You can’t read about travelling to Mexico without hearing about travellers’ diarrhea (TD).
TD can be caused by a number of bacteria. And the other thing to consider is that you’ll likely be eating foods that you’re not used to eating, which could challenge your stomach.
Most sources recommend Imodium (active ingredient loperamide) to fight TD. We purchased some to bring with us, and inevitably used it. But we’re happy to say that the Imodium worked as promised and none of our plans were affected.
If you need to restock while in Mexico (I recommend always having some on-hand), ask for Lomotil at a pharmacy. (In Mexico, Lomotil’s active ingredient is loperamide, while in the US, it’s a different anti-diarrheal medication.) I even saw some at the 7-Eleven store at the Mexico City airport, so it’s worth checking the local 7-Eleven too.
Disclaimer: I’m not a doctor. These are just my experiences. Consult your physician for medical advice.