Bacteriostatic versus sterile water
Sterile water contains no preservative and is intended for single use; once entered, it should be discarded. Bacteriostatic water contains a preservative, which is what permits multiple withdrawals from the same vial over a period.
The regulatory dimension suppliers underestimate
Bacteriostatic water is not an accessory. In the United States it is regulated as a drug product, which affects how it may be listed, sold, and advertised, and marketplaces enforce this inconsistently but seriously. Suppliers have had listings and accounts removed over it while treating it as a commodity add‑on.
Practical note
Treat it as a separate regulatory question from the research product it sits beside, and check the
rules for each channel independently. What is permitted on your own site, on a marketplace, and in
another country are three different answers.