There are two answers to this question. One that covers DOT (FMCSA) employees and owner operators and the other answer that covers Non-DOT employees.
All truck drivers (commercial drivers license) are by law required to get tested for drugs and alcohol. These results must be kept on file.
Many employers require their employees to submit to drug testing both before and after being hired. Most of the time after being hired an employer requires reasonable suspicion before having an employee take a drug test. Even with the recent legalization of marijuana in some states, employees in those states can still be terminated for testing positive. A failed drug test can include the employer requiring the employee to: seek rehabilitation, termination, and losing unemployment benefits.