Doing so makes it clear the employer will not allow drug and alcohol abuse in the work place. But sadly, it can often be only a stopgap measure. What you can do The best protection any employer has is to make certain it's clearly stated in an employee's contracts just what the company's policies are regarding drug and alcohol abuse in the work place. As well as what procedures are already in place to deal with any offenders. But - often the scheduled date for these tests gets known and again you're left dealing with those employees who know how to clean their systems and pass these tests.
Some companies allow a period of probation.
Some even have company paid rehabilitation programs established to help these employees. Most often the employee loses their position. Employers believe if an employee shows they don't care about the policies of their company, to which they agreed when entering into the employment agreement, they can't be trusted. As an employer, it's up to you to know your company's policies and procedures. And to make sure all employees know what they are.