The description of illicit drug use needs to be placed in the context of the time and jurisdiction in which they are defined because many substances that are considered ‘illicit’ in our current time may have been used for legitimate purposes in the past.
Illicit substance – A drug classified by the Drug Enforcement Administration (DEA) as being available only by prescription from a physician or classified as being controlled and having no therapeutic use.
Traditional Illicit Substances are historically identified by drug class and commonly known by substance abusers, and would include drugs such as cocaine, heroin, methamphetamine, etc.