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.