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