The American government classifies all drugs into five different Schedules, Schedule I being considered the worst for addictive tendencies and bodily harm, and Schedule V being considered the least risky.  Where Schedule V drugs are used for medicinal purposes, Schedule I drugs are thought to be of no such use and are subject to harsh legislation concerning use or sale.  The main points of classification are whether the drug can be used in medicine and whether or not it has addictive qualities or long term health side effects:

Schedule I Drugs

This classification includes cannabis, heroin, MDMA and psilocybin, and the guidelines as set down by the American Controlled Substances Act are that these drugs have no medicinal purpose, are highly subject to abuse by users and that there are no safety guidelines set out for their use within the United States.  This is a controversial Schedule since it includes notably dangerous drugs such as heroin with drugs like cannabis and psilocybin which are used widely throughout the country illegally and are considered to have no long term negative effects.  Lobbyists have been trying to have cannabis removed from Schedule I classification for years and so far the federal government has no inclination to do so.

Schedule II Drugs

This classification dictates that the drugs are highly subject to abuse by users, that such abuse can lead to psychological or physical dependence and that despite these drawbacks they might be used for medical purposes within the United States.  Drugs on this list include morphine, cocaine and amphetamines which have all been subject to scrutiny by those researchers who believe they might be better in a different category so that more people might have access to them for medicinal purposes.  These drugs are all subject to very strict medical guidelines and very few people in the country will have access to them.

Schedule III Drugs

These drugs are considered okay for medicinal use in the United States and to have a much lower risk of addiction than Schedule I or II drugs; while it is accepted that these may have moderate risk of physical or psychological dependence, medical guidelines dictate that their use may be helpful in some cases.  Drugs on this list include ketamine and anabolic steroids.

Schedule IV Drugs

These drugs are commonly found in prescriptions as they have an accepted use in medical treatments, are lower risk in terms of addiction (compared to Schedule III drugs) and they are thought to only provoke dependence for a limited time while being used.  These include mebrobamate and zolpidem.

Schedule V Drugs

These are considered very low risk for addiction and abuse, and also very low in terms of physical or psychological dependence.  Such drugs are commonly in use throughout the United States for medical purposes and they include codeine and pregabalin; a pain reliever and anticonvulsant respectively.  

What you need to remember is that the lower the number, the higher perceived risk with using and the more likely it is that use is in fact illegal!