Random Drug Testing

Employees are usually tested prior to employment and occasionally are
randomly tested after hiring. Random testing is especially common in
workplaces containing heavy machinery, toxic materials, and other
potential health hazards. Employees are also routinely tested after
being involved in a workplace accident, in order to determine if the
incident was the result of substance abuse.

http://www.askstar.com.cn/health/Drug-Testing-in-the-Workplace.htm