What I found was that my drug insurer wouldn't allow a six month supply to be paid for at the time of purchase, so what I had to do was buy 6 months, pay for 3 and submit a paper claim. I called the insurance company when I noticed a slight variance between the claims, and I was able to have them increase my allowable claim to 6 months. Last year, when filling my meds before heading south in December, it was slick as a whistle. The pharmacist says, if the doctor prescribes it, they will gladly fill it. Of course, we are talking about cholesterol and diabetes meds, nothing that can be used for mind altering or that sort.
As for Mexico, they might fill them, but we noticed that the US border guards were checking quantities when we walked back.
Ed