Nearly all tourists entering Tibet experience highland altitude sickness. For some the effect is strong, but for most it is just an inconvenience .
The reaction varies form person to person, and experts can not say who will be affected, but statistically old people are more likely to feel stronger altitude sickness than the young, the unfit/unhealthy are more likely than the fit/healthy, and males are affected more strongly than females.
Effects increase with altitude. China Highlights recommends that customers over 65 do not go to Ali in Tibet’s remote west, which is 4,300 meters above sea level in the city, and surrounded by mountains.
You may not feel uncomfortable even several hours after you arrive at a high-altitude place. However most people who feel altitude sickness feel it the first night, and the discomfort may continue for about three days. You may feel the strongest altitude sickness the second day after your arrival.
Most of Tibet is high enough to produce some adverse reaction in most people. For the majority of people, the reaction will be a matter of discomfort, breathlessness, poor sleeping patterns or limited capacity for physical exertion . Poor sleep is most common. Most travelers to Tibet complain they can hardly fall asleep, or fall asleep but wake up frequently on the first two nights.
In some (1%) more serious reactions can develop. These can be potentially life-threatening conditions that may only be relieved by moving to a lower altitude . Travelers should inform themselves about the symptoms so that they can recognize them. A severe, persistent headache, nausea, and loss of coordination or disorientation, are signs of a serious reaction ( Acute Mountain Sickness, AMS for short). This potentially fatal condition requires that you descend immediately (sometimes 300m lower is enough).
Adverse reaction to altitude is usually reduced if one acclimatizes by reaching high altitude over a period of at least a few days (3 days is usually enough).
For this reason, it is often supposed that driving to Lhasa is better than flying. But the altitudes where nights are spent while driving to Lhasa may be far in excess of Lhasa's 3,600m. Roads blocked by landslides or otherwise may require travelers to exert themselves, carrying packs at high altitude, and so increasing the likelihood of an adverse reaction.
Flying to Lhasa, acclimatizing there, and driving out may well be preferable.
On arrival at high altitude, it is possible that no immediate effects of altitude will be felt. Nevertheless, it may be of great importance not to exceed the lowest level of physical exertion on the first day, and only increase exertion very gradually over the following days.
Most of western Tibet is at least 1,000m higher than Lhasa, and is best only approached after several days acclimatizing at a lower altitude.
Here are some of the precautions you may take before entering Tibet according to previous experiences.