The US Controlled Substances Act has classified marijuana as Schedule I, the most restrictive. Schedules IV and V are the least restrictive.
Below is the most recent update of Schedule IV of Title 21 of the US Code, released by the Law Revision Counsel (LRC) of the US House of Representative on Aug. 11, 2006:
"Section 812. Schedules of Controlled Substances
(c) Initial schedules of controlled substances Schedules I, II, III, IV, and V shall, unless and until amended pursuant to section 811 of this title, consist of the following drugs or other substances, by whatever official name, common or usual name, chemical name, or brand name designated: