Prescription painkillers such as Vicodin, OxyContin and codeine kill more people than street drugs such as crack and heroin, according to the first global survey of drug abuse.
More than half of the estimated 78,000 drug-related deaths recorded worldwide in 2010, the most recent year for which comprehensive figures are available, were caused by addiction to painkillers, the study published in the Lancet medical journal found.
The English-speaking world was the worst hit by drug abuse, with Britain, the US and Australia leading the table, along with Russia. The lowest rates of drug abuse were in Asia and Africa.
Countries with strict laws criminalising drug use had higher death rates and other problems linked to drug addiction compared to countries with a more progressive, liberal culture, and initiatives such as needle exchange programmes and methadone clinics.
Men in their twenties had the highest rates of drug abuse for all the drugs included in the study, the research found, while cannabis emerged as the most commonly used illegal drug worldwide.
In addition to cannabis and opioid painkillers, scientists analysed the use of cocaine and amphetamines.
"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.
His co-author Michael Lysnkey, of the National Addiction Centre at King's College London, said the illegal use of prescription drugs was a relatively recent phenomenon.
"The illicit use of prescribed opiates in the US has only happened in the last 10 years or so," he said.
"It's possible in another 20 years, patterns will again change in ways we can't predict."
Ecstasy and hallucinogens were not included in the study, due to a lack of sufficient data.
Vos said people tended to abuse drugs produced close to home, such as cocaine in North America, and amphetamines and opiates in Asia and Australia.
Drug addiction and related mental health problems such as depression and schizophrenia casue more deaths worldwide than AIDS, tuberculosis, diabetes or road accidents, a related study found.
Vikram Patel, of the Centre for Global Mental Health at the London School of Hygiene and Tropical Medicine, urged governments to decriminalise drug use, and instead implement controls similar to those restricting tobacco sales.
"A decriminalised drug policy could potentially transform the public health approach to drug use," he said.
"The enormous savings in the criminal justice system could be used to fund addiction treatment programmes."
On Friday, a coroner in Brighton, south England, ruled that a 23-year-old man who died while running a marathon had suffered an "idiosyncratic reaction" to the over-the-counter painkiller ibuprofen, which he had taken in combination with sports supplements.
Sam Harper Brighouse died of bowel ischemia and a gastrointestinal haemorrhage after collapsing 16 miles into the race.