For those dependent on opioids, the risk of death was 5.71 times higher than healthy individuals in the population of the same age, gender and race. Those with methamphetamine use disorders were next highest with a 4.67-fold risk, followed by those with addictions to cannabis (3.85), alcohol (3.83) and cocaine (2.96). Alcohol dependence was related to the highest number of deaths overall.

The study, available online in the journal Drug and Alcohol Dependence, is the largest North American study to compare mortality rates among different drug users with the longest follow-up. It tracked records of more than 800,000 individuals hospitalized with drug dependence between 1990 and 2005. Of this group, more than 188,000 died during this period.

The findings mean that if 10 individuals in the general population died, then over the same period there would be 57 deaths among people dependent on opioids, which includes prescription opioids as well as heroin.

"One reason for undertaking this study was to examine whether methamphetamine posed a particular threat to drug users, as it has been called 'America's most dangerous drug,'" says CAMH Scientist Dr. Russell Callaghan, who led the study. Globally, methamphetamine and similar stimulants are the second most commonly used class of illicit drugs.

"The risk is high, but opioids are associated with a higher risk. We also wanted to compare mortality risks among several major drugs of abuse, as this comparison hasn't been done on this scale before."

Alcohol dependence affected the highest number of individuals, with 166,482 deaths and 582,771 hospitalizations over the study period. In the methamphetamine group, there were 4,122 deaths out of 74,139 hospitalizations, and for opioids, 12,196 deaths out of 67,104 hospitalizations.

Specific causes of mortality were not examined in this study, so the deaths may not be directly caused by drugs but due to related injuries, infectious disease or unrelated reasons. The researchers are now exploring mortality causes for each drug group, which may also point to reasons why women had a higher risk of death for alcohol, cocaine and opioids than males.