Truth be known, you can buy many drugs in Mexico that you cannot buy without a prescription in the US. Antibiotics are now very difficult to acquire, even just plain ol' Amoxicillin. This is all over Mexico, not just Rocky Point. The Mexican Minister of Health put out an edict on this a year or so ago and it pretty well dried up over-the-counter antibiotics.
One of the things that helps is asking for the drug by its active ingredient, not its trade name.
carisoprodol = SOMA
diazepam = Valium
tramadol = Ultram
These are commonly sold and available for the asking. The legality of bringing them back across the border? I'm not going to touch that -- you take your chances.