Drug classifications allow the criminal justice system to simply and easily punish people who have been accused of a drug crime by labeling certain drugs and substances based on their perceived "severity." How easily a drug may be abused and how much harm it does to an individual are apparent cornerstones of how these "schedules" came to be.
The schedules start at one ("I") and descend in severity to five ("V"). Schedule I substances are what you would "traditionally" think of as being the serious drugs to possess, use and distribute. They consist of cocaine, marijuana, ecstasy, heroin and other similar substances.
Schedule II substances are also severe in nature, but they have an accepted medical use. So morphine, methadone and ritalin are a few of the drugs that qualify as a Schedule II substance.
Schedule III are considered "moderate" when it comes to the potential for abuse. Ketamine and anabolic steroids fall into this category. Schedule IV have a reduced potential for abuse, consisting of drugs such as Xanax, Valium and Ambien. Finally, Schedule V substances are the least-severe substances in the system. Cough syrups with codeine are Schedule V substances.
Now depending on the state, the laws will be a little different -- but this is the scheduling system for federal drug crimes. The punishment and consequences associated with these substances, and the schedule they relate to, will vary as well depending on the circumstances of your case. If you have been accused of a drug offense, you have to fight for your rights.
Source: FindLaw, "Drug Classifications," Accessed June 14, 2016