It is advised that you drink bottled water from larger stores. If you want to drink tap water, you have to boil it for at least 20 minutes as lower air pressure makes for lower boiling points. Otherwise, you can use iodine or purification tablets. Also, make sure not to eat any vegetable or fruits without cooking as they are washed with local water. Use bottled water to brush your teeth and do not drink alcohol until you are acclimatized.