Going south? Better know what drugs to take and those you'd be wise to leave at home.
Rule 1. Leave at home all or almost all patent medicines, since, e.g., the farmacia may not have Tylenol, but they'll have some brand of acetaminophen. And even if they do not speak English, acetaminophen is world-speak. They know what you want. If the farmacia doesn't have it, chances are the local Wal-Mart will. So: take your Wal-Mart card.
Rule 2. You may want to stock up on supplements and your favorite brand of multivitamin you use daily (in addition to any statins or prescription medicines, in which case best take a copy of the prescription with you), since finding kava-kava, for example, might get you socked for making an inappropriate sexual overture. I myself use a prostate aid called saw palmetto and when I search in vain for it in Mexico, some clerks say "from Florida?"
Rule 3. The roadblocks at various places in Mexico are probably less likely in this age to be looking for drugs, but you never know. The customs people in Mexico appear to be looking for movements of arms, but, then, what are the German shepherds there for? I once took a dozen joints to...well, I forget my destination that year, but I do remember smoking in my room in Queretaro, and flushing one joint down at my bus connection in Poza Rica, Veracruz, because I suspected it would be the last stop before the border.
Rule 4. Taking your Wal-Mart card may actually prevent you from daring into a Mexican superstore, which is one heck of an experience let me tell you. One has a French name I can't think of (good one in Merida); the ones I am fairly familiar with are Soriana's, which bought out another, Gigante. To walk around in a good Soriana for about an hour will have you wish you had one where you live. Did I mention they carry lots of patent medicines and all kinds of personal hygiene products?
Rule 5. If you get sick and need additional medications do not hesitate to go to a clinica. Chances are, it's something an internist can take care of, examination and medicine for about $25. (The last time I went to a physician in the U.S., it was $50 or more just for the appointment.) And don't believe Big Pharma's propaganda about "untrustworthy" Mexican drugs. The industry just doesn't want you buying a competitor's goods. The California avocado growers kept Michoacan Hass avocados off the market here for decades by claiming they were "buggy." Avocados or pharmaceuticals, the lie is the same.
Rule 6. Don't drink the water. Always insist on agua potable, which they know to be bottled water, though almost all hotels have large glass bottles you're allowed to tap any time of day or night. Sometimes, you will find only bottled water at the desk -- for a price.