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If a driver is medically unqualified under the Federal Motor Carrier Safety Regulations (interstate commerce) and wants to pursue a waiver, they would have to go through the application process of Subpart B of Part 381 (except for limb impairments, covered in 391.49). This would involve sending a written request to Federal Motor Carrier Administrator who will review the request and notify the applicant as to whether or not the waiver was approved or declined.
If a waiver is granted, a copy must be maintained in the driver’s qualification file per 391.51(b)(8) the driver must carry a copy of the waiver with them while operating a CMV.
If the driver works within intrastate commerce, they would have to pursue state-specific regulations and the individual waiver programs within his/her state.