Employers should have every right to test an employee for drugs when suspected. This is done to prevent employees from doing drugs during their job that might otherwise cause problems or accidents as a result. Drug testing should only be done if employee acts or behaves in a manner that possibly indicates drug or alcohol use.
If the employer feels in any way threatened or harassed by an employee's actions, he has any right to test them for drug use. Even if there has not been any informal conduct, an employer still has the right to test an employee for drug use, for his safety and that of his employees.