Cannabis use during adolescence linked to greater risk of illegal substance abuse in early adulthood

According to the study published online in the Journal of Epidemiology & Community Health, regular and occasional use of cannabis as an adolescent is linked with a greater risk of other illicit drug consumption in early adulthood. The study by Bristol's Population Health Science Institute also found that cannabis use was connected with harmful drinking and smoking. One in five adolescents are at a risk of tobacco dependency, harmful alcohol consumption, and illicit drug use.

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Using data from the Avon Longitudinal Study of Parents and Children (ALSPAC), the researchers from the University of Bristol looked at levels of cannabis use during adolescence and assessed their influence on problematic substances in early adulthood (at the age of 21).

The researchers looked at data on cannabis use among 5,315 adolescents in the age group of 13-18. Information on cannabis use was collected at five time points. Approximately one year apart, cannabis use was categorized into: 1) Do not use, 2) occasional (typically less than once per week), and 3) frequent (typically once a week or more).

When the adolescents reached 21 years, they were asked to provide information on whether they smoke and drink, if so, how much they have smoked and drank, and whether they have been using other illicit drugs in the past three months. Of the 462 who reported recent use of illicit drugs, 176 (38%) used cocaine, 278 (60%) used amphetamines, 136 (30%) used inhalants, 72 (16%) used sedatives, 105 (23%) used hallucinogens, and 25 (6%) had used opioids.

The study's lead author, Dr Michelle Taylor from the School of Social and Community Medicine said:

"We tend to see clusters of different forms of substance misuse in adolescents and young people, and it has been argued that cannabis acts as a gateway to other drug use. However, historically the evidence has been inconsistent."

She continued:

I think the most important findings from this study are that one in five adolescents follow a pattern of occasional or regular cannabis use and that those individuals are more likely to be tobacco dependant, have harmful levels of alcohol consumption or use other illicit drugs in early adulthood."

Complete data were available for 1,571 people. Male sex, mother’s substance abuse, and smoking and alcohol use before the age of 13 were all strongly connected with cannabis use during adolescence. Other potentially influential factors such as housing tenure, maternal education, the number of children the mother had, mother’s drinking and substance use and behavioural problems when the child was 11.

After considering other influential factors, it was observed that those who used cannabis during adolescence were at greater risk of problematic substance misuse by the age of 21, compared with others who did not use cannabis.

By the time the adolescents reached the age of 21:

  • Regular cannabis users were 37 times more likely to be dependent on nicotine, 3 times more likely to have a harmful drinking pattern than nonusers, and 26 times more likely to use other illicit drugs.
  • Occasional cannabis users both who started using it early and those who started much later in adolescence had a heightened risk of dependence on nicotine, harmful drinking, and other illicit drug consumption.
  • The more cannabis the adolescents used the greater was the probability of nicotine dependence.

The study used observational methods and presented evidence for the correlation; however, it does not find out clear cause and effect that the use of cannabis actually causes the use of other illicit drugs. Furthermore, it does not recognize the underlying mechanisms for how this could be. Yet, clear categories of use emerged.

Dr Taylor concluded, "We have added further evidence that suggests adolescent cannabis use does predict later problematic substance use in early adulthood. From our study, we cannot say why this might be, and it is important that future research focuses on this question, as this will enable us to identify groups of individuals that might as risk and develop policy to advise people of the harms.

"Our study does not support or refute arguments for altering the legal status of cannabis use--especially since two of the outcomes are legal in the UK. This study and others do, however, lend support to public health strategies and interventions that aim to reduce cannabis exposure in young people."

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