Cannabis use increases risk of premature death

Heavy cannabis use at a young age increases the risk of early death, according to the longest follow-up study to date on cannabis use. The new study, which was done by researchers at Karolinska Institutet, is published in the American Journal of Psychology.

Cannabis use increases the risk of developing schizophrenia and other psychotic disorders later in life, something that the researchers reported in a previous paper after having followed up all men who were enlisted for military service in 1969–70. At this time, the men answered questions on drug use; today, they are around 60, an age when any harmful long-time effects of cannabis use can start to show.

New follow-up

The researchers therefore made a new follow-up to find out if the mortality rate was higher amongst men who reported using cannabis in their youth.

"A fresh WHO report has shown that cannabis has extensive effects on the health, even if the mental effects are the most salient," says lead author Edison Manrique-Garcia at the Department of Public Health Sciences.

Over 50,000 men were included in the study population, 4,000 of whom had died between 1970 and 2011. The researchers found that men who reported the heaviest cannabis use in their youth (over 50 occasions) ran a 40 per cent higher risk of death than those who reported lighter or no cannabis use (odds ratio 1.4; 95% confidence interval 1.1–1.8). They also found that the correlation remained even when controlling for other factors affecting mortality, such as alcohol use, mental illness and childhood/adolescent social problems.

Treated for psychosis

Since it is well-established that cannabis use increases the risk of psychotic disorders, the researchers studied if those who had suffered psychotic episodes had particularly high mortality rates and if cannabis use affects this. 683 people had been treated for psychosis.

"For the men who had suffered psychosis, the mortality rate was roughly four times higher, but this was independent of earlier cannabis use," says study leader Peter Allebeck, also at the Department of Public Health Sciences. "Our conclusion is that early cannabis use can contribute to premature death, particularly through injuries and suicide, and that this is unrelated to the increased risk of psychosis."

Since the study gave no details of cannabis use during the follow-up time, it is impossible to tell if the increased risk amongst heavy early cannabis users is attributable to strong effects of the drug in their youth or to continued use of cannabis later in life.