Most crimes are handled administratively by the Public Security Bureau (PSB; 公安局; Gōng’ānjú), which acts as police, judge and executioner.
China takes a particularly dim view of opium and all its derivatives. Foreigners have been executed for drug offences (trafficking in more than 50g of heroin can result in the death penalty). It’s difficult to say what attitude the Chinese police will take towards foreigners caught using marijuana – they often don’t care what foreigners do if it’s not political, and if Chinese or Tibetans aren’t involved. Then again the Chinese are fond of making examples of wrongdoings and you don’t want to be the example. If arrested you should immediately contact your nearest embassy, which is probably in Beijing.
In general, as you must travel throughout Tibet with guides, refrain from doing anything that would get them into trouble, such as visiting off-limits monasteries, photographing riot police or military installations, arguing with police or officials, talking politics openly or even visiting private Tibetan homes without special permission.
The PSB is the name given to China’s police, both uniformed and plain clothed. The foreign-affairs branch of the PSB deals with foreigners. This branch (also known as the ‘entry-exit branch’) is responsible for issuing visa extensions and Alien Travel Permits.
In Tibet it is fairly unusual for foreigners to have problems with the PSB, though making an obvious display of pro-Tibetan political sympathies is guaranteed to lead to problems. Photographing Tibetan protests or military sites will lead to the confiscation of your camera or memory card and possibly a brief detention.
Attempting to travel into, through or out of Tibet without a travel permit, or to a destination not listed on your travel permit, is likely to end in an encounter with the PSB, most likely when checking into a hotel in a closed area. If you are caught in a closed area without a permit, you face a fine. Make sure you are friendly and repentant: the only times things get nasty is if you (or the police) lose your cool. Get a receipt to make sure you don’t get fined a second time during your return to where you came from.
If you do have a serious run-in with the PSB, you may have to write a confession of guilt. In the most serious cases, you can be expelled from China (at your own expense).