What purity doesn't tell you: sterility, endotoxin, and injectable risk.

• Purity — a chemical question. Which peptide species are present, and in what proportion. Measured by HPLC, with identity confirmed by mass spectrometry.
• Sterility — a microbiological question. Whether viable organisms are present in the finished preparation. Measured by an incubated culture under controlled conditions, not by any chemical assay.
• Pyrogen-free — a toxicological question. Whether fever-inducing substances, principally bacterial endotoxin, are present below a defined limit. Measured by an LAL or recombinant factor C assay.

A purity figure speaks only to the first. It is not a partial answer to the other two; it is not an answer at all.

Endotoxin is lipopolysaccharide, a structural component of the outer membrane of Gram-negative bacteria. It is released when those bacteria die, and it is remarkably heat-stable — well beyond ordinary autoclaving conditions.

The practical consequence is counterintuitive: a preparation can be sterilized and still be pyrogenic. Killing the organisms does not remove what they left behind. A solution that passes a sterility test can still deliver a dose of endotoxin.

Clinically, a pyrogenic reaction presents within minutes to a few hours of injection: fever, shaking chills or rigors, headache, myalgia, nausea, sometimes hypotension and tachycardia. At the mild end it is mistaken for a virus. At the severe end it requires emergency care.

Endotoxin testing is a distinct assay with its own limits and acceptance criteria, and it is rarely supplied outside a pharmacy or manufacturing setting. Its absence from a document is not evidence of a problem — but it is the absence of evidence about the one property that determines how an injection feels an hour later.

Lyophilized powder becomes an injectable solution at the point of reconstitution, which means the properties of the finished preparation are determined partly by what happens at that step.

• Bacteriostatic water contains roughly 0.9% benzyl alcohol, which inhibits growth of some organisms after mixing. It is a preservative, not a sterilant: it does not sterilize the powder and does not neutralize endotoxin.
• Sterile water contains no preservative. A preparation made with it has no protection against organisms introduced during handling and is intended for immediate use rather than storage.
• Technique determines contamination. Vial septum disinfection, a fresh needle, avoiding touch contamination and avoiding repeated entries all change the microbiological outcome regardless of which diluent is used.
• Stability is finite after mixing. Peptides in solution degrade with time, temperature, light and agitation; refrigeration slows this but does not stop it, and beyond-use dating exists precisely because the reconstituted product is not the same product a month later.

For an injectable preparation, USP <797> governs how it is made: classified air environments, personnel gowning and competency testing, environmental monitoring, documented procedures, and beyond-use dating tied to how the preparation was made and how it will be stored. USP <71> sets out the sterility test itself, applied where the preparation and its beyond-use dating require it.

Nothing in the research-use-only category requires any of that. There is no environmental standard, no sterility requirement, no endotoxin limit, and no obligation to establish beyond-use dating. The gap is not that the standards are lower. It is that they do not apply.

An oral product that is contaminated meets stomach acid, bile, gut flora and an intact intestinal barrier. Those defenses fail sometimes, but they exist, and the usual failure is gastrointestinal and self-limiting.

An injection bypasses all of it. Whatever is in the syringe is delivered into subcutaneous tissue or the bloodstream directly, with no filtration step and no opportunity for a defense to intervene. The failure modes are therefore systemic and fast: cellulitis or abscess at the site, a pyrogenic reaction within hours, and in the worst case bacteremia.

That asymmetry is why sterility standards exist for injectable preparations and why the absence of them is a different order of question than it would be for a capsule.