Please do pay attention to the following notes before you set off to Tibet.
Not every one is physically suitable for staying at some place with average 4, 000 meters elevation for several days or even longer although Tibetan are very hospitable. PTT suggests that a body exam is necessary before you enter Tibet. Alternatively, you can pick up some place with lower elevation to travel. That is safe and responsible.
Normally, different degrees of swirl, headache, nosebleed, and being indulge in sleep happen to every one. Big cities such as Lhasa, Shigatse, and Nyingchi can help you to get over the altitude sickness and give you standard first aid with well medical facilities and hospitals. However, situation is quite different in Ngari, Nagqu, and Chamdo. High elevation, far distance between cities, low first aid efficiency made the sick died before. 
Five groups of people are strongly excluded on 3,000-meter-above travel: first, people who have all types of heart attacks, blood diseases, cerebrovascular problems; second, people who have chronic breath system diseases such as bronchi asthma and emphysema; third, people who have heavy cold and upper respiratory tract infection; fourth, people who have high-blood-pressure, plateau lung edema, plateau high blood and plateau heart attack; fifthly, terminal pregnant women.
PTT suggests you can transfer from low elevation place to high one if you are not sure about your physical condition. For instance, firstly, you can fly to Nyingchi prefecture with 3,000 meters elevation from Chengdu. Then, after two or three days, you can go to Lhasa or some places else. During the trip, you may be used to oxygen-lack environment.
Ask guide to help you use oxygen bag, if altitude sickness appears suddenly. Do not always depend on oxygen bag. That makes your travel dull. Please remember: healthy people can get used to oxygen-lack environment step by step. Do not over worry high altitude. Millions of tourists enter into Tibet annually. Few serious cases lead to irreversible result.
Generally speaking, there will be reaction for you in high altitude area for shortage of oxygen. The level of reaction is different from one to another. Some will feel badly only for 1-2 hours. But some will have it for long time. The normal symptoms of oxygen shortage will be chest tightness, dyspnea and headache etc. So if you have those symptoms, you need to lie down on hotel bed so that to adapt yourself to the elevation fast. And you may take some medicine like Rhodiola oral liquid, inosine oral liquid before you set to Tibet and take aspirin which is safe and effective if you have headache.
The ways to treat altitude sickness have been proved to be effective through the pactice of Tibet travel groups for long time organized by PTT.

Aspirin	Rhodiola Oral Liquid	Inosine Liquid