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 (GVW), or gross combination weight (GCW) of 10,001 pounds (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.

If they transport hazardous materials, in a quantity requiring placards, in interstate commerce.

However, an individual operating a covered farm vehicle, as defined in §390.5, is not required to have a DOT Medical Certificate