Marijuana is the most popular illegal drug used worldwide, but addictions to popular painkillers like Vicodin, Oxycontin and codeine kill the most people, according to the first-ever global survey of illicit drug abuse.

In addition to cannabis and opioid painkillers, scientists analyzed abuse of cocaine and amphetamines in 2010, largely based on previous studies. Ecstasy and hallucinogens weren't included, because there weren't enough data. The researchers found that for all the drugs studied, men in their 20s had the highest rates of abuse. The worst-hit countries were Australia, Britain, Russia and the U.S. The study was published online Thursday in the journal, Lancet.

But there were few concrete numbers to rely on and researchers used modeling techniques to come up with their estimates.

"Even if it is not very solid data, we can say definitely that there are drug problems in most parts of the world," said Theo Vos, of the Institute for Health Metrics and Evaluation at the University of Washington, the study's senior author. Vos said people tended to abuse drugs produced close to home: cocaine in North America, amphetamines and opioids in Asia and Australia. The lowest rates of drug abuse were in Asia and Africa. Of the estimated 78,000 deaths in 2010 because of illegal drug use, more than half were because of painkiller addictions.

Vos said countries with harsh laws against drugs had worse death rates for addicts when compared to countries who relied on other policies to wean people off drugs, such as needle exchange programs and methadone clinics.

Other experts warned officials needed to be on their toes to address potential health problems from drug abuse.

"The illicit use of prescribed opiates in the U.S. has only happened in the last 10 years or so," said Michael Lysnkey, of the National Addiction Centre at King's College London, who co-authored an accompanying commentary. "It's possible in another 20 years, patterns will again change in ways we can't predict."

In a related study, scientists found mental health and drug abuse problems including depression, schizophrenia and cocaine addiction kill more people worldwide than AIDS, tuberculosis, diabetes or road accidents.

In some developing countries such as India, attempts to stop AIDS have also slowed drug abuse as they focus on helping people kick their addictions, according to Vikram Patel, of the Centre for Global Mental Health at the London School of Hygiene and Tropical Medicine. Patel recommended an approach to drug use similar to current controls on tobacco.

"A decriminalized drug policy could potentially transform the public health approach to drug use," he wrote in an email. "The enormous savings in the criminal justice system could be used to fund addiction treatment programs."