The US government keeps drugs on a schedule as determined by the abuse rate as follows:
Schedule I drugs are the most dangerous with highest potential for abuse and high potential for severe psychological or physical dependence—heroin, LSD, ecstasy, etc.
Schedule II drugs have less potential for abuse and include cocaine, methadone, and OxyContin.
Schedule III drugs have a lower abuse rate and include Tylenol with certain amounts of codeine, anabolic steroids, and testosterone.
Schedule IV drugs have an even lower potential for abuse, and include Xanax, Valium, and Ambien.
Schedule V drugs have the lowest potential for abuse, and include Robitussin AC with certain amounts of codeine, Lyrica, and Lomotil.
If you are living through chemistry, be sure to keep your prescription handy. Though a scheduled drug may be legally prescribed to you, if you do not have that prescription with you, you could still be arrested for possession of a controlled substance and have that arrest on your permanent record even if you can show later that you had a doctor’s orders to take that drug.