Medicinal Herbs and Natural Alternatives
In the Potrero Chico there is a spring where you can get delicious and clean water. However, Montezuma's Revenge or traveller's sickness (upset stomach and diarrhea due to foreign bacteria or parasites) is still a possibility.
If you head to a local doctor or clinic for a cure, they will inject you with antibiotics (i.e. penicillin) in the butt. Although effective for eliminating the symptoms of the sickness, antibiotics weaken your immune system and burden your liver.
Healthier alternatives for preventing and curing traveller's sickness are available and can be obtained inexpensively at a natural foods store before leaving home.

Grapefruit Seed Extract (GSE) - GSE is a powerful parasite killer and water purifier. A small bottle will last for months and can be used to sterilize drinking water, for dish washing and taken internally to eliminate parasites. As a preventative measure, a few drops in your water bottle daily are a good idea. Recommended dosage to cure traveler's sickness is 10 to 15 drops in a glass of water or tea three times a day. Care must be taken not to take high concentrations of GSE continuously for long periods of time as it also kills friendly bacteria in the stomach. GSE can be taken alone or mixed with other remedies.

Herbal Remedy - Green Black Walnut Hulls (either liquid extract or dried husks)
Cloves (freshly ground)
Wormwood (either liquid extract or dried)
A powerful blend of bitter herbs that is highly effective eliminating harmful bacteria or parasites responsible for traveller's sickness. Make a tea by steeping for 10 minutes a tablespoon of the dried herb and about a dozen ground cloves. Drink a couple cups of tea three times a day for four days.

Local Remedies - Eat loads of Chiles, Garlic and Onions (raw as possible) Mexico is famous for Chiles and the locals eat them abundantly for good reason. Spicy Chiles create a fiery climate in you stomach that parasites and harmful bacteria cannot tolerate, so they leave. Add these spicy foods to every meal for a week or for extreme cases try fasting on them for four days.