Drugs that are illegal are classified into five schedules on the Controlled Substances Act (CSA). This schedule is based on what medicinal use the drug has, as well as the impacts that it has on the people who use the drug, including the likelihood of addiction.
The CSA's scheduling decisions aren't always without debate. One of the biggest debates in the country now is whether marijuana should continue to be listed as a Schedule I drug, which is the category reserved for drugs without medicinal use and that are considered very unsafe. Drugs in this category, which also includes heroin, are said to never have acceptable use.
As states decriminalize marijuana, they have to consider the fact that the drug is still illegal on a federal level. A person who possesses marijuana legally under state laws could still break federal law by having the drug.
Schedule II drugs are legal in limited cases but they are heavily regulated. Medical professionals can write prescriptions for drugs in this category, which includes drugs like methadone and morphine.
Schedule III drugs have some chance of dependence, but they also have proven uses. Just like Schedule II drugs, you can get a prescription for these drugs, which include ketamine.
Schedule IV drugs, which include Xanax and Valium, are typically available via prescription only, but they don't have a high probability of abuse and dependence. Schedule V drugs, such a cough syrups with codeine, can also be prescribed. These are considered the safest of the drugs on the CSA schedules.
If you are facing a drug charge for a controlled substance, you must think carefully about your defense options. These charges can sometimes come with considerable penalties that might include long term incarceration.
Source: FindLaw, "Drug Classifications," accessed Feb. 09, 2018