* 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 (2 to 4% of world population) 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.
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