New research led by the University of Queensland has found that regular cannabis use may have harmful effects later in life, irrespective of the age the person first started using it. The findings showed that life outcomes for regular users were worse by age 35 compared to those who had not regularly smoked. 

The 20 year-long prospective cohort study, led by Dr Gary Chan from UQ’s National Centre for Youth Substance Use Research, compared those who initiated cannabis use during high school to those who started after high school, comparing their life outcomes to non-users by age 35. 

The findings of the study are published in the journal Drug and Alcohol Review.

“Compared to non-users, regular cannabis users were more likely to engage in high-risk alcohol consumption, smoke tobacco, use other illicit drugs and not be in a relationship at age 35,” Dr Chan said in a statement. “These outcomes were more common among those who started using cannabis regularly in adolescence."

“They were also at higher risk of depression and less likely to have a paid job." Dr Chan continued. “Overall, regular use of cannabis – more than weekly and especially daily use – was found to have harmful consequences, regardless of the age people began using it.”

The study began back in 1992, following 1,792 Australian high school students that were aged 15 at the time. The study assessed patterns of cannabis use as time proceeded, as well as various other life outcomes such as tobacco smoking, drinking, relationship status, employment, financial hardship, the use of other illicit drugs. They also looked at health outcomes, such as depression and anxiety. 

“Two-thirds of people who use cannabis regularly started use in their early 20s," Dr Chan said. "Because adult-onset is a lot more common than adolescent on-set, most of the harms associated with cannabis are in fact in the group who begin later on. Those who began regular use as a young adult accounted for the highest proportion of subsequent illicit drug use and tobacco use in the population, and a much higher proportion of high-risk drinking." 

As the study authors wrote in the paper: "Cannabis users who began regular use in their teens had poorer later life outcomes than non‐using peers. The larger group who began regular cannabis use after leaving high school accounted for most cannabis‐related harms in adulthood."

Although some compounds in cannabis, such as CBD, may have medicinal benefits as touted by various studies over the years – such as treating epilepsy, killing superbugs, and maybe even helping to alleviate mental health problems – more work needs to be done to understand the long-term effects of cannabis use. This is because cannabis contains an array of different compounds such as THC – which is responsible for the psychoactive experience associated with smoking cannabis – and whether this or other compounds present in cannabis contribute to the negative effects of long-term smoking remains debatable.

Nevertheless, Dr Chan said that their work in the new study illustrated to the public that there are harmful risks involved in regular cannabis use, and that health legislation should take these findings into account.

“Public health agencies and policy makers need to deliver a clear and strong message to the public that regular cannabis use is harmful, regardless of when an individual initiates its use,” he said.

“This is particularly important for jurisdictions that have already legalised recreational cannabis, such as Canada and some US states.” Dr Chan concluded.