A former neighbor of mine used to take a trip or two a year to Mexico. She was a registered nurse at the time. But she moved away in the mid-90's so my info is dated.
She used to go to some town that is nothing but pharmacies and doctors. She also said she never had any problems transporting drugs back over the border, but she also admitted she didn't buy anything but antibiotics for herself and her family. I don't know how she got her stuff across the border or how much scrutiny she fell under each time she returned to the US.
She said that most drugs that were prescription in the US were sold over the counter in Mexico. She also said that one had to be knowledgeable about drugs because the labelling differed in Mexico. If you did not know what to look for, you could end up buying the wrong stuff.
If I remember rightly, most drugs are labelled by their common name. So instead of looking for
Viagra, you would look for
sildenafil citrate on the label.
As an aside, she also shopped for drugs at the local farm supply store. She told me most drugs for animals were manufactured to USP standards. Again she said all she had to do was read the label and she could buy what she needed to treat herself without paying the high pharmacy costs. All the same, I don't think there is a bovine version of Viagra.