Drug misuse is widespread in the UK and causes significant morbidity and mortality. It is clearly important to know how to assess and manage substance use disorders, particularly to know about treatments for heroin addiction. In this article, we examine the principles of identifying and managing drug addiction, and consider in some detail the presentation, risks, harms and treatment for misuse of opioids, stimulants, benzodiazepines, cannabis and novel psychoactive substances, which constitute the main classes of substance misused in the UK.