Across the country, there have been several initiatives to decriminalize and even legalize marijuana, including efforts here in Vermont. A common sentiment behind these movements is that cannabis in not really addictive or harmful. These efforts have reignited the debate about the potential dangers associated with cannabis use, particularly among adolescents. A recent review paper by Hurd and colleagues in the journal Neuropharmacology examined the literature on the link between early cannabis use and later addiction and provides some practical conclusions that can be useful in discussions with patients.
Adolescence is a period during which there is a lot of brain plasticity, thus rendering the stage as potentially susceptible to the influence of substances such as cannabis. Cannabaniod receptors are highly expressed in the brain, particularly in regions such as the prefrontal cortex, cerebellum, amygdala, and hippocampus that are critical for cognitive and emotional functioning.
Regarding the “gateway hypothesis,” which states that early cannabis use increases the risk of addiction for other drugs, there is good evidence from multiple studies that the intensity of cannabis exposure is directly related to the use of ‘heavy’ drugs. Further, early cannabis use has been linked to poorer outcomes in a number of areas including educational achievement, employment, rule-breaking behaviors, and assuming more adult roles. Human studies of cannabis often have methodological flaws that make it difficult to demonstrate a clear causal action of cannabis use on later outcomes. In other words, it can be quite difficult to determine if cannabis is truly the problem itself or if a common genetic or environmental factor drives both cannabis use and psychopathology (Harder et al., 2008). Animal studies, however, are often free of these complications and have demonstrated a direct relation between cannabis exposure and increased intake of opiates. Animal studies also show links between THC exposure and later behavioral changes (although the study about pot smoking rats being less likely to attend college is inconclusive). In humans, behavioral effects tend to be seen in a subset of cannabis abusing adolescents and include negative affect, decreased goal directed behavior, aggression, and less frequently psychosis.
Overall, then, the available evidence does point to cannabis use in adolescents being related to increased vulnerability to future addiction and poorer outcomes; however, there is much that remains to be learned about how cannabis interacts with other factors in development. The article provides a great deal of useful information about the risks associated with cannabis use while not glossing over the significant gaps in knowledge that need to be addressed.
Harder VS, et al. Adolescent cannabis problems and young adult depression: Male-female stratified propensity score analyses. Am J Epidemiol 2008; 168:592-601.
Hurd YL, et al. Trajectory of adolescent cannabis use on addiction vulnerability. Neurophrarmacology 2013. Epub ahead of print.