How the ICD-11 and the CDDR address the public health dimensions of substance use

The use of psychoactive substances is highly prevalent and contributes substantially to risk behaviours, morbidity and mortality. The United Nations Office on Drugs and Crime World Drug Report¹ estimated that, in 2021, one in every 17 people aged 15-64 in the world had used an illicit drug in the year before. Users increased from 240 million in 2011 to 296 million in 2021, substantially more than accounted for by population growth.

Cannabis continued to be the most used illicit drug (219 million users, 4.3% of the global adult population); 36 million people had used amphetamines, 22 million cocaine, and 20 million methylenedioxymethamphetamine (MDMA or “ecstasy”) or related drugs in the previous year. An estimated 60 million people engaged in non-medical opioid use, 31.5 million of whom used opiates (i.e., non-synthetic opioids; mainly heroin).

Globally, there is very limited implementation of efficient and effective prevention strategies for substance use², and there is a substantial treatment gap for disorders due to this use³. Global evidence has called attention to the need for a new and comprehensive conceptualization of substance use disorders that incorporates the full range of relevant conditions, from risky consumption to mental disorders linked to harmful drug use⁴.

In response to these challenges, the World Health Organization (WHO) adopted a public health approach to the development of the classification of disorders due to substance use in the ICD-11. By public health approach, we refer to a broader perspective that integrates health and social aspects, aiming to benefit affected individuals and their community, and focusing on population well-being⁵.

From a public health perspective, it is essential to identify persons who exhibit a hazardous use of substances that increases the risk of harmful psychological or medical consequences, but whose symptoms do not meet the diagnostic requirements for substance use disorders. These individuals can benefit from education, prevention, and community interventions. People with diagnosable disorders need harm reduction and treatment services of differing intensities and settings, depending on the nature of their condition and the substance involved. Those who suffer physical or psychological harm due to others’ substance use should also be identified and may require services⁶.

In line with this perspective, the range of psychoactive substances classified in the ICD-11 section on disorders due to substance use has been expanded, reflecting changes in the substances associated with public health impact in different parts of the world. An extended set of substance classes will help track patterns more accurately, in order to formulate appropriate clinical and social policy responses nationally and globally. For example, a new set of categories for disorders due to synthetic cannabinoids has been added. Synthetic cannabinoids are sprayed on natural herb mixtures to mimic the euphoric effect of cannabis, and can produce respiratory depression⁷. Their use is reported in high-income countries, but little information is available for low- and middle-income countries¹.

As described in the Clinical Descriptions and Diagnostic Requirements for ICD-11 Mental, Behavioural and Neurodevelopmental Disorders (CDDR)⁸, four primary conditions are identified for each class of psychoactive substances, which are hierarchically and mutually exclusive from one another: a) hazardous substance use, which is conceptualized as a pattern of substance use that is sufficient in frequency or quantity to increase the risk of harmful physical or mental health consequences to the user or to others; since it involves incremental risk for harm that has not yet occurred, it is not considered a mental disorder (rather, it appears in the ICD-11 chapter on “Factors influencing health status or contact with health services”, facilitating early attention and advice from health professionals); b) episode of harmful substance use, which refers to an episode that has already caused harm to a person's physical or mental health or has resulted in behaviour leading to harm to the health of others, but in the absence of a known pattern of substance use; c) harmful pattern of substance use, a sub-dependence diagnosis, characterized by a persistent and repetitive pattern of substance use that has directly caused harm to the person or to someone else through the person's behaviour; and d) substance dependence, when a disorder of substance use regulation has arisen from repeated or continuous use of a substance, typically accompanied by a strong internal drive to use it.

In the ICD-11, the substance dependence diagnosis has been simplified with respect to the ICD-10. It is based on the presence of at least two of three key features: a) impaired control over substance use, b) increasing priority given to substance use over other activities, and c) physiological features of tolerance or withdrawal. Physical and mental harm is very commonly seen in substance dependence, but is not a required feature.

The CDDR indicate that clinicians may assign other substance use diagnoses in addition to one of the four primary diagnoses, depending on the specific clinical situation, including substance intoxication, substance withdrawal, and a range of substance-induced mental disorders (delirium; psychotic, mood, anxiety, obsessive-compulsive, and impulse control disorders)⁸. Additional medical diagnoses can be assigned as appropriate to describe the consequences of substance use. Clinicians can also apply a range of specifiers offering more precision in diagnosis according to the severity, course, or other manifestations of the primary and additional diagnoses.

The classification of conditions related to substance use in the ICD-11 clearly corresponds to different types of intervention needs, consistent with the WHO services pyramid framework describing the optimal mix of services for mental health⁹. Hazardous use is an appropriate target for brief interventions as well as for public health programs and primary prevention. Harmful use can be responded to in generalist settings, such as primary care, using mild or more intensive interventions depending on whether the problem is a single episode or a harmful pattern of use, and on the substance involved. The most severe cases of substance dependence are appropriately treated in more intensive specialized settings, but they represent only a small portion of the overall disease burden related to substance use. Accordingly, the ICD-11 and the CDDR will help clinicians conceptualize and communicate the most appropriate forms of treatment for specific disorders, and support public health interventions for more common but less severe presentations.

Overall, the ICD-11 and the CDDR are valuable tools for helping to reduce the gap between those who need treatment and those who receive it. They will also support improvements in drug and health policies through better characterization of different groups of people affected by substance use, who experience different types of harm and have different needs. This includes improvements in the treatment system to provide more effective alternatives for severe alcohol and drug dependence.

Implementing the new diagnostic requirements can also support a better referral system that matches the needs of different users to the services provided. It can also support improved epidemiological studies and generate more valuable data for WHO member states by providing better categories that accurately reflect substance use outcomes. Finally, and importantly, the new classification supports implementing a public health model rather than focusing only on punishment and incarceration.