The routes into harmful use of Cannabis or other drugs are multiple, and this is not dissimilar from other problems like Conduct problems, Depression or Psychosis. This list is by no means intended to be exhaustive, but may act as a helpful checklist if considering the complexity of substance use disorder in a client.

Biological

• time-critical changes are occurring in the adolescent brain, meaning that the dopamine "reward system" and the hormone oxytocin (amongst other mechanisms, some less well understtod) interact with neural development so that this is a sensitive period of brain development, where new experiences are powerfully encoded and patterned behaviours become more easily ingrained than at other times in the life cycle.
• The relative underperformance of the frontal lobes (if the brain were a company, the Frontal lobe would be the "board of executives" in relation to the rest of the brain) in adolescence means that the young person is liable to be more impulsive, and also more prone to exploratory risk-taking behaviour as part of a biological imperative to explore the wider world.
• It is known that in genetically prone/vulnerable individuals Cannabis can trigger schizophrenia or other psychotic illnesses, or induce these to arise earlier than would have been the case - and the earlier the onset of psychosis, the worse the outlook.

Cognitive - individual

• Early positive appraisals of an experience ("this is nice!") make it more likely that this experience will be repeated, and that such patterned responses will become harmful to the individual.
• Early trauma (a need to blank out painful memories, etc) is a frequent finding in young substance users.

Systemic factors:

• Children of families where drug use is common and normalised, or where one or both parents have significant Substance Use Disorder, are at far greater risk.
• Peer group influences are increasingly significant in the teenage years.
• Social-ecology - the pervasiveness of drugs in the local community/society; the availability of dealers, money to afford drugs, etc. are major factors.