49 CFR 655.41: Pre-employment drug testing.

(a)(1) Before allowing a covered employee or applicant to perform a safety-sensitive function for the first time, the employer must ensure that the employee takes a pre-employment drug test administered under this part with a verified negative result. An employer may not allow a covered employee, including an applicant, to perform a safety-sensitive function unless the employee takes a drug test administered under this part with a verified negative result.

(2) When a covered employee or applicant has previously failed or refused a pre-employment drug test administered under this part, the employee must provide the employer proof of having successfully completed a referral, evaluation and treatment plan as described in § 655.62.

(b) An employer may not transfer an employee from a nonsafety-sensitive function to a safety-sensitive function until the employee takes a pre-employment drug test administered under this part with a verified negative result.

(c) If a pre-employment drug test is canceled, the employer shall require the covered employee or applicant to take another pre-employment drug test administered under this part with a verified negative result.

(d) When a covered employee or applicant has not performed a safety-sensitive function for 90 consecutive calendar days regardless of the reason, and the employee has not been in the employer's random selection pool during that time, the employer shall ensure that the employee takes a pre-employment drug test with a verified negative result.