I reconstituted with plain sterile water two weeks ago — what actually happens from here?

I grabbed the wrong ampoule. A 10 mg research vial was reconstituted with 2 mL of plain Sterile Water for Injection USP, not bacteriostatic water, and it has been at 4 °C for fourteen days. It has been entered four times with a fresh 31 G fixed-needle syringe each time. The solution is still completely clear against a black card and there is no film on the meniscus.

Most of what I can find online is either "it is totally fine, water is water" or "bin it immediately", with no reasoning attached to either. I would like to understand the actual risk model:

• Is the peptide itself affected by the absence of benzyl alcohol, or is this purely a microbiological question?
• If it is microbiological, what is the realistic organism and what is its realistic time course at 4 °C in an essentially nutrient-free unbuffered aqueous solution?
• Is there anything I can inspect that would change my assessment, or is "looks clear" worthless as evidence?
• Can I add benzyl alcohol now, or is that closing the barn door?

Research use only, and I understand nobody here can tell me a vial is safe. I want the reasoning so I can make my own call and design the workflow so it does not happen again.

Slightly harder line than the accepted answer, on a specific point: the fact that you cannot detect the failure is an argument for conservatism, not against it.

Consider the asymmetry. If the vial is fine and you discard it, you lose the cost of one vial. If the vial has 10^4 CFU/mL of a Gram-negative environmental organism and you do not discard it, the peptide content is irrelevant because the endotoxin load is the problem, and endotoxin is not something a clear appearance rules out at all. Pyrogenic response is driven by nanograms per kilogram; an organism count far below visual turbidity can carry a biologically significant endotoxin burden even if every cell is now dead.

Two related points that follow:

• Sterile filtration would not fix this either. A 0.22 µm filter removes cells; it does not remove free lipopolysaccharide, which passes straight through. Filtering a suspect vial gives you a clear solution with the same endotoxin content and a false sense of resolution. That is arguably worse than not filtering.
• Refrigeration is not protection, it is delay. The vial went into the fridge cold and comes out for each entry. If it sat on the bench for twenty minutes four times, that is over an hour at permissive temperature — plenty for Pseudomonas at generous growth rates, and the cold interval afterwards does not reverse anything.

None of this means your specific vial is contaminated. On priors it very likely is not, particularly with fresh needles. It means the evidence you can gather does not distinguish the good case from the bad case, so the decision has to be made on consequence rather than on inspection. Reasonable people land differently on that. Just make sure you are choosing rather than assuming.

On the tonicity point, since it comes up in every one of these threads and is usually stated wrongly.

People say "use bacteriostatic water because sterile water stings". The mechanism is not tonicity difference between the two products, because both are hypotonic — adding 9 mg/mL benzyl alcohol to water contributes negligible osmolality. If a preserved diluent is reported as more comfortable than an unpreserved one, the likely reason is benzyl alcohol's weak local anaesthetic activity, not osmolality.

What actually reduces the hypotonic contribution is the solute in your vial: buffer salts, bulking agent and peptide from the lyophilisation cake. A cake containing mannitol or a phosphate buffer system pulls the reconstituted solution toward isotonic on its own. That is why a 2 mL reconstitution of a well-formulated cake behaves differently from 2 mL of the same diluent alone, and why generalisations about "water stings" are unreliable across products.

If tonicity is genuinely the thing you want to control, the correct product is Bacteriostatic 0.9 % Sodium Chloride Injection, which is both preserved and isotonic. Be aware that for some peptides higher ionic strength changes solubility and aggregation propensity, so it is not a free swap — check what the supplier's own reconstitution instruction says and follow that, because they have presumably tested their own formulation and you have not.

Practical workflow note that prevents the recurrence, which is the part of the question worth more than the disposition of this one vial.

Ampoule confusion happens because Sterile Water for Injection and Bacteriostatic Water for Injection labels are visually near-identical: same size, same clear liquid, same typography conventions, and the distinguishing word is set in the same weight as everything else. Three things that reliably fix it:

1. Never store the two in the same box. Different container, different shelf, physically separated.
2. Mark the preserved bottles yourself with a broad marker stripe on first receipt. You are not trying to be legible, you are trying to make the two piles distinguishable at arm's length in bad light.
3. Write the diluent identity and the date on the vial you reconstitute, not just the concentration. If this had been written on the vial, the question would have been asked on day zero when the answer was simply "reconstitute a fresh one" and the cost was one vial.

The last one is the highest-value habit in this entire subject area and it costs nothing. Compound, concentration, diluent, date. Four fields.

A related failure worth designing out at the same time: the diluent bottle itself going unrecorded. Write the first-entry date on the bottle when you pierce it, because the in-use clock on a preserved multiple-dose container starts then and not at purchase. A bottle whose first-entry date you cannot recall is a bottle of unknown provenance, and it will be the common factor across every vial you make from it — which turns one undated container into an entire batch of unanswerable questions.