At the risk of sounding like a braggart, I must confess I have never had any problems with the food in Mexico. We have been to PV at least 30 times and stay south of town in Mismaloya. We eat at the local restaurants, but just use common sense.
My wife, on the other hand, had troubles for years until she talked it over with her Doctor. She suggested my wife start out two weeks before the trip taking FiberCon tablets on a daily basis. FiberCon is a natural laxative, which also can be described as a \"stool solidifier\".
This is more of a natural solution, and should be followed only on the advice of your personal health care provider. As so many of the previous writers stated, do not self medicate. TALK TO YOUR DOCTOR FIRST! You may save your vacation and enjoy the Sun, warmth and natives as we do.