Depends on the med and your doctor. Just tell him you want to have some extra ("just in case," or "leave at my parent's house" or something).

Have him fill out a second prescription. take it to your local Walgreen's, or Walmart, or Target. They all have $4 scrips for a month supply (different lists, but relatively comprehensive overall). You could just pony up the extra cost yourself.

Or, next time you're in Mexico...