Reading Around… Cannabis

First published: 03 August 2016 | Last updated: 26 November 2020

Prevalence of use

Cannabis is the most popular illicit drug in the UK: Finding from the 2013/14 Crime Survey for England and Wales demonstrate that cannabis is the most commonly used drug in the UK with 6.6% of adults (aged between 16 and 59) using it in the previous year.

Key findings from the 2013/14 Crime Survey for England and Wales:

  • Falling long-term trends in cannabis use: Cannabis use dropped by approximately 3% between 2002/2003 and 2009/ 2010 in 16-59 year olds.
  • Use has remained stable (at 6-7%) since 2009/ 2010.
  • Use is higher in young people (between 16-24 years old): Approximately 15% using cannabis in the last year.
  • Cannabis is also the most prevalent drug globally with 3.5% using the drug (Gowing, 2015).


The Adult Psychiatric Morbidity Survey (2007) suggests that approximately 3% of the population report dependence upon cannabis – the highest out of all substances.

Harms (Hall, 2015)

  • Driving whilst under the influence of cannabis doubles the risk of being involved in a car crash.
  • Increased risk of low birth weight in babies if cannabis was used during pregnancy.
  • Cardiovascular problems.
  • Acute adverse effect such as anxiety, panic and psychotic symptoms (see section on mental health).
  • Poor psychosocial and mental health outcomes.

Department of Health: A summary of health harms in drugs (2011)

Key findings from the report:

  • No cases of fatal overdose reported from cannabis use.
  • Cannabis dependence syndrome is widely documented.
  • Associated with chronic respiratory diseases.

Harms are mediated by:

  •  Route of administration (oral vs smoking).
  • Concurrent tobacco use.
  • Availability.
  • Individual variability such as genetic factors (e.g. family history of schizophrenia increases adverse mental health outcomes).
  • Evidence of adverse mental health outcomes (see below).

Harms in young people  (Silins et al. 2014)

  • Earlier use associated with adverse outcomes: Data from three large, long-running longitudinal studies from Australia and New Zealand shows a dose-response relations between the frequency of adolescent cannabis use and all adverse young adult outcomes.
  • Those who were daily users before the age of 17 years old were less likely to complete school, get a university degree and more likely to develop dependence, use other illicit drugs and attempt suicide.

 Cannabis and mental health

  • Frequent and heavy cannabis use has been linked with affective and psychotic illnesses. There has been considerable debate over the causal pathways between cannabis use and psychotic illnesses, in particular whether: (1) Cannabis use can cause schizophrenia that would not have occurred in its absence; (2) People with psychotic illnesses are more likely to use cannabis; (3) Cannabis-use precipitates psychosis in vulnerable people or (4) Whether cannabis-use exacerbates psychotic symptoms. A selection of key papers exploring this topic are outlined below:
  • A systematic review in the Lancet (Moore et al. 2007) of cannabis-use and risk of psychotic outcomes reported that there was an increased risk of any psychotic outcome in individuals who had ever used cannabis. There was also more risk in those who used cannabis more frequently. Whilst this paper demonstrates the association between cannabis and psychotic illnesses, due to the methodology, the paper could not determine the direction of the causal pathway.
  • Genetic predisposition to schizophrenia and cannabis use: A study by Power et al. (2014) demonstrated that those with a genetic predisposition to schizophrenia are more likely to use cannabis. This supports the hypothesis that people with psychotic illnesses are more likely to use cannabis (see (2) above), however, this is not to suggest that this is the only causal pathway as it only accounts for a small proportion of the variance in cannabis use.
  • Cannabis use and earlier onset psychosis: A systematic review and meta-analysis (Large et al. 2011) reported strong evidence for the use of cannabis and earlier onset of psychotic illnesses, supporting the hypothesis that cannabis is a causal factor in psychotic illnesses and supports the view that cannabis-use precipitates psychosis in vulnerable people (see (3) above).

A review by Hall et al. (2009) explores the various causal pathways and concludes that:

  • There is reasonable evidence that cannabis use exacerbates the symptoms of schizophrenia (see (4) above).
  • Also consistent evidence that cannabis-use precipitates psychosis in vulnerable people (see (3) above).
  • The evidence whether cannabis use can cause schizophrenia that would not have occurred in its absence is the most contentious and firm conclusions are difficult to draw.

Cannabis use and depression

  • Evidence is less clear: Findings for depression, suicidal thoughts, and anxiety outcomes in the systematic review from the Lancet (Moore et al. 2007) were not consistent and conclusions could be drawn.
  • Another systematic review and meta-analysis on the association between cannabis use and depression reported that cannabis-use and in particular heavy use may be associated with increased risk of depressive disorders but that more research is needed (Lev-Ran et al. 2014).


  • Public Health England (PHE): Drug treatment in England 2013/2014
  • There are no medications suitable for the treatment of cannabis misuse.
  • Recommends psychosocial interventions:
    • Brief motivational interventions for mild users.
    • Structured key-working for dependent users.
    •  Cognitive behavioural therapy (CBT) for users with comorbid depression and anxiety.
  • See also: The National Institute for Health and Care Excellence (NICE)  guidance: Drug misuse psychosocial interventions.



  • Under the Misuse of Drugs Act 1971 cannabis is classified as a Class B drug.
  • There is some debate over the classification of cannabis in the UK: Between 1971 and 2004 cannabis was classified as a Class B drug. Between 2004 and 2008 it was reclassified as a Class C.
  • From 2008 – present it was reclassified again as a Class B drug in light of evidence for it’s association with schizophrenia.


United States (US)

  • Recreational cannabis-use is legal in four states in the US (Washington, Colorado, Oregon and Alaska), allowing users to grow up two a specified number of plants at home for personal use. It is still illegal to purchase cannabis on the black market, but it is likely that regulation around the selling of cannabis will change.
  • Medical marijuana use is legal in 25 states.


  • On 10th December 2013, Uruguay became the first country to legalize marijuana. Citizens are permitted to grow their own cannabis plants and the Government is on the process of setting up it’s own dispensing network.


Further reading

  • See key paper by Hall et al (2014) ‘What has research over the past two decades revealed about the adverse health effects of recreational cannabis use?’


The opinions expressed in this commentary reflect the views of the author(s) and do not necessarily represent the opinions or official positions of the Society for the Study of Addiction.