Addiction (2000) 95(4), 485–490

EDITORIAL

Cannabis use and public health:assessing the burden

Cannabis plant derivatives have been recognizedfor centuries as a relatively safe way to achieve lesparadis arti� ciels but troubling evidence hasemerged from time to time suggesting that theeffects of cannabis use may not be benign for allindividuals and populations. Increasing preva-lence of regular cannabis use and new epidemio-logical research on its health hazards areprompting us to see cannabis in a new light atthe end of the 20th century.

We believe that more attention should be paidto the public health impact of cannabis use,especially by young adults in many contempor-ary developed societies such as Australia, Britain,Canada, the European Union and the UnitedStates (Hall, 1995). Our concern is based on acombination of scienti� c evidence about thelong-term effects of cannabis use, and the grow-ing prevalence of regular marihuana usethroughout the world. According to the WorldDrug Report (UNIDCP, 1997) illicit drug usehas become a global phenomenon, with cannabisthe most widely used illicit drug (Hall, Johnston& Donnolly, 1999). In many societies, cannabisis now smoked by adolescents and young adultsfor its intoxicating effects which, like alcohol,include cognitive and psychomotor impairment(Smiley, 1999). It is likely that cannabis smokingshares at least some of the adverse health effectsof alcohol and tobacco (Hall & Solowij, 1998),both of which have a substantial impact onpublic health (English et al., 1995).

The addiction � eld has paid much less atten-tion to cannabis than the prevalence of its usemight suggest is warranted. One reason has beenthat under current social policies most cannabisuse is intermittent and the majority of usersdiscontinue their use in their mid- to late twen-ties (Chen & Kandel, 1995; Bachman et al.,

1997). Only a small proportion of users adoptthe pattern of use that poses the greatest risk ofadverse health effects, namely, weekly or morefrequent use over months or years (Hall et al.,1999). American research on cohorts of adoles-cents who initiated cannabis use in the mid-1970s suggests that around one in 10 cannabissmokers used on a daily basis beyond their mid-to late twenties (see Hall et al., 1999). However,given the prevalence of use, this small proportioncan affect a substantial number of people: 4.2%of US adults at some time in their lives,according to the National Comorbidity Survey(Anthony, Warner & Kessler, 1994).

A second reason for the neglect of cannabisuse has been that the adverse effects of heavycannabis use do not appear to be as serious asthose of heavy opioid and stimulant use. Manymore opioid and stimulant users seek treatmentfor problems caused by their drug use than docannabis users. The dependence symptoms andhealth problems experienced by opioid andstimulant users are also more severe and serious(e.g. overdose deaths and infectious diseases)than the respiratory, psychological and interper-sonal problems often reported by dependentcannabis users (e.g. Stephens & Roffman, 1993).Thirdly, addiction researchers have also reactedagainst well-motivated but exaggerated claimsabout the seriousness of the health risks ofcannabis use.

It would be unwise to assume that the appar-ently benign consequences of current patterns ofcannabis use are eternal verities. The prevalenceof life-time cannabis use by young adults hasincreased in many developed countries over thepast several decades (Hall et al., 1999). Theproportion of young adults who use cannabisregularly has also increased and the age of in-

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itiation has declined in a number of countries(Hall et al., 1999). The average D -9-tetrahydro-cannabinol (THC) content has probably alsoincreased, although by how much remains indispute in the absence of good purity data (Hall& Swift, 1999). More risky patterns of smokinghave also developed in some countries, such asAustralia, with increased popularity of water-pipes or bongs which permit the delivery of largebolus doses of THC (Hall & Swift, 1999). Thereare reports of similar practices in Britain.

The rising prevalence of cannabis use byyoung adults has probably contributed to agreater social tolerance of cannabis use in somecountries, which have reduced the penalties forpossession and use (e.g. Australia, Italy, Spain).The ease of cultivating the Cannabis sativa planthas ensured that a substantial black market in-dustry has developed to cultivate, distributeand sell cannabis products in Australia, theNetherlands and the USA (UNIDCP, 1997).

If these trends continue over the next fewdecades, then cannabis will become the thirdmost widely used recreational drug after alcoholand tobacco, as it now is in the United Statesand Australia (UNIDCP, 1997). Indeed, givendeclines in the prevalence and social toleranceof tobacco use, the prevalence of regularcannabis smoking in Australia and the UnitedStates may rival that of daily tobacco use. Ourexperience with alcohol (Edwards et al., 1994)suggests that it would be remarkable if increasedavailability, lower prices and greater social toler-ance of cannabis use did not increase the pro-portion of the population who use regularly. Itmay also increase the duration of regularcannabis use beyond the mid-20s (Bachmanet al., 1997). If the use of cannabis were legal-ized, then its sale would eventually have tobe permitted to meet demand for the product.The promotion of use that would accompany alegal market would very probably increase theprevalence of regular cannabis use (MacCoun &Reuter, 1997).

On the available epidemiological evidence,with all its imperfections and uncertainties (Hall,1999), there are a number of probable adversehealth effects related to both acute intoxicationand chronic ingestion that could potentially pro-duce a substantial public health burden if theprevalence of cannabis use increased (Hall,1995). The nature of these risks needs to bebetter understood.

Acute health effectsMotor vehicle accidentsCognitive and psychomotor impairment are hall-marks of cannabis intoxication (Chait & Pierri,1992; Beardsley & Kelly, 1999). While cannabisimpairs performance in laboratory and simulateddriving settings, studies of on-road driving per-formance have found at most modest impair-ments (e.g. Robbe, 1994; Chesher, 1995),perhaps because of restrictions on the dosesstudied.

Controlled epidemiological studies have notestablished that cannabis-only users are at in-creased risk of motor vehicle accidents (Smiley,1999). Uncertainty about the role of cannabis inmotor vehicle accidents will be dif� cult to re-solve because case–control studies are dif� cult toconduct. Blood levels of cannabinoids do notindicate whether a driver or pedestrian was in-toxicated with cannabis at the time of an acci-dent, and many drivers with cannabinoids intheir blood were also intoxicated with alcohol atthe time of the accident (Chesher, 1995).

Given the high rates of cannabis use amongyoung adults who are at highest risk of beinginvolved in motor vehicle accidents, accidentalinjury and death is a potential public healthissue. At present, cannabis seems to make asmall contribution in its own right to motor ve-hicle accidents (Gieringer, 1988). Its major pub-lic health signi� cance for road safety may be inexacerbating the adverse effects of alcohol on theperformance of those drivers who combine al-cohol and cannabis intoxication. This is the ma-jority of cannabis users who are involved in fatalaccidents (Hall, Solowij & Lemon, 1994), and itmay be the majority of cannabis users in thecommunity (Norton & Colliver, 1988).

Chronic health effectsDependencePeople who use cannabis daily over months oryears may experience dif� culty in abstainingfrom or controlling their cannabis use (Anthony& Helzer, 1991; Stephens & Roffman, 1993).Daily cannabis users develop tolerance to itssubjective and cardiovascular effects, and mayexperience withdrawal symptoms on the abruptcessation of cannabis use (Compton, Dewey &Martin, 1990; Wiesbeck et al., 1996). Some seekhelp because they experience dif� culty in con-trolling their cannabis use, and continue to usecannabis despite experiencing adverse personal

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consequences of use (Kandel & Davies, 1992;Stephens & Roffman, 1993; Swift et al., 1998a;Swift, Hall & Copeland, 1998b).

According to the Epidemiologic CatchmentArea Study, 3% of the adult US population metdiagnostic criteria for cannabis abuse or depen-dence (Anthony & Helzer, 1991). Similar esti-mates have been reported in the NationalComorbidity Survey (Kessler et al., 1994) and anAustralian extension of the NCS (Hall et al.,1999). The prevalence of cannabis dependencein the general population has not been re� ectedin the number of people seeking treatment,which is relatively small perhaps because there isa high rate of remission of symptoms in theabsence of treatment, or because of a reluctanceon the part of cannabis users to be treated insettings designed for heroin and cocaine users.There may also be fewer adverse personal andsocial consequences of cannabis than of alcoholand opioid dependence, with which existingtreatment services are traditionally orientated todeal.

Cannabis dependence deserves attention forone reason if for no other: it prolongs cannabisuse and thereby places users at increased risk ofexperiencing other adverse health effects ofchronic use. In addition, some populations maybe at increased risk of speci� c adverse effects,such as adolescents, pregnant women andpeople with serious mental illnesses such asschizophrenia.

Respiratory diseasesBecause cannabis is usually smoked, long-termcannabis users are at increased risk of respiratorydiseases (Bloom et al., 1987; Tashkin, 1999).Chronic heavy cannabis smoking impairs thefunctioning of the large airways, and causessymptoms of chronic bronchitis, such as cough-ing, sputum and wheezing (Bloom et al., 1987;Tashkin, 1999). Given the adverse effects oftobacco smoke, which is qualitatively very similarin composition to cannabis smoke (Tashkin,1999), chronic cannabis smokers also showhistopathological changes in lung tissues thatprecede the development of lung cancer in to-bacco smokers (Tashkin, 1999). The possibilitythat cancer may be induced by chronic cannabissmoking has been raised by case reports of can-cers of the aerodigestive system in young adults

with a history of heavy cannabis use (e.g. Taylor,1988; Donald, 1991).

SchizophreniaThere is evidence that large doses of THCcan produce acute psychotic symptoms, such asconfusion, amnesia, delusions, hallucinations,anxiety, agitation and hypomania (Channabasa-vanna, Paes & Hall, 1999). There is more mixedsupport for the hypothesis that cannabis use cancause an acute psychotic disorder (a “cannabispsychosis”) that persists after the excretion ofTHC (Thornicroft, 1990; Hall, 1998).

There is reasonable evidence that cannabis usemay precipitate schizophrenia in vulnerable indi-viduals (Andreasson et al., 1987) and there iseven better evidence from prospective studiesthat continued cannabis use worsens the prog-nosis of people with schizophrenia (Hall, 1998).Thus cannabis use may make symptoms moredif� cult to control in individuals affected withschizophrenia (Hall, 1998). These may be smallin number but costly in terms of the suffering ofaf� icted individuals and their families and intheir impacts on health service costs.

Cognitive impairmentThe available evidence suggests that the long-term heavy use of cannabis does not producesevere or grossly debilitating impairment of cog-nitive function such as that produced by chronicheavy alcohol use (Solowij, 1998; Lekitsos et al.,1999). Nevertheless, evidence is emerging thatlong-term cannabis users show subtle cognitivechanges that can be detected using sophisticatedelectrophysiological methods (Solowij, 1998).

Uncertainty remains about the public healthsigni� cance of the subtle cognitive impairmentseen in chronic cannabis users. Impairment isnot readily apparent in the everyday cognitiveperformance of most experienced users, althoughcannabis use may impair the performance ofpeople who are employed in highly demandingjobs that pose a risk to the safety of others (e.g.aeroplane pilots, air-traf� c controllers and heavytransport drivers). It also remains uncertainwhether these effects reverse with abstinencefrom cannabis (Solowij, 1998).

There is community concern about the educa-tional consequences of cognitive impairmentamong adolescents who are heavy cannabis

488 Editorial

users. Cannabis use is common in this agegroup, which may be especially vulnerable to theadverse effects of cannabis (and other types ofintoxication) on learning and school perform-ance. The task of appraising the seriousness ofthese risks is complicated by the fact that adoles-cents whose school performance is poorest arethe most likely to become involved in heavy use(Fergusson & Horwood, 1997). However, thereis suf� cient evidence from longitudinal studies toinfer plausibly that heavy cannabis use furtherreduces the educational achievement of adoles-cents whose school performance is poor to beginwith (e.g. Newcomb & Bentler, 1988; Fergus-son, Lynskey & Horwood 1996; Tanner, Daviesand O’Grady, 1999). Given the importance ofeducational achievement to the transition in roleof childhood to adulthood, this is a concern.

ImplicationsFrom a scienti� c perspective, we have notidenti� ed the kinds of toxic effects that produceda policy consensus in the public health com-munity on policy towards tobacco products 30years ago. It took decades of systematic researchto establish the pathophysiology of tobaccosmoking, long after the link with cancer had � rstbeen observed (Proctor, 1999). Similar delays inachieving a scienti� c consensus have occurredwith fetal alcohol syndrome and other healthconditions linked to popular psychoactive sub-stances.

Our experience with tobacco products andalcoholic beverages suggests that an effectivepublic health policy is much more dif� cult toimplement when a substance is widely used andaccepted. One can predict that only as the preva-lence of cannabis use increases will the acute andchronic health problems it causes be better docu-mented and understood. By then its entrencheduse will make public health measures much moredif� cult to implement.

Any major change in the legal status of can-nabis seems unlikely at present, although thereare well-organized groups currently advocatingfor it. In the interim, we would advocate thefunding of well-designed epidemiological andclinical research studies on the effects of chroniccannabis use. These should include prospectivestudies of the effects of cannabis use on ado-lescent development, and clinical trials of thera-peutic interventions for cannabis dependence in

primary care and specialized treatment settings(Programme on Substance Abuse, 1997). Epi-demiological research on the personal and publichealth consequences of chronic cannabis useshould include research on the correlates andconsequences of heavy sustained cannabis use,especially among young people who initiate useearly, and who display symptoms of dependenceon cannabis. The two papers included in thisissue of the journal by Fergusson & Horwoodand McGee et al. exemplify the value of this typeof research. Longitudinal studies of large andrepresentative cohorts of cannabis users (e.g.Sidney et al., 1997) are also needed to assess theeffects of cannabis use on rarer adverse healthoutcomes, such as accidental injuries, cancers,serious respiratory diseases and premature deathfrom various causes.

ConclusionThe current neglect of cannabis by the addic-tions � eld may prove to be at odds with its publichealth signi� cance in the 21st century. It may bea truism that more and better research is neededbut it is no less true for that. The key question is:What can we do to learn more about the healthhazards of this fascinating substance before itswidespread use impedes a more rational policyresponse?

Some have argued that the subject of mari-huana toxicity has been exhausted over the pastseveral decades, and that we should now con-sider it a “soft drug” whose widespread useshould be tolerated for its recreational and med-icinal properties (Zimmer & Morgan, 1997). Weadvocate a more cautious approach of seekingbetter information. We think that this is moreappropriate in the light of the history of prema-ture closure on social policies towards psychoac-tive substances that has characterized ourexperiences with alcoholic beverages, tobacco,heroin and cocaine over the past millennium.

WAYNE HALL

Australasian Regional EditorTHOMAS F. BABOR

Regional Editor for the Americas

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