FYI - "Montazuma's Revenge" is totally preventable. On the islands, only drink bottled water, and chilled soda from a can or bottle, do not order a soda in a can and then add it to a glass of ice. Stay away from fresh fruits and veggies unless you rinse them yourself in bottled water.
Alcoholic drinks are usually ok, the alcohol kills the bacteria.
If you take these measures and you still get it, then Immodium is the stuff to take.