Am planning a 6 week trip in Mexico, starting with Cancun. I have heard probiotics are a good precaution but I wonder if I can pick up a suitable non-prescription probiotic from a local Bartell's or Rite-Aid or Walgreen's that would work. Apparently not all probiotics are the same. Also, until I figure out more where I can eat or not eat the first few days I am there, I may prefer an American type Subway Sandwich or McDonald's type store. I know that I lose adventure that way but I would prefer safety for the first few days. Can I just assume that all of the vegetables on the sandwich in a place like a Subway's in Cancun is good? For the long term though I may just need a good probiotic. Any thoughts on picking one up.
Here's an article with some information:
http://www.probiotics-lovethatbug.com/travelers-diarrhea.html
Florastor is the probiotic mentioned. The active ingredient is Saccharomyces Boulardii which is also available in other brands, often for a lower price.
I also try to eat yogurt regularly. I'm careful about the water that I drink but not so much with the food. If it looks fresh and well cooked, and the food prep area looks clean (street vendors/markets), I enjoy. One suggestion that I was given about eating street food or market food is to go to a crowded place. If the locals eat there regularly its probably good. A place won't stay in business long if it doesn't have return customers that don't get sick.
Any place that caters to tourists is going to have good food prep practices. In this day and age of internet, one report on line of someone blaming a restaurant for them getting sick goes a long way in ruining a business.
Following all of the above, I rarely get sick when I travel, although everyone is different.

While I don't know anything about probiotics, I would say restricting your diet to American-based fast food chains is a little extreme. There are many "nice" restaurants that are as every bit as hygienic (and, possibly more so) as Subway but would still allow you to try Mexican food. For example, there are well known local chains like Sanborn's, Tok's, and Vips. While they're not anything amazing, they'll give you types of food you won't find at home and at prices similar to fast food. There will be hundreds of other local restaurants that serve tourists and locals and have no interest in skimping on pesos but running the risk in making their clients sick. You can use Yelp or TripAdvisor to find these. Certainly nothing is foolproof but I wouldn't say the risk at fast food is any less. Just because something comes from America doesn't necessarily mean it is better or safer.

Last time I got sick was after eating in a Subway. Doesn't mean necessarily that's where I picked up the bug, but more than likely. Why not bring your probiotics with you?
There´s a McDonald´s restaurant right across from the ADO bus station in Cancun centro. I recommend it. It even has a Mexican menu, featuring McMolletes, and McBurritos, among other fine dining.

Indeed yogurt is found throughout México. But with very few exceptions, it's no better than sugar pops. Very little good bacteria but plenty of sugar.

I don't believe any of the US drugstore chains do business in Mexico. But I'm open to correction. But Costco for sure has probiotics, plus their hot dogs and pizza are a pretty safe bet. They now have (at least in Querétaro and Celaya where I shop) a pizza al pastor, which is the bomb.
But I also agree with mclarjh, a McMuffin is tough to beat. Carl's Jr a close second. The thing with Subway is that I'm always leery of those veggies just kinda sitting around all day. Tho we do grab a sando a couple of times a month.