Management of substance abuse

The health and social effects of nonmedical cannabis use

New WHO publication on cannabis


Chapter 9. Conclusions

9.1 What do we know?

In summary there is less knowledge about the health and social effects of nonmedical cannabis use than about the use of alcohol and tobacco. On the basis of the current review by experts, the following conclusions of the known and unknown effects can be made.

9.1.1 What do we know about the neurobiology of cannabis use?

We know the following:

·       CB1 receptors (which respond to THC) are widely distributed in the brain, including areas that control attention, decision-making, motivation and memory.

·       These receptors modulate the effects of a variety of other neurotransmitter systems.

·       Short-term and long-term cannabis use down-regulates these receptors in ways that may explain the short-term and long-term effects of cannabis on working memory, planning and decision-making, response speed, accuracy and latency motivation, motor coordination, mood and cognition.

9.1.2 What do we know about the epidemiology of cannabis use and cannabis dependence?

We know the following:

·       Cannabis is the most widely used illicit drug globally. In 2013, an estimated 181.8 million people aged 15-64 years used cannabis for nonmedical purposes globally (uncertainty estimates 128.5–232.1 million).

·       Cannabis use appears to be more common in developed countries than in developing countries, although we lack good data on prevalence of use in the latter.

·       Young people often use cannabis, with the mid-teens being the age of first use in many developed countries.

·       There has been an upward trend in the mean THC content of all confiscated cannabis preparations in the USA and some European countries.

·       Cannabis dependence exists and is a cluster of behavioural, cognitive and physiological phenomena that develop after repeated cannabis use. There are some indications that the prevalence of cannabis dependence increased worldwide between 2001 and 2010.

·       There is a major demand for addiction treatment systems for cannabis-use disorders in many high-income countries and in some low- and middle-income ones.

9.1.3 What do we know about the short-term effects of cannabis use?

We know the following:

The most obvious short-term health effect of cannabis is intoxication marked by disturbances in the level of consciousness, cognition, perception, affect or behaviour, and other psychophysiological functions and responses.

·       A minority of first-time cannabis users become very anxious, have panic attacks, experience hallucinations and vomit. These symptoms may be sufficiently distressing to prompt affected users to seek medical care.

·       Acute use impairs driving and contributes to an increased risk of traffic injuries.

·       There is some evidence that cannabis use can trigger coronary events. Recent case reports and case series suggest that cannabis smoking may increase CVD risk in younger cannabis smokers who are otherwise at relatively low risk.

9.1.4 What do we know about the long-term effects of regular cannabis use?

We know the following:

·       Regular cannabis users can develop dependence on the drug. The risk may be around 1 in 10 among those who ever use cannabis, 1 in 6 among adolescent users, and 1 in 3 among daily users.

·       Withdrawal syndrome is well documented in cannabis dependence.

·       Growing evidence reveals that regular, heavy cannabis use during adolescence is associated with more severe and persistent negative outcomes than use during adulthood.

·       In a number of prospective studies there is a consistent dose-response relationship between cannabis use in adolescence and the risk of developing psychotic symptoms or schizophrenia.

·       The association between cannabis use and psychosis or schizophrenia has been recognized for over two decades in at least four ways:

1.     Cannabis produces a full range of transient schizophrenia-like positive, negative and cognitive symptoms in some healthy individuals.

2.     In those harbouring a psychotic disorder, cannabis may exacerbate symptoms, trigger relapse and have negative consequences on the course of the illness (Manrique-Garcia et al., 2014).

3.     With heavy cannabis use, susceptible individuals in the general population develop a psychotic illness which is associated with age of onset of use, strength of THC in the cannabis, frequency of use and duration of use.

4.     Cannabis use is associated with lowering the age of onset of schizophrenia. It is likely that cannabis exposure is a "component cause" that interacts with other factors to precipitate schizophrenia or a psychotic disorder, but is neither necessary nor sufficient to do so alone. Symptoms of schizophrenia increase with cannabis use and strength. The magnitude of the symptoms is associated with the amount used and the frequency of use. Daily use in adolescence and young adulthood is associated with a variety of negative health and psychological outcomes. These include:

-      early school-leaving

-      cognitive impairment

-      increased risk of using other illicit drugs

-      increased risk of depressive symptoms

-      increased rates of suicidal ideation and behaviour.

It remains to be determined which of these associations are causal.

·       Long-term cannabis smoking produces symptoms of chronic and acute bronchitis, as well as microscopic injury to bronchial lining cells, but it does not appear to produce COPD.

·       Long-term heavy cannabis smoking can potentially trigger myocardial infarctions and strokes in young cannabis users.

·       Smoking a mix of cannabis and tobacco may increase the risk of cancer and other respiratory diseases but it has been difficult to decide whether cannabis smokers have a higher risk, over and above that of tobacco smokers.

·       There is suggestive evidence that testicular cancer is linked to cannabis smoking and this potential link should be investigated further.

9.1.5 What do we know about prevention and treatment?

We know the following:

·       Evidence-based preventive interventions should cover the whole prevention chain from universal and selective to indicated prevention.

·       Comprehensive family prevention that involves training for parents, children and the family collectively is found to be effective in reducing both lifetime cannabis use and past-year use in adolescents.

·       Life skills programmes that combine both a social competence curriculum and social influence approaches are shown to reduce cannabis use at 12-month follow-up and beyond.

·       Interactive social programmes targeting vulnerable young people is found to be effective in reducing past-month cannabis use.

·       A single-session brief psychological intervention of 30-45 minutes increases the chances of cannabis cessation if people are not dependent on cannabis.

·       Many people with cannabis-use disorders cease cannabis use without treatment.

·       For people who are dependent on cannabis, family interventions are effective for adolescents, and CBT, MET and PPS are effective in adults.

9.2 Priority areas for future research

One of the objectives of the expert meeting held in 2015 was to identify areas for future research to enable us to learn more about both the association with and causality of cannabis use and health and social consequences. The areas identified by the experts were as follows 9.2.1-9.2.6).

9.2.1 Substance content and prevalence

We need to know more about:

·       the THC content of cannabis products used by most cannabis users in different countries;

·       the typical dose of THC received by regular cannabis users, and whether users titrate their dose of THC when using more potent cannabis products;

·       whether increased rates of treatment-seeking are influenced by higher THC content in cannabis, whether cannabis products with higher THC content affect the adverse health effects of cannabis use, and whether increased THC content has been accompanied by a reduction in the CBD content of cannabis products;

·       the prevalence of use in many low- and middle-income countries;

·       the extent to which household and school surveys reach all cannabis users;

·       global data on the prevalence of harmful patterns of cannabis use;

·       the prevalence of changing routes of cannabis administration (e.g. use of vaporisers and edible cannabis products);

·       the global prevalence of heavy cannabis use and cannabis-use disorders.

9.2.2 Neurobiology of cannabis use

We need to know more about:

·       the extent to which neurobiological changes and especially cognitive impairments are reversible in heavy cannabis users;

·       the duration of acute impairments produced by cannabis (the length of time after using cannabis that psychomotor and cognitive performance are impaired);

·       the possible results of longitudinal studies combining epidemiological and neuroimaging methods to study the effects of cannabis use on brain functioning;

·       the possible replicability of neuroimaging studies of cannabis users by using standardized imaging methods, better statistical analyses and larger samples;

·       whether genetics explain the observation that persons who score higher on sensation-seeking, aggression and antisocial behaviour have increased risks of cannabis-use disorder?

9.2.3 Health consequences

We need to know more about:

·       case-control studies on the effects of cannabis use on motor vehicle accidents, and the relationship between cannabis use and other types of injury;

·       how tolerance to cannabis in regular users affects the ability to drive;

·       the triggering effects of cannabis on coronary heart events, especially myocardial infarction;

·       the effects of cannabis use during pregnancy or conception through investigations using better methods of assessing cannabis use;

·       the effects of regular long-term cannabis use on various cancer risks, specifically

-      upper aerodigestive tract cancers, while taking into account the effects of concomitant alcohol and tobacco use,

-      respiratory cancers that better control for the effects of tobacco smoking,

-      head and neck cancers that stratify for HPV status;

·       in countries with a high prevalence of cannabis use, the link between cannabis smoking and CVD in young adults, specifically

-      cardiac syndromes and infarctions,

-      strokes and cerebral ischaemic events;

·       the potentially causal effects of long-term cannabis use on the risks of mental disorders, specifically

-      psychoses and particularly schizophrenia

-      major depression and bipolar disorders

-      anxiety disorders;

·       the effects of acute and regular cannabis use on suicide ideation, suicide attempts and death by suicide, while examining dose-response relations and controlling for other drug use.

9.2.4 Social costs

We need to know more about:

·       epidemiological estimates of the social and economic costs of cannabis use.

9.2.5 Prevention

We need to know more about:

·       the effect of preventive programmes for children of cannabis-affected families (as a result of more longitudinal research);

·       how best to scale up prevention, targeting persons of different age groups and in different settings;

·       what works in indicated prevention.

9.2.6 Treatment

We need to know more about:

·       the effectiveness and cost-effectiveness of screening and brief interventions for hazardous and harmful cannabis use, including in educational settings;

·       the effectiveness and cost-effectiveness of mobile telephone and Internet-based interventions for cannabis-use disorders;

·       the effectiveness and cost-effectiveness of family interventions for cannabis-use disorders;

·       potential effective pharmacotherapy for cannabis-use disorders.