Zero Tolerance Policy
The first thing in this regard is to have a zero tolerance policy in place. Let the employees know that if they use narcotic drugs and are caught, they will be shown the door without any further consideration. Such a policy in place sends a strong message to all employees who refrain from such acts as a result.
Part of Contract
Make it a part of the contract that the employee can be tested a certain number of times during the course of their employment. Many companies also request employees to take drug tests while they apply for a job and are being seriously considered. This will allow you not only to test the employees at any point in time but also help you not hire people who abuse drugs in the first place.
Some organisations have mandatory tests in place for every employee. An example of this is Home Depot that requires every employee who is up for a promotion to take a drug test as a mandatory part of the promotion process. The company also takes its employees take a drug test if a serious accident takes place at work. They also reserve the right, under the contract, to drug test any employee at any given time twice during the period of employment.