((a)) If it is shown at a hearing under this chapter that a holder of a license, certificate of registry, or merchant mariner’s document issued under this part, within 10 years before the beginning of the proceedings, has been convicted of violating a dangerous drug law of the United States or of a State, the license, certificate, or document shall be suspended or revoked.

((b)) If it is shown that a holder has been a user of, or addicted to, a dangerous drug, the license, certificate of registry, or merchant mariner’s document shall be revoked unless the holder provides satisfactory proof that the holder is cured.