Elsevier

The Lancet

Volume 379, Issue 9810, 7–13 January 2012, Pages 55-70
The Lancet

Series
Extent of illicit drug use and dependence, and their contribution to the global burden of disease

https://doi.org/10.1016/S0140-6736(11)61138-0Get rights and content

Summary

This paper summarises data for the prevalence, correlates, and probable adverse health consequences of problem use of amphetamines, cannabis, cocaine, and opioids. We discuss findings from systematic reviews of the prevalence of illicit drug use and dependence, remission from dependence, and mortality in illicit drug users, and evidence for acute and chronic effects of illicit drug use. We outline the regional and global distribution of use and estimated health burden from illicit drugs. These distributions are likely to be underestimates because they have not included all adverse outcomes of drug use and exclude those of cannabis—the mostly widely used illicit drug. In high-income countries, illicit drug use contributes less to the burden of disease than does tobacco but a substantial proportion of that due to alcohol. The major adverse health effects of cannabis use are dependence and probably psychotic disorders and other mental disorders. The health-related harms of cannabis use differ from those of amphetamine, cocaine, and opioid use, in that cannabis contributes little to mortality. Intelligent policy responses to drug problems need better data for the prevalence of different types of illicit drug use and the harms that their use causes globally. This need is especially urgent in high-income countries with substantial rates of illicit drug use and in low-income and middle-income countries close to illicit drug production areas.

Introduction

Illicit drugs are drugs for which non-medical use has been prohibited by international drug control treaties for half a century because they are believed to present unacceptable risks of addiction to users.1, 2 International control has since been extended from plant-based drugs—heroin, cocaine, and cannabis—to synthetic drugs, such as amphetamines and methylenedioxymetamfetamine (MDMA), and pharmaceutical drugs such as buprenorphine, methadone, and benzodiazepines (panel 1).

In this paper, we summarise data for the prevalence, correlates, and probable consequences of use of the amphetamines, cannabis, cocaine, and opioids—the most commonly used and studied illicit drugs. We discuss findings from systematic reviews of data for the prevalence of illicit drug use and dependence,3, 4, 5, 6, 7, 8 remission from dependence,9 and mortality in illicit drug users (panel 2).10, 11, 12, 13 We attribute adverse health effects to these drugs using findings from reviews of published studies of the evidence on a range of acute and chronic harms of illicit drug use.8, 14, 19, 35, 36, 37, 38, 39, 40, 41 We provide a brief summary of adverse health effects for different drug types referencing other reviews (webappendix pp 3–5 for more details). We also summarise earlier global burden of disease studies that estimated the regional and global distribution of health burden from illicit drug use and compared this with the burden attributable to alcohol and tobacco use.29, 30, 31, 32, 33

We do not discuss the prevalence of or disease burden related to MDMA (ecstasy), hallucinogenic drugs, inhalants, or the non-medical use of benzodiazepines and anabolic steroids because information about the prevalence of their use and quantification of their harms is more scarce than it is for the drugs included in this paper (webappendix p 1).42, 43, 44, 45, 46 Their exclusion is because of the scarcity of evidence rather than any judgment about the contribution of these drugs to disease burden. We were also unable to separately discuss the magnitude of adverse outcomes attributable to prescribed pharmaceutical opioids. Although increased prescription of these drugs has been accompanied by increases in morbidity and mortality in some countries,47 data for the magnitude of risks of iatrogenic dependence and mortality in users are not available.47, 48 In countries where use of these drugs has been studied, a substantial proportion of problem users had pre-existing problems with opioids. In these countries, estimates of opioid-dependent people include both heroin and pharmaceutical opioid users.47

Key messages

  • The illegality of opioids, amphetamines, cocaine, and cannabis precludes the accurate estimation of how many people use these drugs, how many people are problem users, and what harms their use causes.

  • An estimated 149–271 million people used an illicit drug worldwide in 2009: 125–203 million cannabis users; 15–39 million problem users of opioids, amphetamines, or cocaine; and 11–21 million who injected drugs.

  • Levels of illicit drug use seem to be highest in high-income countries and in countries near major drug production areas, but data for their use in low-income countries are poor.

  • Cannabis use is associated with dependence and mental disorders, including psychoses, but does not seem to substantially increase mortality.

  • Illicit opioid use is a major cause of mortality from fatal overdose and dependence; HIV, hepatitis C, and hepatitis B infections from unsafe injection practices are important consequences in people who inject opioids, cocaine, or amphetamines.

  • Adverse health outcomes such as mental disorders, road-traffic accidents, suicides, and violence seem to be increased in opioid, cocaine, and amphetamine users. To what extent these associations are causal is unclear, because confounding variables are not always controlled and quantification of risk is poor.

  • Global burden of disease estimates suggest that in high-income countries, the contribution of illicit drug use is a substantial proportion of that attributable to alcohol.

  • These estimates probably underestimate the true burden because only a few effects of problem use of opioids, cocaine, and amphetamines are included. The global burden of disease 2010 study will address these limitations.

Section snippets

The prevalence of drug use and dependence

Major challenges exist in the accurate estimation of the prevalence of an illegal, and often stigmatised, behaviour like illicit drug use. This is especially so in cultural settings where illicit drug use can lead to imprisonment, and where research participants cannot be assured of confidentiality or freedom from reprisals for disclosing their drug use behaviours. By necessity, a range of imperfect methods have to be used to estimate the prevalence of use in such areas (panel 3).

The

The natural history and risk factors for use and dependence

Studies in high-income countries, with high levels of cannabis use, have reported a common temporal ordering of drug initiation—alcohol and tobacco, followed by cannabis use, and then other illicit drugs. This pattern persists after control for possible confounders.19, 61, 62 This pattern is not consistent across countries.25 Use of other illicit drugs is more prevalent than is use of cannabis in some countries (eg, Japan), and the association between initiation of alcohol, tobacco, and

Health consequences of illicit drug use

Four broad types of adverse health effects of illicit drug use exist:1 the acute toxic effects, including overdose; the acute effects of intoxication, such as accidental injury and violence; development of dependence; and adverse health effects of sustained chronic, regular use, such as chronic disease (eg, cardiovascular disease and cirrhosis), blood-borne bacterial and viral infections, and mental disorders (Table 4, Table 5). Many people who use illicit drugs will use more than one of the

Burden of disease attributable to illicit drug use

Since 1993, estimates of the causes of global disease burden have used disability-adjusted life years (DALY)1243to combine disease burden from premature mortality with that from disability. This metric allows a comparison of the contribution across diseases, injuries, and risk factors. In 2002, the comparative risk assessment exercise124 estimated the proportion of disease burden attributable to alcohol, tobacco, and injecting drug use. These estimates explicitly accounted for variations in

Discussion

A substantial proportion of young adults in developed countries have used an illicit drug at some time in their lives. Worldwide, around one in 20 people aged 15–64 years might have done so in the past year. Cannabis is the drug most often used and the most widely available because of widespread domestic production in many countries. A minority of individuals who use illicit drugs become dependent on or inject them. The prevalence of dependence on these drugs has rarely been directly assessed,

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