Horror stories about Montezuma's Revenge abound for any visitor to Central or South America. With a little precaution, however, you can avoid most stomach problems that can make your trip to Peru a nightmare.
Is tap water safe to drink in Peru?
Tap water in Peru, and basically in all of South America, is not safe to drink. Most families boil their water to purify it, and then keep a bottle or pot of clean water available on the stove or in the fridge. If you boil your water, many travel sites recommend that you bring the water to a boil and let it boil for five minutes before letting it cool. While this method is meant to cover all your bases and is sure to kill 100% of all bacteria and parasites, it isn't truly necessary. Once your water has come to a boil it is safe to drink and you don't really need to let it boil for another 5 minutes, especially if you are camping and need to preserve gas. If you don't want to boil your water or don't have the use of a stove, then you can use chlorine tablets or drops to purify your water. Pick some up at a camping store before you go, or find an adventure gear store once you arrive in Peru.
Can you use tap water to brush your teeth in Peru?
Many people worry about brushing their teeth with tap water while in Peru. Whether you brush with tap water or bottle water partly depends on your own body and what you are comfortable with. Many people do use tap water to brush their teeth. They spit out the water and don't drink any. While you don't swallow the water, it is impossible not to leave some residue in your mouth and you do ingest a small amount of un-purified tap water this way. The people who continue to brush with tap water obviously do not get sick from this small exposure to un-purified water. Others are religious about avoiding contact with un-purified water and only use bottled water to brush their teeth. If this is your first time to South America and you don't know how your body will react, sicking with bottled water is probably a safer bet. Especially if you are only here for a week or two. You don't want to spend a few days puking your guts up if those few days equal half the time of your trip. If you're going to be in Peru for an extended visit and like to experiment, try brushing with tap water but be prepared to suffer the consequences if you react poorly. Chances are, though, you'll be perfectly fine.
The real danger to your stomach, however, comes from food. Most high-end restaurants, especially tourist restaurants, adhere to a very clean standard and you shouldn't have any problems with the food they serve. More economic options, less touristy places, and especially market stall restaurants and street vendors, are the places you need to watch out for. This doesn't mean you shouldn't eat there. It just means you need to know what you can eat and what you probably shouldn't eat if you don't have a big bottle of Pepto-Bismol waiting for you at home and 24/7 access to a toilet.
In restaurants of dubious quality, you need to watch out for fresh salads, fresh fruit, and ice. This is because the fresh salads and fruit might have been washed in unclean water or could still harbor bacteria if they weren't cleaned properly. You need to watch out for ice because it could have been made with un-purified tap water. If you are concerned, either don't eat or drink it, or politely ask the waiter if they use purified or bottled water to clean their produce and to make ice. In lower-end restaurants also be on the look out for dirty silverware, cups and plates. It is not considered rude at all to send back dirty items. If your silverware comes to you wet, dry it off with a napkin before you use it.
When it comes to street vendors, this decision really is up to you. Most of the typical food you find for sale on the street is truly delicious and uniquely Peruvian. On the other hand, it is also probably the most dangerous in terms of your health. If you are only here for a brief visit, it might be better to skip out on street-vendors. If you are here for longer and your stomach has the time to adjust, cautious experimentation probably won't cause too much damage. In general, however, meat bought on the street is probably not a good option. It's likely been left outside in the open air all day, subject to heat and flies, before being cooked on an open grill in the street. Fruit juices and other beverages are truly delicious and quiet thirst quenching after a long walk through city streets, but might bring you into contact with un-purified water in one form or another. Have fun but use your own judgment!
If you do get sick, Pepto-Bismol will be your best friend. Make sure you pack a bottle with you. Remember to drink plenty of clear fluids as it is easy to get dehydrated if you have consistent diarrhea. If you don't get better after a few days, it is recommended that you seek professional medical help. There are both public hospitals and clinics, and private health clinics. Ask for a recommendation to the best private health clinic in the area. It will be more expensive than a public clinic, but you will get much better service. Your health is not something to mess with.