The story

Multi-dose vials created a problem the moment they existed: every entry of the needle is a fresh chance to carry an organism in, and a vial opened on Monday is still being used on Friday. Benzyl alcohol became the answer across the twentieth century because it is simple, stable, and works at a low strength. Clinics then found a second use for it by accident, noticing that patients flinched less when a preserved solution went in, and that observation was tested in its own right for eye injections and for the sting of propofol and lidocaine. One hard limit came out of the same century of use: benzyl alcohol is unsafe for newborns, so preservative-free water is used for them. That is the one place this vial is set aside.