It is not wise to drink tap water or ice made from tap water. Most hotels in urban areas including Lhasa boil the water first before serving them hot or cold. However, when trekking in the more remote areas you should boil your own water or treat it with water-purification tablets. Tea is always safe to drink but you are advised to refrain from locally brewed alcohol as it’s often made with contaminated well water. Large 5-liter bottles of drinking water are available in most supermarkets.
The water in Tibet is ‘hard water’ so you need to boil it at least for 10 minutes to purify it. Consider purchasing a water filter for a long trip (often more economical than buying bottled water). Total filters take out all parasites, bacteria, and viruses, and make water safe to drink.
Chlorine tablets (eg Puritabs or Steritabs) will kill many pathogens, but not giardia and amoebic cysts. Iodine is more effective for purifying water and is available in liquid (Lugol’s solution) or tablet (eg. Potable Aqua) form. Follow the directions carefully and remember that too much iodine can be harmful.