Developing a habit

The legacy of cannabis use during the teenage years.

ADOLESCENCE is a time of change, experimentation and exploration, not only physically and cognitively but also psychologically.

It is when the importance of the peer group emerges and when risky behaviours can be seen, particularly initiation of alcohol and tobacco use, increased motor vehicle accidents, and sexually transmitted infections due to unsafe sex practices.

Cannabis use is also often initiated during the adolescent years, with the 2004 Australian Institute of Health and Welfare National Drug Strategy Household Survey reporting 18% of 14 to 19 year olds and 26% of 20 to 29 year olds using cannabis in the previous year.

The literature suggests early initiation and regular (weekly) use of cannabis predicts harmful use of cannabis into the future. A study by Swift et al in Addiction this month attempts to clarify “the relationship between the pattern of adolescent use and the frequency of problematic cannabis use at 24 years” (2008;103:1361-70).

The data comes from the Victorian cohort study of health in adolescents with a total of 2032 students participating over a 10-year period from 1992 to 2003, using self-administered online questionnaires at school or computer-assisted telephone interviews for the latter years.

The average age at commencement was 14.9 years and on completion 24.1 years. From the total sample of 2032 students, 1520 or 78% were interviewed at age 24 years. Early cannabis use was defined as initiating before the age of 16 years.

A third of the 24-year-old age group reported using cannabis in the adolescent years, including 12% using weekly/daily. More than half the adolescent users had continued to use cannabis and two-thirds started at a mean age of 15.9 years. Cannabis use was more common among males and those whose parents were separated and divorced or those who lived with a parent who smoked cigarettes.

At age 24, 509 participants had used cannabis in the past year, with 37% weekly users and 20% meeting the criteria for cannabis dependence. Those who had early initiation in adolescence were at two- to three-fold risk of problematic cannabis use at 24 years. The authors reported that adolescent males were not only more likely to experiment with cannabis but “to become long-term entrenched users”.

Adolescent cigarette smoking was significantly and independently associated with problematic cannabis use; in particular, dependence at age 24 years. However, there was no association with persistent adolescent alcohol intake.

There may well be common genetic, environmental or neuropharmacological factors that predispose to both cannabis and nicotine dependence. Adolescent cannabis users with persistent symptoms of anxiety and depression also had increased rates of problematic cannabis use at 24 years.

Interestingly, however, the 24-year-old dependent cannabis users comprised equal proportions of occasional, weekly and daily adolescent users. Therefore, while high-dose exposure during adolescence is strongly predictive, there are other important factors that moderate the development of problematic use.

The authors emphasise that it is not possible to designate a “safe” level of cannabis exposure during the adolescent years. Prevention of initiation needs to remain a priority, with education around the risks of cannabis use, particularly focusing on the vulnerable groups: males, those with ongoing persistent symptoms of anxiety or depression, and persistent smokers. 

Dr Susan Towns, FRACP, is head of adolescent medicine, The Children’s Hospital, Westmead, NSW.