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Chapter 8 Harm reduction policies for cannabis PDF Print E-mail
Written by Administrator   
Monday, 11 October 2010 00:00

Chapter 8 Harm reduction policies for cannabis

Wayne Hall and Benedikt Fischer


This chapter reviews the limited evidence on strategies for reducing the harms arising from
cannabis use and from criminal penalties to control its use. It summarises evidence on the
harms arising from cannabis use, namely, increased risks of: car crashes among users who
drive while intoxicated; the development of cannabis dependence among regular users;
psychosis and poorer adolescent psychosocial outcomes; and increased risks of respiratory
disease from smoking. Strategies for reducing these risks to users are described, such as,
roadside drug testing to deter cannabis-intoxicated driving, and education of users about
patterns of use that increase risks of dependence, poor mental health and respiratory
problems. The chapter also briefly discusses depenalisation and decriminalisation of
cannabis use as strategies to reduce harms arising from cannabis prohibition. It concludes
with suggestions for research priorities in how to reduce harms arising from cannabis use
and the policies adopted to reduce such use.

Keywords: cannabis-impaired driving, cannabis dependence, respiratory risks, cannabis


Cannabis is the most widely used illicit drug globally, and its use has increased over the past
decade. In 2005, around 160 million adults (4 % of the global adult population) were estimated
to have used cannabis in the previous year, 10 % more than in the mid 1990s (UNODC, 2007).
In the recent World Mental Health Surveys, the lifetime use of cannabis was higher in the
United States and New Zealand than in Europe, which, in turn, reported higher rates of use
than the Middle East and Africa or Asia (Degenhardt et al., 2008). Because of their larger
populations, Asia, Africa and the Americas account for an estimated 31 %, 24 % and 24 % of
global cannabis use compared to 19 % in Europe and 2 % in Oceania (UNODC, 2007).

In the United States in 2005, 40 % of the adult population reported using cannabis at some
time in their lives and 13 % of adolescents reported use in the past year (SAMHSA, 2006).
Cannabis use in most countries begins in the mid to late teens and is most common among
people in their early 20s (Degenhardt et al., 2008). Most use is intermittent and time-limited
(Bachman et al., 1997), with about 10 % of those who ever use cannabis becoming daily
users, and another 20 % to 30 % using weekly (Hall and Pacula, 2003). Cannabis use
declines from the early and mid 20s to the early 30s, reflecting major role transitions in early
adulthood (e.g. entering tertiary education or full-time employment, marrying, and having
children) (Anthony, 2006; Bachman et al., 1997).

Cannabis use in Europe

Cannabis is the most widely used illicit drug among European adolescents and its use is so
common that it has been described as an ‘illegal everyday drug’ (Essau, 2006). In the late
1990s and early 2000s, the median rate of lifetime cannabis use among European adults
aged between 18 and 64 years was 15 %, with a range between 31 % in the Czech Republic
and 2 % in Romania (EMCDDA, 2006). Rates of lifetime use were higher among younger
adults (aged between 15 and 34 years), with a median rate of 21 %, and a range between
3 % in Romania and 45 % in Denmark (EMCDDA, 2006).

Smart and Ogborne (2000) have summarised data on illicit drug use among high school
students in 36 European countries during the mid-1990s (circa 1995). The highest prevalence
of lifetime cannabis use was found in Scotland (53 %), which was higher than the overall
prevalence in the United Kingdom (41 %), followed by the Netherlands (22 %). These rates
increased during the 1990s in those countries that have undertaken a series of surveys over
that time, namely, the Netherlands, Switzerland, and Norway (Harkin et al., 1997). These
trends mirror those in Australia, Canada and the United States (Room et al., 2008).

More recent survey data collected by EMCDDA suggests that cannabis use rates have
increased throughout Europe since then, and have recently begun to stabilise. Out of a total
population of almost 500 million, 74 million Europeans aged 16 to 64 years have had
lifetime experience with cannabis, 23 million in the past year, and 12 million in the past
month (see the General Population Survey Tables in EMCDDA, 2009). Highest rates of use
were in young adults aged 15–34 years (31 % lifetime, 13 % past year). These rates vary
between countries (ranges 3 %–50 % and 1 %–21 % respectively). Average European rates
were lower than in the United States (49 % and 21 %), Canada (58 % and 28 %) and
Australia (48 % and 20 %) in the mid 2000s.

The probable harms of cannabis use

As argued in more detail elsewhere (Hall and Pacula, 2003; Room et al., 2008), there is
reasonable evidence that cannabis use can harm some users. In this chapter we summarise
the evidence on those adverse effects most commonly attributed to cannabis use and best
supported by epidemiological evidence. We then describe strategies that could be used to
reduce these harms arising from cannabis use. We also briefly discuss alternative policy
approaches that aim to reduce harms arising from current criminal control policies towards
cannabis use.

Cannabis and motor vehicle crashes

Cannabis intoxication produces dose-related impairments in cognitive and behavioural skills
that may affect driving (Mannet al., 2008; Ramaekers et al., 2004; Solowij, 1998). Older
studies that measured inactive metabolites of tetrahydrocannabinol (THC) could not assess
whether drivers were impaired at the time of accidents (Ramaekers et al., 2004). Recent

studies measuring THC in blood suggest that cannabis-affected drivers are at a higher risk of
being involved in crashes (e.g. Drummer et al., 2004; Gerberich et al., 2003; Mura et al.,
2003). Cannabis use appears to increase the risk of motor vehicle crashes by two to three
times (Ramaekers et al., 2004) compared with 6 to 15 times for alcohol. It has been
estimated that cannabis-affected driving accounted for 2.5 % of fatal accidents in France,
compared to 29 % for alcohol (Laumon et al., 2005).

Cannabis dependence

A cannabis dependence syndrome develops in some daily or near-daily users of cannabis
(Budney, 2006; Roffman and Stephens, 2006). Cannabis dependence is characterised by
marked distress resulting from impaired control over cannabis use and difficulty in ceasing
use despite harms caused by it. After tobacco and alcohol, cannabis was the most common
form of drug dependence in the US in the 1990s and early 2000s (Anthony, 2006) and in
Australia in the late 1990s (Hall et al., 1999). The risk of developing cannabis dependence in
the United States is similar to that for alcohol but lower than that for nicotine and the opioids
(Anthony et al., 1994). Around 10 % of those who ever use cannabis meet criteria for
dependence (Anthony, 2006). This rises to 16 % in persons who initiate in early adolescence
(Anthony, 2006).

Over the past two decades, increasing numbers of people have sought professional help for
their cannabis use in the United States, Europe and Australia (Hall and Pacula, 2003). In
Europe in 2006 there were 390,000 requests for treatment for cannabis dependence
(EMCDDA, 2008). This was 21 % of all cases requesting assistance for illicit drugs and
second only to opioids (EMCDDA, 2008). Some of this increase may be explained by
increased diversion of cannabis users apprehended by the police into treatment programmes,
but not all, because increases have also occurred in the Netherlands where cannabis
possession has been decriminalised de facto (Dutch National Alcohol and Drug Information
System, 2004).

Cannabis and schizophrenia

A 15-year prospective study of 50 465 Swedish conscripts (Andréasson et al., 1987) found
that the risk of schizophrenia increased with the number of times cannabis had been used by
age 18. A 27-year follow-up of the same cohort (Zammit et al., 2002) also found a doseresponse
relationship between frequency of cannabis use at baseline and risk of
schizophrenia during the follow-up. These relationships persisted after controlling for other
drug use and other confounding factors. These findings have been supported by longitudinal
studies in the Netherlands (van Os et al., 2002) and Germany (Henquet et al., 2004) and by
two small New Zealand cohort studies (Arseneault et al., 2002; Fergusson et al., 2003). The
most plausible explanation appears to be that regular cannabis use acts with a variety of
other unknown risk factors to precipitate psychoses in vulnerable individuals (Degenhardt
and Hall, 2006; Moore et al., 2007).

The respiratory risks of cannabis smoking

Regular smokers of cannabis who do not smoke tobacco have more symptoms of chronic
bronchitis and poorer lung function than people who do not smoke either cannabis or
tobacco (see Tashkin, 1999). People who smoke cannabis with or without tobacco also seem
to be more susceptible to respiratory infections (Tashkin, 1999).

Cannabis smoke is carcinogenic (Marselos and Karamanakos, 1999), making cannabis
smoking a potential cause of cancers of the lung and mouth, tongue, and oesophagus (Hall
and MacPhee, 2002). Epidemiological studies of head and neck cancer have produced
conflicting results: one case control study found an association (Zhang et al., 1999) but a
longitudinal study (Sidney et al., 1997) and two other case control studies failed to do so
(Llewellyn et al., 2004; Rosenblatt et al., 2004). Case control studies of cannabis smoking and
lung cancer have found associations but they have not been able to separate the effects of
cannabis from tobacco smoking because most cannabis users in these studies were also daily
cigarette smokers (Mehra et al., 2006).

Potential harm reduction strategies for cannabis

The following sections outline some potential harm reduction strategies for cannabis. Some
are based on adaptations of similar policies that have been used to reduce harm from other
drugs, such as alcohol. In other cases we outline the type of advice that could be given to
users to avoid patterns or practices of use that increase the risk of experiencing adverse
health outcomes (Swift et al., 2000). With few exceptions, there is little evidence on their
effectiveness. Research into the effectiveness of these proposals should be a priority for harm
reduction policies for cannabis.

Motor vehicle accidents

It is obvious that cannabis users can avoid cannabis-related vehicle crashes by not driving
while intoxicated, but it is uncertain whether cannabis users have responded to education
campaigns that urge them not to drive after using. Australia, Norway and Sweden have
adopted random roadside drug testing in an effort to discourage cannabis-impaired driving.
In Australia, the Victorian state government introduced random roadside saliva testing for
cannabis and other drugs in December 2004; other Australian states and territories have
since followed (Butler, 2007). Australian legislators have assumed that this policy will
substantially reduce cannabis-related road crashes in the same way that random breath
testing reduced alcohol related crashes in Australia (Henstridge et al., 1997). Other European
countries have adopted the more focused policy of testing for cannabis in saliva or urine on
suspicion of use or evidence of impaired driving (Mann et al., 2008).

The illegality of cannabis use has prompted the adoption of a ‘zero tolerance’ approach in
Australia, Norway and Sweden, with the presence of any detectable amount of THC defined
as an offence (Butler, 2007). Any road safety benefits of this policy are a by-product of the
deterrent effect of enforcing prohibitive drug laws. Proponents of drug testing argue that it

will save lives (Jones et al., 2008) but so far there is no evidence that it has done so. This
policy needs to be properly evaluated to see if it reduces cannabis-impaired driving at an
acceptable social and economic cost (Hall and Homel, 2007). Other approaches that focus
on harm reduction would include: developing measures of cannabis-impaired driving, as
advocated by Grotenhermen et al. (2007), and encouraging cannabis users to adopt
‘designated driver’ programmes like those advocated for alcohol users.

Cannabis dependence

An essential first step in reducing the risk of cannabis dependence is informing users of the
risk. This can be done by explaining that the risk increases with regular use and is greatest
when cannabis is used daily for weeks or months, as is true for alcohol and tobacco
dependence. Priorities for research include assessing whether users will accept this advice or
what the most persuasive way of delivering it would be.

Screening and brief advice for excessive alcohol consumption in general practice, hospital or
even non-medical settings reduces consumption and the problems caused by alcohol (e.g.
Shand et al., 2003). The same approach could be adopted for cannabis use disorders in
primary care settings, for example among young adults with respiratory problems or
symptoms of anxiety and depression, all of which are common among cannabis-dependent
individuals who seek help from family physicians (Degenhardt et al., 2001).

Similarly, brief interventions for frequent cannabis users could be targeted at populations and/or
settings where cannabis use is known to be high, for example youth mental health services,
juvenile justice centres, and among college students (Hall et al., 2008a). Such interventions could
advise users to reduce the frequency of cannabis use and not to use it before driving. A ‘checkup’
approach modelled on the Brief Drinker Check-up (Miller and Sovereign, 1989) provides a
promising model for raising the issue of health risks of cannabis use in a non-confrontational
way (see Berghuis et al., 2006). This approach has been trialled and evaluated with promising
results in a number of studies (Martin and Copeland, 2008; Stephens et al., 2007).

The question of how best to inform young people about the risks of cannabis dependence
requires research on young people’s views about the type of information that they find most
persuasive. In the interim the following are suggestions about what advice could be given:

• Cannabis users can become dependent on cannabis. The risk (around 10 %) is lower than that
for alcohol, nicotine and opiates, but the earlier that a young person begins the higher the risk.
• Using cannabis more than weekly increases the risks of developing dependence and other
health problems.
• Regular use probably also increases the risk of psychosis in young people who have a
family member with a psychosis or other mental disorder, or who have unusual
psychological experiences after using cannabis.
• Driving within a few hours of cannabis use increases the risk of both fatal and non-fatal
motor vehicle accident involvement and should be avoided, especially after drinking

Cognitive behavioural therapy can be used to treat cannabis dependence on an outpatient
basis. Rates of abstinence have been modest — for example, around 15 % reported continuous
abstinence at six-month follow-ups, according to Copeland et al. (2001) — but cannabis use
and cannabis-related problems are substantially reduced (Denis et al., 2006; Roffman and
Stephens, 2006). A recent review by Nordstrom and Levin (2007) concluded that while a
number of psychotherapies have been found to be effective in treating this disorder, none has
been found to be more effective than any other, although offering vouchers as a reward to
reinforce negative urine toxicology screens improved abstinence during treatment.

Informing young people about the mental health risks of cannabis use

A major public health challenge will be finding effective ways of explaining the mental health
risks of cannabis use to young people. In addition to a possible increased risk of psychosis,
young people also need to be informed about the risks of developing dependence, impairing
their educational attainment, and possibly increasing their risk of depression (Hall, 2006;
Patton et al., 2002). These risks add weight to the prudential argument for discouraging
cannabis use by young people.

Policymakers need to be realistic about the impacts of educational messages (Caulkins et al.,
2004; White and Pitts, 1998). Small, statistically significant reductions in cannabis use may
be observed in well-conducted programmes (Caulkins et al., 2004; Gorman, 1995; Tobler,
Lessard, Marshall et al., 1999; White and Pitts, 1998) but the primary impact is on
knowledge rather than behaviour (White and Pitts, 1998). Any behaviour change is more
likely to occur among less frequent rather than heavier users (Gorman, 1995). Given this, the
nature and delivery of the advice may need to differ for different groups facing different
levels of risk (Toumbourou et al., 2004). The best way to deliver the advice will depend upon
good social marketing research on the views of young people (Grier and Bryant, 2005).

Education about the risks of cannabis use should explain the mental health risks of regular
intoxication with alcohol and cannabis; and define the high-risk groups, namely those with a
family history of psychosis and those who have had bad experiences with cannabis. Such
education needs to be directed not only at cannabis users but also at their peers to increase
recognition of these problems among young people so that they can encourage affected
peers to cease using or seek help earlier than might otherwise be the case.

A major challenge is framing the magnitude of the risk of psychosis. The risk for any
individual increases from around 7 in 1 000 (Saha et al., 2005) to 14 in 1 000, but the
consequences of psychosis for those individuals who are vulnerable are serious. The
temptation for parents and health educators is to play up the risk, arguing that everyone is at
risk because it is difficult to predict which young people are most vulnerable. This strategy is
of doubtful effectiveness and may undermine the credibility of the message by being seen to
exaggerate the risk.

It is prudent to encourage young people who use cannabis and experience psychotic
symptoms to stop, or at the very least to reduce the frequency of their cannabis use. The

challenge in implementing this goal is finding effective ways of persuading persons with
schizophrenia to stop doing something that they enjoy and to help those who want to stop
but find it difficult to do so. Many persons with schizophrenia have characteristics that predict
a poor outcome from psychological interventions for cannabis dependence, namely, they
lack social support, may be cognitively impaired, are often unemployed, and do not comply
with treatment (Kavanagh, 1995; Mueser et al., 1992). There are very few controlled outcome
studies of substance abuse treatment in schizophrenia (Lehman et al., 1993). A recent
Cochrane review identified only six relevant studies, four of which were small (Jeffery et al.,
2004) and found no clear evidence that supported substance abuse treatment in
schizophrenia over standard care.

Reducing respiratory risks

The respiratory risks of cannabis smoking could be eliminated if cannabis users adopted
eating or ingesting rather than smoking cannabis. This is unlikely to happen, because most
long-term users find smoking a more efficient and easier way to titrate their dose of THC than
the oral route (Grotenhermen, 2004; Iversen, 2007).

Putatively ‘safer’ forms of cannabis smoking, such as water pipes, are popular among
younger cannabis users in Australia (Hall and Swift, 2000) but United States and Australian
(Gowing et al., 2000) research suggests that water pipes deliver more tar per dose of THC
than do joints. It is also unclear how much the respiratory risks of cannabis smoking might be
reduced if users were to smoke lesser amounts of the more potent cannabis products
(Melamede, 2005). There has been too little research to determine whether users can reliably
titrate their dose and, if they can, whether in fact they do so (Hall and Pacula, 2003).

It is reasonable to advise cannabis smokers to avoid breath-holding or ‘deep inhalation’
techniques to maximise the absorption of THC in the lungs. This practice increases the
quantities of tar and particulate matter that are retained in the lungs without necessarily
increasing the THC delivered. It is also advisable for cannabis users to eliminate the use of
tobacco in smoked cannabis preparations because of tobacco’s addictiveness and

Vaporisers appear to be a more promising way of reducing the carcinogens and toxicants
inhaled when cannabis is smoked (Gieringer et al., 2004; Grotenhermen, 2004; Melamede,
2005). These devices are designed to deliver inhaled THC without carcinogens and toxicants.
They do so by heating cannabis to a temperature (180oC), which releases THC without burning
the plant material. A study by Gieringer et al. (2004) found that vaporisers achieved a similar
efficacy in delivery of THC to smoking a cannabis cigarette while very substantially reducing
levels of carcinogens. Hazekamp et al. (2006) evaluated the performance of the same device in
delivering pure THC and found that it had acceptable safety properties. However, Bloor et al.
(2008) found that while vaporisers reduced levels of released ammonia, compared to smoked
cannabis these levels (170 ppm) were still well above recommended safe levels (35 ppm) for
short-term occupational exposures. These levels of ammonia increase respiratory irritation, but
the respiratory effects of long-term intermittent exposure in daily users are unknown.

Abrams et al. (2007) compared the effects of varying doses of cannabis vaporised and
smoked in a joint in 18 subjects under double blind conditions. They found that the vaporiser
delivered similar amounts of THC and produced similar psychological effects. Sixteen of the
18 subjects preferred the vaporiser. They did not test for delivery of tars and carcinogens but
did find lower CO levels in blood when using a vaporiser. Earleywine and Barnwell (2007)
found suggestive evidence that vaporisers had reduced respiratory symptoms in a
convenience sample of 6 883 cannabis users interviewed via the Internet. The rate of
respiratory symptoms (bronchitis, wheeze, breathlessness) among the 150 who reported only
using vaporisers was 40 % of that reported by cannabis smokers (after controlling for
cigarette smoking, duration of use and amount typically used). The reduction in symptoms
among vaporiser users appeared to be larger in heavier cannabis users. More work is
needed to evaluate the long-term safety and efficacy of vaporisers in reducing the
respiratory risks of cannabis use.

Reducing the harms arising from cannabis control policies

Under current criminal cannabis control policies in many European and other developed
countries, cannabis users can nominally be sentenced to prison if caught in possession of
cannabis. Even if prison sentences are rarely imposed, the acquisition of a criminal conviction
or record for the personal use of cannabis can adversely affect the lives of otherwise lawabiding
users (Lenton, 2000) in ways that some have argued are more serious than any
harms that result from using cannabis (Wodak et al., 2002), for example, by impeding
professional or travel opportunities and adversely affecting personal relationships (Room et
al., 2008). The limited research (Erickson, 1980; Lenton et al., 1999a; Lenton et al., 1999b)
suggests: that many persons convicted of cannabis offences have no other criminal records;
that a criminal conviction adversely affects their employment prospects and their reputations;
and that it has a negligible effect on their cannabis use.

The enforcement of cannabis control laws is also often applied in a highly selective, if not
discriminatory, way. In Australia in the early 1990s cannabis offenders appearing before the
criminal courts were more likely to be unemployed and socially disadvantaged males than
were cannabis users in community surveys (Advisory Committee on Illicit Drugs, 1993). Recent
US studies show higher rates of arrests for cannabis offences among Hispanic and Black
minorities (Gettman, 2000; Human Rights Watch, 2000). It is uncertain to what extent the same
is true in European countries with substantial ethnic minorities or immigrant populations.

The non-enforcement or removal of criminal penalties for personal use is one way of
reducing the adverse effects of the law on users. The Netherlands was one of the first
European countries to do so in 1976 (see box ‘De facto cannabis decriminalisation in the
Netherlands’, p. 243), and Portugal has more recently done so among other European
countries (see box ‘Cannabis decriminalisation in Portugal’, p. 243). In several Australian
states, personal cannabis use is subject to a non-criminal ‘infringement’ or ‘expiation’ notice,
an offence similar to a speeding ticket and punished by a limited fine (Room et al. 2008).
Studies of the impact of these changes have typically found that reductions in the severity of
penalties for cannabis use have little, if any, impact on rates of population cannabis use in

Australia (e.g. Donnelly et al., 1999), the United States (Pacula et al., 2004) and Europe
(Greenwald, 2009; Room et al., 2008). The lack of any evidence of a large impact on rates
of use also suggests that this policy may have little or no effect on cannabis-related harms,
while at the same time reducing enforcement costs and effects on users (Room et al., 2008).

De facto cannabis decriminalisation in the Netherlands
The Netherlands decriminalised cannabis possession for personal use on a ‘de facto’ basis
from 1976. This means that while personal possession is still formally prohibited by criminal
law, the law is not enforced. The Dutch system tolerates cannabis users possessing and
buying small amounts of cannabis for personal use, most notably in several hundred ‘coffee
shops’ across the country. Also in the Netherlands, no major changes in cannabis use rates
have been observed that could be unambiguously attributed to this policy, and use rates are
lower than the EU average. This approach aims to separate the cannabis market from that of
other illicit drugs. While this de facto decriminalisation has been well-supported politically
and socially in the Netherlands, it has recently come under some pressure from neighbouring
countries concerned about ‘drug tourism’ (Chatwin, 2003; MacCoun and Reuter, 1997;
Pakes, 2004; Room et al., 2008). The Dutch government has responded to these concerns by
reducing the number of coffee shops and the amount of cannabis that can be sold.

Cannabis decriminalisation in Portugal
Portugal formally decriminalised use of all illicit drugs by changing its drug control laws in
2001. Cannabis use and possession remains illegal but it is treated as an ‘administrative
violation’. Drug use offenders are brought to the attention of ‘Dissuasion Commissions’ who
typically suspend any punitive proceedings. In serious cases, such as those of repeat offenders,
administrative penalties — like fines, suspension of driver’s licence or community service
orders — can be imposed and problematic users can be referred to treatment. Since these
reforms, no significant changes have been observed in cannabis use, which remains low
compared to other EU countries and North America. The number of drug use infractions has
been stable since the reforms, which have been well-accepted politically and by the general
public (Greenwald, 2009; Hughes and Stevens, 2007; Room et al., 2008).

An unintended consequence of depenalisation via civil penalties can be an increase in
numbers of persons fined or diverted into non-criminal interventions (e.g., education or
treatment measures) by the police, an effect referred to as ‘net widening’. This occurs because
the police find it easier and less time-consuming to enact non-criminal measures (e.g. impose
a fine) than to formally arrest and process a criminal charge. If a substantial proportion of
offenders do not pay their fines, more cannabis users may end up in prisons for fine-default
than would be the case if cannabis use remained as a criminal offence (Room et al., 2008).
The removal or the non-enforcement of any penalties for personal use (as in the Netherlands)
avoids this problem (Hall and Pacula, 2003; Room et al., 2008), as does allowing noncustodial
ways to enforce the payment of fines (Room et al., 2008).

Research priorities for cannabis harm reduction

Research is needed on the effectiveness of these policies that aim to reduce the harms of
cannabis use. Among the priorities for future inquiry are the following questions:

• What do cannabis users believe are the harms of using cannabis?
• Does the type of evidence presented about these adverse effects persuade them?
• Are they prepared to act on advice about how to reduce these harms?
• Does roadside drug testing deter cannabis users from driving while intoxicated? If so, does
this reduce motor vehicle accident fatalities? Does it do so at an acceptable social and
economic cost? Are there better ways than deterrence policies to reduce risks related to
cannabis and driving?
• Do adolescent users accept that cannabis use can be harmful? Are they prepared to act
on harm reduction advice? Are brief interventions in medical or non-medical settings
effective in changing risk patterns of use or practices?
• Does the use of vaporisers substantially reduce the respiratory risks of cannabis smoking?
• Do cannabis users titrate their doses of cannabis products?
Among priorities for research on the effects of harm reduction measures such as
depenalisation and decriminalisation are the following:
• Do depenalisation or decriminalisation policies result in changes in patterns or rates of
cannabis use, or attitudes towards cannabis use, especially among vulnerable/high-risk
populations (e.g., youth/students)?
• Will more tolerant policies for cannabis use reduce access or exposure to other illicit drugs?
• Do decriminalisation approaches result in tangible savings of public resources (e.g.,
enforcement time) without increasing the prevalence of harmful cannabis use (e.g.,
numbers seeking treatment for cannabis dependence)?


Cannabis is the most widely used illicit drug in Europe, as it is globally. While cannabis use
clearly does not result in harms that are comparable to those of alcohol or tobacco, its use is
associated with significant potential risks and harms. Based on existing evidence, a number
of these risks and harms are modifiable by harm reduction approaches directed at users. This
more pragmatic, ‘public health’ approach that builds on experiences from the alcohol field
requires substantial additional research and policy engagement. Its utility is still hindered by
the century-old illegal status of cannabis in most European jurisdictions.
Driving under the influence of cannabis has been given considerable attention in recent years.
Governments in Australia, Norway and Sweden have implemented random roadside saliva
testing to detect the presence of cannabis in drivers to reduce cannabis-impaired driving and
prevent accidents as a primary harm. However, the scope of this policy might be overly punitive
in penalising drivers who are not actually impaired by cannabis while driving. Thus, the
effectiveness, cost-effectiveness and social effects of this policy remains to be evaluated.

Given the existing knowledge around the acute and long-term harms associated with
cannabis use, and key predictors of these effects, there appears to be considerable room for
interventions with or advice to cannabis users towards reducing the odds or severity of
problems resulting from use. For example, harm reduction advice that could be given to
current cannabis users includes the following:

• Avoid more than weekly use to minimise the risks of developing mental health problems or
• Avoid smoking as a route of administration or use a vaporiser instead, rather than smoke
a bong or joint.
• If you smoke cannabis, avoid deep inhalation or breath-holding practices in order to
reduce the risks of respiratory problems.
• Do not drive or use machinery when intoxicated.

There is a need for research on how to effectively convey such messages, and to
measure their potential impacts on individual and/or population levels of harm from
cannabis use.

There is reasonable evidence that removing criminal penalties for personal possession
and use of cannabis reduces some of the harms of current control policy incurred by
users who come to the attention of criminal control. This policy can reduce the extensive
social and economic harms of use prohibition (rather than the effects of cannabis use)
without producing large increases in the prevalence of cannabis use, as recent policy
reform experiments in a number of countries have suggested. Such efforts would also
help to bring cannabis use more into a policy framework of public health rather than
repressive control. They may facilitate steps towards a more integrated and rational
regulation of all commonly used psychoactive substances guided by their potential to
cause harm and evidence on the benefits and costs of different interventions (Nutt et al.

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