First, we recommend every traveller to consult their personal doctor three months prior to coming to Tibet. Your doctor knows you the best and can recommend you, the things you will need to do before coming to Tibet and could also recommend you some medicines too.
Here is the list of medicine, We find commonly with our travellers; Aspirin, flu tablets, throat lozenges, and nasal decongestants. Diamox is the most common medicine for high altitude sickness. And it is not available for sale in China. So if you think you will need it, you can bring it from home.
Here is wisdom from 14 century years old Tibetan medical science; Water is the best medicine, please drink plenty.