"We wanted to find out why thousands of people in the US and Canada are dying from prescription painkillers every year and why these rates have climbed steadily during the past two decades," explained Nicholas King, professor at the faculty of medicine, McGill University in Canada.
In an effort to identify available evidence, King and his team conducted a systematic review of existing literature, comprehensively surveying the scientific literature and including only reports with quantitative evidence.
They found evidence for at least 17 different determinants of increasing opioid-related mortality - mainly dramatically increased prescription and sales of opioids.
It included increased use of strong, long-acting opioids like Oxycontin and methadone; combined use of opioids and other (licit and illicit) drugs and alcohol; and social and demographic factors.
Currently, the US and Canada rank no.1 and no.2, respectively, in per capita opioid consumption.
"We found little evidence that internet sales of pharmaceuticals and errors by doctors and patients - factors commonly cited in the media - have played a significant role," King added.
The findings point to a complicated "epidemic" in which physicians, users, the health care system and the social environment all play a role, according to the researchers.
"Our findings might be useful in preventing other countries from following the same path as the US and Canada," King added.