The Controlled Substances Act of 1970:

"(a) Establishment There are established five schedules of controlled substances, to be known as schedules I, II, III, IV, and V ...."

"(b).... 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 psychologicaldependence.

"(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."

Source

U.S. Code. Title 21, Chapter 13 -- Drug Abuse Prevention and Control -- Section 812, Schedules of Controlled Substances, p. 384.