49 CFR 199.103
Use of persons who fail or refuse a drug test.
(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.#