measured amount, as of September 28, 2026:

Two things stand out immediately. First, underfill is the minority outcome: 115 vials, about 8%, ran more than 5% under label, against 824 vials, roughly 57%, that ran more than 5% over. Second, "exactly on label" is itself a minority result — only about a third of vials land within ±5%. The median across all 1,441 vials is +6.5% over label, not zero.

So the more common failure mode, in a strictly literal sense, is overfill, not underfill. Thirty-seven vials came in more than 20% short, which is real and worth taking seriously. A hundred and fifty-eight came in more than 20% over, which is a larger group, and worth understanding for a different reason.

Why overfill happens, and why it's still worth knowing

We don't have a way to know any individual vendor's intent, and we're not going to guess at it. But there are a few plausible, unremarkable reasons overfill shows up this often across a market this size.

Peptide synthesis and lyophilization — the freeze-drying step that turns a liquid solution into the powder in the vial — aren't precise to the microgram. Hitting a target fill weight exactly, batch after batch, is harder than hitting a range around it, and a manufacturer may prefer to err generous rather than risk a vial testing short. Overfilling by 10–20% may work as a buffer against that variability, and against exactly the kind of underdosing complaints that drive people to order lab tests in the first place.

That doesn't make overfill a non-issue. If you're relying on a label to know what's actually in a vial — for consistency between batches, for comparing one vendor's product to another's, or simply for knowing what you're paying for per milligram — a vial that runs 20% or 40% over label is just as inaccurate as one that runs 20% under. We're not making a dosing point here; we're making a measurement point. The label is a claim about contents, and a lab report is the only way to check that claim, in either direction.

The purity-vs-quantity trap

This is the finding worth remembering longer than any single percentage. Fifty-two vials in our data tested at 99% purity or higher while measuring more than 10% under their labeled amount. Twenty of those fifty-two were more than 20% under — a scoring failure by our threshold, sitting behind a purity number that looks close to perfect.

If you read only the purity line on a certificate, every one of those 52 vials looks excellent. The purity result isn't wrong — it's answering a real question, just not the one that matters for whether the vial holds what it claims. This is the single best argument for reading a full report rather than the headline purity percentage, and for asking specifically whether quantity was tested at all when a certificate doesn't show it.

By peptide: who runs short, who runs over

Aggregated market-wide numbers can hide a lot. Here's how the twelve peptides in our database with 15 or more vials with a quantity result compare, again as counts rather than the site's weighted scoring rates:

A few patterns are worth calling out individually.

Tesamorelin, NAD+ and ipamorelin have the highest share of badly short vials. About one in ten of each came in more than 20% under label: 3 of 30 tesamorelin vials, 3 of 28 NAD+ vials and 2 of 20 ipamorelin vials. With samples this small, one or two new results can move these shares a lot.

Ipamorelin is also the only peptide on this list with a negative median — the typical ipamorelin vial in our data ran 1.6% under label rather than over it. It's a small sample, but it's the one compound here that doesn't follow the market-wide pattern of running generous.

GHK-Cu is the overfill outlier. Ten of its 28 measured vials — more than a third — came in over 50% above label, which counts as a scoring failure on our site even though it's an overfill, not a shortfall. Nothing in our data suggests this is deliberate rather than a fill-weight or concentration-labeling issue upstream, but it's a large and consistent enough pattern to flag on its own.

BPC-157, MOTS-C, and TB-500 all run well over label on average. BPC-157's median is +17.8%, MOTS-C's is +13.5%, and TB-500's is +9.0%. None of the three has a single vial in our data that measured more than 20% under label — whatever their manufacturing process is doing, it isn't producing shortfalls.

Tirzepatide and retatrutide, the two largest samples by far, sit closer to the market median — +5.4% and +8.2% — with a wider absolute spread simply because there are so many more of them tested. Tirzepatide has the most vials more than 20% under label of any peptide, 12, but that is 1.6% of its 764 measured vials, a smaller share than tesamorelin (3 of 30), NAD+ (3 of 28) or ipamorelin (2 of 20).

What overfill and underfill share

It's worth naming the thing both ends of this distribution have in common: they both mean the number on the label isn't the number in the vial. A buyer comparing price per milligram between two vendors, or comparing one batch with the next, is relying on that label being accurate in a specific, narrow sense — not as guidance for how to use anything, but as a factual description of contents, the same way a food label states ingredient weights. A vial that runs 30% over label breaks that reliability just as much as one that runs 30% under, even though only one of them would show up as a complaint in a community thread.

That asymmetry in what gets talked about — underfill generates complaints, overfill mostly doesn't — is probably part of why "underdosed" is the word that circulates, while overfill rarely gets discussed as its own category. The lab data doesn't share that asymmetry. Both are measurable, and both appear on every report we publish, whichever direction they run.

A caution about small samples

Every peptide on that table except tirzepatide and retatrutide has fewer than 35 measured vials, and several have fewer than 20. A pattern built on 15 to 30 vials from a market of unknown total size can shift meaningfully with the next handful of test results, especially for a peptide like epitalon or SS-31 where one or two additional bad results would move the median noticeably. Treat every row below tirzepatide and retatrutide as a real but provisional picture, not a settled verdict on that compound. We keep these numbers current on each peptide's page, and they'll shift as more buyers submit reports.

Reading a blend or multi-peptide product

Everything above assumes a single-compound vial, which is most of what's in our data. Products that combine two or more peptides in one vial complicate the quantity question further, because a report needs to measure each component separately for a "vs. label" comparison to mean anything. A single combined weight, or a purity figure that doesn't break out each ingredient, can hide one component running well under label while another runs over, with the total landing somewhere in between that looks unremarkable. We don't have enough blend-specific data in our database yet to publish separate figures for combination products, but the same reading rules apply: look for each component listed and measured on its own, not folded into a single number.

What to check on a report

If you're trying to work out whether a vial's label is trustworthy, the quantity line is the one to look for specifically, separate from purity:

• Is quantity even reported. A certificate that only shows purity hasn't told you anything about fill accuracy, in either direction.
• What it's measured against. The lab should state the labeled amount it's comparing to, not just a raw number with no reference point.
• The batch and date. A quantity result is specific to the batch tested; it doesn't carry over to a different lot from the same vendor.
• How many vials, not just one. A single measurement tells you about one vial. A vendor with multiple independently tested vials landing in a consistent range tells you more than any individual report can.

We publish every measured quantity result, matched against its label, on each vendor's page and each peptide's results page. None of this is scored as a dosing recommendation, and none of it should be read as one — it's a record of what independent labs found in the vials sent to them, mostly by buyers, nothing more.

Figures in this article are from the VendorVerify database as of September 28, 2026. See also how to read a peptide COA for how purity and quantity results are calculated, and endotoxin and sterility in research peptides for what a quantity result doesn't tell you.