Benzyl alcohol stalls bacteria

Benzyl alcohol slips into the fatty membrane of a bacterial cell and loosens it, which disturbs the cell's ability to hold its contents and keep its pumps working. At 9 to 11 mg per mL it does not sterilise the vial, it holds growth back, which is what the word bacteriostatic means. That is enough to cover the small number of organisms a needle can carry in on a repeat entry. It is the reason one vial can serve a month of daily draws.