I live in the US and every once a year or so I go visit family in Mexico, and I always end up getting the stomach illness known as Montezuma's Revenge which is caused by the country's infamously poor sanitation. It's basically extremely powerful nausea which becomes better after vomiting.

What's the best way to handle it?

May be that drinking water of the area doesn't suit you. Next time you go there, try using boiled water.

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Akram
Homeopathy
Reiki Training and Treatment

ONe cannot prescribe based on one symptoms. Look for at least 2 more charactersitics to complete the remedy choice. IT may be a choice a between 4-5 more commonly indicated remedies for others in MExico as well.
HEre are the basic remedies...

GENERALITIES; VOMITING; amel. (K1411, SII-676, G1163): acon., agar., ail., anac., ant-t., ars., asar., carbn-s., card-m., coc-c., colch., dig., eup-per., helia., hell., hyos., kali-bi., lat-m., nat-m., nux-v., op., plb., puls., sang., sanic., sec., tab., vip., xan.

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http://www.homeopathy2health.com

The sanitation is so horrendously bad in Mexico that no one uses the tap water for drinking. Everyone always drinks bottled water. So it couldn't be it.

Ah but what do they use to wash e.g lettuce in ?

You bet it most certainly could be the tap water.

My experience is that most of such cases are the result of unsuitable drinking water. People may be taking bottled water but for cooking, washing and bathing etc. using tab water is not uncommon. So, we still cannot totally rule out the possibility. Can we?

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Akram
Homeopathy
Reiki Training and Treatment

So other than avoiding tap water as much as possible, what can I do?

There are homeopathic remedies that can take care of this problem very well. I could have recommended a medicine right away but probably it is out of procedure to do so on the board, I am not sure. If you like you can email me. I'll recommend the name.

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Akram
Homeopathy
Reiki Training and Treatment