If a drug or medication makes a CMV driver unable to drive safely, operating the vehicle is:
Correct answer: B – Prohibited
Federal rules prohibit operating a CMV while under the influence of a substance to a degree that makes safe operation impossible. The prescription exception applies only when a licensed medical practitioner familiar with the driver’s medical history has advised that the substance will not affect safe operation. Prescription and over-the-counter medications can both cause impairment.
Source: Commercial Driver License Manual — § 2.22.2, “Other Drugs”, 49 CFR § 392.4 — Drugs and Other Substances


