A controlled substance is a drug or chemical whose production, possession, supply or use is restricted by law because of its potential for misuse, harm or diversion. In England and Wales, Scotland and Northern Ireland, the term is primarily governed by the Misuse of Drugs Act 1971 and associated regulations, which classify controlled drugs into Classes A, B and C and set out offences and licensing regimes. In Ireland, the Misuse of Drugs Acts 1977–2016 and regulations perform a similar function, with schedules listing controlled drugs.The term covers illicit drugs (such as heroin, cocaine and MDMA) and many prescription medicines (such as certain opioids, benzodiazepines and stimulants) when handled outside lawful authority. It is central to criminal offences (possession, supply, production, importation), regulatory compliance for healthcare and pharmaceutical entities, and professional discipline for clinicians and pharmacists.While specific classifications and schedules differ slightly between the UK and Ireland, the underlying concept and practical usage of “controlled substance” or “controlled drug” are broadly consistent, and practitioners must consult the relevant national legislation and current schedules for precise status.