How is it possible that the employer can drug test an employee without drug testing another what form of discrimination is this?

How is it possible?  Well, they have one employee drug tested and not the other. What form of discrimination is this?  There is no information here that provides an answer so it may not be discrimination at all.  However, it really will not matter whether it is discrimination or not unless it is discrimination based on race, color, sex, national origin, religion, disability or age 40 or older. Then it will only matter if the discrimination leads to an adverse employment action such as termination.