Final answer:
The most commonly used illicit substance is marijuana, overshadowing other substances such as oxycodone, heroin, methamphetamine, and cocaine in terms of prevalence.
Step-by-step explanation:
The most commonly used illicit substance is marijuana. Recreational use of marijuana intends to alter consciousness and provide a euphoric feeling known as a 'high'. In comparison to other listed substances like oxycodone, heroin, methamphetamine, and cocaine, marijuana has seen widespread illicit use despite varying legal status across jurisdictions. Notably, marijuana-related offenses account for a significant number of drug abuse violations, although arrests for its manufacture have declined as legalization and decriminalization progress.
Opioids such as oxycodone and heroin have caused an opioid crisis, stemming from their over-prescription for pain management. Methamphetamine use has increased due to ease of production and accessibility, and cocaine is known for its highly compulsive use pattern.
Heroin, although a major drug of abuse, is less commonly used than marijuana. Unlike substances like methamphetamine which can be manufactured, heroin is taken in forms such as snorting, smoking, or intravenous injection. Its intense euphoric effects have made it a notorious drug of abuse.