The objection FDA keeps making
Across all seven of its 2026 peptide briefing packages, FDA raised the same scientific objection more consistently than any other. It is almost entirely absent from community discussion, and it explains something the purity number on a COA cannot.
The objection is immunogenicity: the risk that your immune system learns to recognise an injected peptide and mounts a response to it.
Why a peptide provokes a response at all
Your body produces thousands of peptides and does not attack them, so the intuition is that a synthetic copy should be equally invisible. Three things break that intuition.
Route. FDA's framing is explicit: subcutaneous and nasal administration carry higher immunogenicity risk than oral or rectal. The immune surveillance under the skin is designed to sample what arrives there. It is doing its job.
Aggregation. Peptides in solution can clump. Aggregates present the same molecule in a repeating, ordered array, and repeating arrays are exactly the pattern the immune system evolved to treat as foreign. This is the reason handling matters: shaking a vial, freeze-thaw cycles, storing a reconstituted vial past its window.
Impurities. Peptide synthesis produces related species: truncated chains, deletion sequences, oxidised variants. These are chemically close to the target and immunologically different from it.
The point that changes how you read a COA
Aggregation and impurities are separate axes from purity.
A vial can be 98% pure by HPLC and still be substantially aggregated, because HPLC purity measures how much of the sample is the target molecule, not what physical state it is in. Two vials with identical purity numbers can carry different immunogenicity risk, and the purity figure will not distinguish them.
That is why the community shorthand, "check the COA says ≥98%", is necessary and not sufficient. The fuller read is identity by mass spectrometry, purity by HPLC, endotoxin below 0.5 EU/mg, and a manufacturing story that makes aggregation less likely: real GMP facilities, cold-chain shipping, batch-specific testing rather than a representative certificate.
What the consequences actually look like
FDA's own description of the range is worth quoting, because it neither dismisses nor catastrophises: consequences run "from antibody responses with no apparent clinical manifestations to life-threatening and catastrophic reactions."
Most of that range is silent. An anti-drug antibody response that produces no symptom is the common case, and you would never know it happened. What it can do is quietly reduce the effect: your body clears the compound faster than it used to, and a protocol that worked in month one stops working in month four for a reason that has nothing to do with tolerance in the usual sense.
The severe end is rare and real. It is also the reason this is one of FDA's stated objections to compounding these substances at all.
Why it is missing from the conversation
Immunogenicity is invisible without a lab test that nobody in this market offers. It has no felt symptom in the common case, and no vendor has a commercial reason to raise it. So the discussion settles on the number that is easy to print, which is purity.
You cannot test for it yourself. What you can do is reduce the inputs. Buy from sources with real manufacturing behind them. Handle vials so they do not aggregate: swirl rather than shake, avoid freeze-thaw, respect the beyond-use window. And treat a declining response over months as information worth recording rather than a reason to raise the dose.