Drivers are required to have a DOT Medical Certificate:
If they operate a motor vehicle with a gross vehicle weight rating (GVWR), gross combination weight rating (GCWR), gross vehicle weight (GCW), or gross combination weight (GCW) of 4,536 kilograms or more in interstate commerce.
If they operate a motor vehicle designed or used to transport more than 15 passengers (including the driver) in interstate commerce.
If they operate a motor vehicle designed or used to transport between 9 and 15 passengers, for direct compensation, beyond 75 air miles from the driver's normal work-reporting location, in interstate commerce
If they transport hazardous materials, in a quantity requiring placards, in interstate commerce.
CFR 49 Part 391.41
CFR 49 Part 390.5