49 C.F.R. § 655.33 – Pre-duty use.

(a) General. Each employer shall prohibit a covered employee from using alcohol within 4 hours prior to performing safety-sensitive functions. No employer having actual knowledge that a covered employee has used alcohol within four hours of performing a safety-sensitive function shall permit the employee to perform or continue to perform safety-sensitive functions.
(b) On-call employees. An employer shall prohibit the consumption of alcohol for the specified on-call hours of each covered employee who is on-call. The procedure shall include:
1. (1) The opportunity for the covered employee to acknowledge the use of alcohol at the time he or she is called to report to duty and the inability to perform his or her safety-sensitive function.
2. (2) The requirement that the covered employee take an alcohol test, if the covered employee has acknowledged the use of alcohol, but claims ability to perform his or her safety-sensitive function.