The package literature on Frozen Vancomycin injection instructs the
user not to refreeze but does not indicate why this practice is
discouraged. I would like to know why refreezing is prohibited.

The thawed solution in GALAXY plastic container (PL 2040) remains
chemically stable for 72 hours at room temperature (25°C/77°F) or for
30 days when stored under refrigeration (5°C/41°F).

http://72.14.207.104/search?q=cache:4SNDT4jllcsJ:www.rxlist.com/cgi/generic2/vancociniv_ids.htm+Vancomycin+injection+frozen+pharmacology+refreezing&hl=en&gl=au&ct=clnk&cd=1

It may be assumed that the structure degrades on thawing and so if it
were refrozen, one would be (next time) using an unstable product. The
results of which (depending on the chemistry) may result in a anything
from a useless product to a dangerous one.

For example look what happens to ice cream (particularly cheap
icecream) when you buy it frozen, let it thaw and then refreeze...the
water component separates from the fat. The water refreezes into icy
crystals, crunchy, but not the same ! In other words the chemical
structure is changed by refreezing  with (unlike simple water) some
permanent changes ocurring that change the nature of the drug.