Apart from the driving skills and the theoretical knowledge that are required to operate a commercial vehicle, people who want to obtain a commercial driver's license have to meet certain health requirements, as well.
These requirements are not the same in all states, so you have to check what your state's DMV requires from CDL applicants. However, there are several general requirements that apply in almost every state.
Some sort of medical exam is required in all states, since it's a federal requirement that all commercial drivers are physically prepared to operate a commercial vehicle in a safe manner. In order to obtain a medical examiner's certificate, you have to pass a couple of tests, which are mainly focused on your blood pressure, blood sugar levels, vision, and hearing.
When it comes to blood pressure, it has to be between 140 - 159/90 - 99 to obtain a certificate that is valid for a year. If your blood pressure is higher than 160 - 179/100 - 109, you will receive a one-time certificate for 3 months, and you are going to have to take prescription medication.
In order to pass vision tests, you must have a 20/40 vision in both eyes, and if you use corrective lenses, it has to be noted on the certificate. You have to prove that you can distinguish among traffic lights that show red, green and amber colors. To be eligible for a CDL, you must be able to perceive forced whispered voice from a distance of 5 feet or more, and if you have hearing loss in one ear, it must be lower than 40 dB.
Furthermore, your blood sugar levels must be under 200, and if you have diabetes that has to be controlled with insulin shots, you can't obtain a CDL. Also, you must not use amphetamines, some sort of narcotics, or any other habit-forming drugs if you want to be eligible for a CDL.