21 U.S. Code § 954. Transshipment and in-transit shipment of controlled substances
Notwithstanding sections 952, 953, and 957 of this title—
(1) A controlled substance in schedule I may—
be imported into the United States for transshipment to another country, or
be transferred or transshipped from one vessel, vehicle, or aircraft to another vessel, vehicle, or aircraft within the United States for immediate exportation,
if and only if it is so imported, transferred, or transshipped (i) for scientific, medical, or other legitimate purposes in the country of destination, and (ii) with the prior written approval of the Attorney General (which shall be granted or denied within 21 days of the request).
A controlled substance in schedule II, III, or IV may be so imported, transferred, or transshipped if and only if advance notice is given to the Attorney General in accordance with regulations of the Attorney General.