Normal developmental processes might result in higher risk for drug use at some stages of the lifecycle than others. Experimentation, as well as the process of addiction, most often starts in adolescence,166 a period during which the brain undergoes important developmental changes.167 Beginning in preadolescence and continuing into the mid-20s, cortical grey matter volumes reduce, which reflects a normal pruning process;168 white matter volume increases over the course of adolescence, reflecting increases in connectivity, including axonal extension and myelination.168 Drug exposure during adolescence is associated with more chronic and intensive use and greater risk of a substance use disorder than is initiation at older ages.167,169–172 Normal adolescent-specific behaviours (eg, risk-taking, novelty-seeking, and high sensitivity to peer pressure) increase the propensity to experiment with legal and illegal drugs,167 which might reflect the incomplete development of brain regions (eg, myelination of frontal lobe regions) that are involved in the processes of executive control and motivation.168 Heavy alcohol use during adolescence is associated with a range of neurobehavioural sequelae, including impairments in visuospatial processing, attention, and memory,173,174 and adolescents who had engaged in
