Altitude Sickness: Known as soroche, you'll likely encounter altitude sickness at Cusco's 3,500-meter (11,500-foot) elevation. Drink lots of fluids but eliminate or minimize alcohol and caffeine consumption. Most large hotels have an oxygen supply for their guests' use. The prescription drug acetazolamide can help. Check with your physician about this, and about traveling here if you have a heart condition or high blood pressure, or are pregnant.
Warning: Sorojchi pills are a Bolivian-made altitude-sickness remedy whose advertising pictures a tourist vomiting at Machu Picchu. Its safety has not been documented, and we don't recommend trying it.
Water: Tap water is not safe to drink here. Stick with the bottled variety, con gas (carbonated) or sin gas (plain).
Safety: Security has improved dramatically in Cusco. A huge police presence is on the streets, especially around tourist centers such as the Plaza de Armas. Nonetheless, petty crime, such as pickpocketing, is not uncommon: use extra vigilance in crowded markets or when getting on and off buses and trains. Robbers have also targeted late-night, early-morning revelers stumbling back to their hotels.