21 U.S.C. § 859

(a) First offense

Except as provided in section 860 of this title , any person at least eighteen years of age who violates section 841(a)(1) of this title by distributing a controlled substance to a person under twenty-one years of age is (except as provided in subsection (b)) subject to (1) twice the maximum punishment authorized by section 841(b) of this title , and (2) at least twice any term of supervised release authorized by section 841(b) of this title , for a first offense involving the same controlled substance and schedule. Except to the extent a greater minimum sentence is otherwise provided by section 841(b) of this title , a term of imprisonment under this subsection shall be not less than one year. The mandatory minimum sentencing provisions of this subsection shall not apply to offenses involving 5 grams or less of marihuana.

(b) Second offense

Except as provided in section 860 of this title , any person at least eighteen years of age who violates section 841(a)(1) of this title by distributing a controlled substance to a person under twenty-one years of age after a prior conviction under subsection (a) of this section (or under section 333(b) of this title as in effect prior to May 1, 1971 ) has become final, is subject to (1) three times the maximum punishment authorized by section 841(b) of this title , and (2) at least three times any term of supervised release authorized by section 841(b) of this title , for a second or subsequent offense involving the same controlled substance and schedule. Except to the extent a greater minimum sentence is otherwise provided by section 841(b) of this title , a term of imprisonment under this subsection shall be not less than one year. Penalties for third and subsequent convictions shall be governed by section 841(b)(1)(A) of this title .