While it is best to test all candidates for a position at the same point in the process to avoid discrimination issues, you do not necessarily need to test all applicants for all jobs. You could choose to test only those who are applying for a job in the same job category. For example, a drug test might be given to applicants who will be driving a forklift, but not to employees entering clerical jobs. You can decide to whether it is best to test all candidates, or to test only those in certain job categories.