21 U.S.C. Sec. 841(A)(1) Definition of Controlled Substance is a federal law that defines controlled substances as drugs and other substances that are regulated by the U.S. government due to their potential for abuse. It includes the five schedules of controlled substances, which are classified according to the substance's potential for abuse, medicinal use, and safety. These five schedules are: Schedule I, Schedule II, Schedule III, Schedule IV, and Schedule V. Schedule I substances are those with the highest potential for abuse and no accepted medical use, while Schedule V substances are those with the lowest potential for abuse and accepted medical use.