This is the first of three articles that will look at the issue of adolescent substance use. This article introduces different substances and how they are used. The second article will focus on adolescent substance use, the harm it can cause and the best prevention methods available. The final article will focus on treatment of adolescent substance use. This article starts with an overview of categories of substances followed by a brief overview of how substances interact with the body. Specific substances that are most relevant to those working with adolescents in Ireland today are then discussed. Alcohol and cannabis are the two most frequently used substances, while cocaine and nitrous oxide are also discussed as they have received significant media attention in recent years. But first, when talking about substance use it is worth thinking about the language that is used; words and phrases such as addiction, addict, alcoholic, drug dependent, drug abuse, drug misuse etc. are used frequently. Addiction, strictly speaking, is not a medically defined term but tends to be used to describe someone who meets the criteria for a substance use disorder (SUD). The criteria for what constitute a substance use (or any mental health) disorder are set out by two organisations: the World Health Organisation (WHO) and the American Psychiatric Association (APA). The WHO publish a book called the International Classification of Diseases which is currently in its eleventh edition (ICD-11) and the APA publish one called the Diagnostic and Statistics Manual, currently in its fifth edition (DSM-5). The DSM-5 tends to be used in North America and Australia while the ICD-11 is used almost everywhere else, including Ireland and so I will focus on this. Within the ICD-11 system there are four main substance use diagnoses which are summarised in T able 1. These are diagnosed in relation to a particular substance such as an episode of harmful alcohol use, or harmful cannabis use. A criticism that is often made of these criteria is that they do not consider the person’s age, so the same criteria are used to diagnose harmful alcohol use in a twelve-year-old as a forty-year-old.

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