Publishing in the Journal of Epidemiology & Community Health scientists report that cannabis use in adolescence is associated with an increased risk of harmful substance use in early adulthood.
Dr Michael Bloomfield, Clinical Lecturer in Psychiatry at UCL, said:
“This is a well conducted study by a team of authors who include internationally respected epidemiologists. The study used data from the Avon Longitudinal Study of Parents and Children, which has yielded a wealth of medical research.
“This new study reports that cannabis use in adolescence is associated with an increased risk of other drug taking, including tobacco and alcohol in adulthood. Whilst this study finds that there is an increased risk of other drug use associated with adolescent cannabis use, there are many possible explanations for this beyond the ‘gateway hypothesis’. These include shared risk factors for drug use more generally. That said, there are plausible mechanisms by which cannabis use might increase the risk of other drug use, such as interactions with the brain chemical dopamine.
“Whilst at this stage, we cannot say that cannabis is a ‘gateway drug’, this study adds to growing research that the potential risks associated with cannabis use appear to be heightened during adolescence, a key time when the mind and brain are still developing.”
Dr Tom Freeman, King’s College London and Society for the Study of Addiction Fellow, said:
“This is a high quality study using a large UK cohort followed from birth to early adulthood. It provides further evidence that early exposure to cannabis is associated with subsequent use of other drugs. A key strength was the repeated assessment of cannabis use throughout adolescence, allowing the researchers to thoroughly classify different patterns of use.”
“What is striking about this study is the strength of the relationship between early cannabis use and subsequent tobacco dependence. Regular cannabis users were 37 times more likely to become dependent on tobacco compared to those who didn’t use cannabis, but only 3 times more likely to develop harmful patterns of drinking. This could be because cannabis is almost exclusively smoked together with tobacco in ‘joints’ in the UK. As a result, cannabis use often exposes young people to tobacco for the first time, leading them directly to the more harmful problem of tobacco dependence. At a time when smoking uptake is declining, dissuading young people from mixing their cannabis with tobacco is a critical message for public health.”
Ian Hamilton, Lecturer in Mental Health at the University of York, said:
“The authors have gone to considerable lengths and used sophisticated statistical analysis in this study but there are still some cautionary points worth considering when interpreting the results.
“The loss to follow up particularly at the end point when those included in the study were aged 21 is significant, with data available for only 1 in 3 of those who provided data at the first measurement point aged 13 years old. Although not unusual in this type of observational cohort study, missing data has the potential to skew the outcomes and therefore the results should be treated with caution.
“This study adds to the evidence that cannabis acts as a ‘gateway’ to nicotine dependence, as the majority of people using cannabis in the UK combine tobacco with cannabis when they roll a joint. This habit represents one of the greatest health risks to the greatest number of young people who use cannabis.
“This research suggests that adolescent cannabis use serves as a ‘gateway’ to a harmful relationship with drugs as an adult. However it is unclear whether this is due to social, biological or behavioural factors or a mix of all three.
“It is possible that the adolescents in this study were exposed to higher potency cannabis often referred to as ‘skunk’ and this may have influenced the results reported in this study.
“There has been a surge in the number of adults seeking specialist drug treatment citing cannabis as their primary problem. This study supports the case for intervening early and we could start by providing public health messages that clearly communicate the risks associated with using cannabis in adolescence.
“Observational studies such as these are unable to show causation; rather, as this study does, they can point to an association which is likely to be due to a complex interaction between environment, genetic vulnerability and social factors in the link between adolescent cannabis use and having a dependence on drugs as an adult.”
Dr Amir Englund, Researcher in Psychopharmacology at the Institute of Psychiatry, Psychology, and Neuroscience, King’s College London, said:
“The gateway hypothesis is a vague concept first popularised by the US Federal Bureau of Narcotics Commissioner Henry Anslinger who, after alcohol prohibition, made cannabis his new target. The claim then was that the harm of cannabis was that it made the person desire more harmful drugs like heroin.
“However, the concept of gateway is confused as people can sometimes mean that one drug leads to the use of another – rather than from a ‘lighter’ to a ‘heavier’ one. Animal studies have explored the desire among rats to repeatedly take cocaine and heroin when they were administered THC during ‘adolescence’, and found no greater likelihood compared to a placebo treatment. Specifically, a Swedish study on rats that received THC during adolescence found that these rats consumed a larger dose of heroin as adults, indicating a blunting effect (i.e. they needed more of it to feel the rewarding effects). Interestingly, the THC rats gave up sooner on the heroin when they had to work harder for it compared to the rats who had not received THC previously (Ellgren et al., 2007).
“This is a well-designed large-scale study which followed up adolescents into early adulthood and found that those who started using cannabis early and regularly were also more likely to be dependent on tobacco, using harmful amounts of alcohol and having tried other illegal drugs. This suggests that those who start using cannabis early and often are also more likely to engage in other risky behaviours in the future.
“However, many other psychosocial factors were related to cannabis use as well, and these seemed to explain some of these effects. Hence, it remains difficult to establish if it is specifically a pharmacological effect of cannabis which leads to this (although unlikely based on previous animal research) or if it has more to do with the psychosocial environment the adolescent grows up in. While aiming to reduce adolescent cannabis use remains an important strategy, it will likely not be enough as many other factors are also linked to future risky behaviours.”
* ‘Patterns of cannabis use during adolescence and their association with harmful substance use behaviour: findings from a UK birth cohort’ by Michelle Taylor et al. published in the Journal of Epidemiology & Community Health on Wednesday 7 June 2017.