The US Controlled Substances Act (Title 21 United States Code, Subchapter I, Part B Section 812), regulates the availability of drugs in the US, whether they can be prescribed, and to what extent. Drugs are classified into one of five schedules. The most restrictive schedule is Schedule I, which includes marijuana. Below is Title 21 of the US Code, released by the Law Revision Counsel (LRC) of the US House of Representatives on Aug. 11, 2006:

Section 812. Schedules of Controlled Substances.

(a) Establishment
There are established five schedules of controlled substances, to be known as schedules I, II, III, IV, and V. Such schedules shall initially consist of the substances listed in this section. The schedules established by this section shall be updated and republished on a semiannual basis during the two-year period beginning one year after October 27, 1970, and shall be updated and republished on an annual basis thereafter.

(b) Placement on schedules; findings required
Except where control is required by United States obligations under an international treaty, convention, or protocol, in effect on October 27, 1970, and except in the case of an immediate precursor, a drug or other substance may not be placed in any schedule unless the findings required for such schedule are made with respect to such drug or other substance. The findings required for each of the schedules are as follows:

(1) Schedule I.
(A) The drug or other substance has a high potential for abuse.
(B) The drug or other substance has no currently accepted medical use in treatment in the United States.
(C) There is a lack of accepted safety for use of the drug or other substance under medical supervision.

(2)Schedule II.
(A) The drug or other substance has a high potential for abuse.
(B) The drug or other substance has a currently accepted medical use in treatment in the United States or a currently accepted medical use with severe restrictions.
(C) Abuse of the drug or other substances may lead to severe psychological or physical dependence.

(3)Schedule III.
(A) The drug or other substance has a potential for abuse less than the drugs or other substances in schedules I and II.
(B) The drug or other substance has a currently accepted medical use in treatment in the United States.
(C) Abuse of the drug or other substance may lead to moderate or low physical dependence or high psychological dependence.

(4)Schedule IV.
(A) The drug or other substance has a low potential for abuse relative to the drugs or other substances in schedule III.
(B) The drug or other substance has a currently accepted medical use in treatment in the United States.
(C) Abuse of the drug or other substance may lead to limited physical dependence or psychological dependence relative to the drugs or other substances in schedule III.

(5)Schedule V.
(A) The drug or other substance has a low potential for abuse relative to the drugs or other substances in schedule IV.
(B) The drug or other substance has a currently accepted medical use in treatment in the United States.
(C) Abuse of the drug or other substance may lead to limited physical dependence or psychological dependence relative to the drugs or other substances in schedule IV.”

The US Drug Enforcement Agency (DEA) Drug Rescheduling Criteria details the requirements that marijuana, currently a Schedule I drug, must meet before the DEA will agree to permit physicians to easily prescribe it to patients.

Aug. 11, 2006 - US Code Title 21 Section 812