Jan. 5, 2012 -- Between 149 million and 271 million people worldwide used an illicit drug at least once in 2009, according to a new review of studies attempting to estimate the extent of the problem. That translates to 1 in 20 people aged 15 to 64 taking an illegal drug.

But this global figure is likely to underestimate the number of users, the researchers warn, since people might not want to admit to illegal use in surveys, and data from the poorest countries is limited.

Even so, two Australian researchers reviewed studies from around the globe to determine the scope of illegal drug use in people aged 15 to 64 and understand its health effects on problem-users in these countries.

Marijuana and hashish (cannabis) use topped the list with between 125 million and 203 million users worldwide in 2009. The highest levels of use were seen in North America, Western Europe, Australia, and New Zealand.

In North America, nearly 11% of the population aged 15 to 64 used cannabis that year. Between 14 million and 56 million people aged 15 to 64 worldwide used amphetamine-type stimulants, such as speed and crystal meth.

Cocaine use was highest in North America, and it had 14 million to 21 million users worldwide.

Opioid use, including heroin, had an estimated 12 million to 21 million users globally. The highest rates of use were in the Near and Middle East, where up to 1.4% of the population aged 15 to 64 had tried the drug at least once that year.
Similar Health Risks to Alcohol

Not surprisingly, the researchers found that the highest level of illicit drug use was seen in the wealthiest countries or those areas nearest to the drugs' production.

Although illicit drug use was linked with about 250,000 deaths worldwide in 2004, alcohol claimed roughly 2.25 million lives globally during that same time period, while tobacco use led to an estimated 5.1 million deaths. Drug deaths tend to occur in younger people, while alcohol and tobacco abuse tends to claim lives in middle-aged and older people.

In addition, the findings suggest that the disease burden from people's use of illicit drugs in wealthier countries is less than what's seen with tobacco, but it may be similar to the effects from alcohol.

This research is the first in a three-part special series on addiction in The Lancet.