49 CFR § 655.42 - Pre-employment alcohol testing.
An employer may, but is not required to, conduct pre-employment alcohol testing under this part. If an employer chooses to conduct pre-employment alcohol testing, the employer must comply with the following requirements:
(a) The employer must conduct a pre-employment alcohol test before the first performance of safety-sensitive functions by every covered employee (whether a new employee or someone who has transferred to a position involving the performance of safety-sensitive functions).
(b) The employer must treat all covered employees performing safety-sensitive functions the same for the purpose of pre-employment alcohol testing (i.e., you must not test some covered employees and not others).
(c) The employer must conduct the pre-employment tests after making a contingent offer of employment or transfer, subject to the employee passing the pre-employment alcohol test.