What drug and alcohol tests are required for a CDL license?

Drug and alcohol tests are required in the following situations:

Pre-Employment
Before a motor carrier allows a driver to perform a safety sensitive function. The pre-employment test is only required for controlled substances. Alcohol testing is permitted.
Reasonable Suspicion
When a trained supervisor/employer has reasonable suspicion to believe that the driver has used alcohol and/or controlled substances.
Random
Each year, the number of random tests conducted by the employer must equal at least 10% of average number of driver positions subject to the regulations. If you had 10 or less drivers, you would have at least 1 per year. If you have between 11 and 20 drivers, you would need 2 random tests each year.
Post Accident
CDL drivers who are involved in fatal crashes, or who are cited for moving violations arising in a crash that requires a vehicle being towed or an injury requiring medical attention away from the scene. The alcohol test must be conducted within 8 hours and the controlled substances test must be conducted within 32 hours of the crash.
Return-to-duty and follow-up
Required after certain violations and crashes