(a) An operator may not knowingly use as an employee any person who—

(1) Fails a drug test required by this part and the medical review officer makes a determination under DOT Procedures; or

(2) Refuses to take a drug test required by this part.

(b) Paragraph (a)(1) of this section does not apply to a person who has—

(1) Passed a drug test under DOT Procedures;

(2) Been considered by the medical review officer in accordance with DOT Procedures and been determined by a substance abuse professional to have successfully completed required education or treatment; and

(3) Not failed a drug test required by this part after returning to duty.