Roughly 200 million people worldwide use illicit drugs such as marijuana, amphetamines, cocaine and opioids each year, according to a new study. The figure represents about one in 20 people between the ages of 15 and 64.

Using a review of published studies, Australian researchers estimated that as many as 203 million people use marijuana, 56 million people use amphetamines including meth, 21 million people use cocaine and 21 million people use opioids like heroin. The use of all four drug classes was highest in developed countries.

"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," reads the report, published today in The Lancet. "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."

The 200 million number does not include people who use ecstasy, hallucinogenic drugs, inhalants, benzodiazepines or anabolic steroids - just one reason it's likely a vast underestimate of illicit drug use, according to lead author Louisa Degenhardt of the Sydney-based National Drug and Alcohol Research Center.

"Drug use is often hidden, particularly when people fear the consequences of being discovered for using drugs, such as being imprisoned," Degenhardt said in a press conference.

Up to 39 million people are considered "problematic" or dependent drug users and up to 21 million people inject drugs, according to the report.

"It's likely that injectable drug users have increased," said Degenhardt, adding that the practice "is a major direct cause of HIV, hepatitis C and to some extent hepatitis B transmission globally." Cocaine, amphetamine and heroin can be injected either alone or in combination.

Illicit drugs can have dangerous health effects, including overdosing, accidental injury caused by intoxication, dependence and long-term organ damage. While they may not cause immediate death, they're thought to shave 13 million years of the life spans of users worldwide, according to the report.  A 2000 report by the World Health Organization attributed roughly 241,000 deaths to illicit drug use - double the number from 1990.