the public health significance of cannabis use in australia

The public health significance of cannabis use in Australia Wayne Hall National Drug and Alcohol /-hearch Cmtru, University of New South Wales, Sydnq

Abstract: A fair appraisal of the public health significance of cannabis use has been hampered by the polarised opinions about its health effects expressed by partisans on both sides of the debate on its legal status. The findings of a recent review of the literature on the adverse health and psychological effects of cannabis are used to estimate the major probable public health risks of cannabis use in Australia. These appear to be, in order of approximate public health importance: adverse psychological effects; motor vehicle accidents; cannabis dependence; respiratory disease; precipitation and exacerbation of schizophrenia in vulnerable individuals; low-birthweight babies; and perhaps subtle cognitive impairment. On current patterns of use, cannabis use is a modest public health concern by comparison with alcohol and tobacco, although given the scale of public health damage caused by the latter drug?, and the currently low prevalence o f regular cannabis use, this is not cause for complacency. (AuslJ f u b h c ffeabh 1993; 19: 235-42)

11F.m is a reasonable cause to be concerned about the public health significance of T cannabis use in Australia. Cannabis is widely

used by Australian adolescents and young adults, and because it is an intoxicant like alcohol that is usually smoked like tobacco, it is a reasonable assumption that i t shares at least some of the adverse health effects of these two drugs, both of which have a substantial effect on public health.l

There are a number of technical and social obstacles to appraising with confidence the public health significance of cannabis use. The technical obstacles concern difficulties in making causal inferences about the connections between cannabis use and the adverse health and psychological consequences that have been attributed to it.

Causal inferences about the long-term effects of chronic cannabis use are especially difficult. I t takes time for adverse effects to develop, it may take a long time for suspicion to be raised about a connection between drug use and the adverse effect, and the longer the interval between cannabis use and the health consequence, the more numerous the alternative explanations of the association that need to be excluded. There is also a tension between the rigour and relevance of the available evidence of an association between chronic cannabis use and adverse health outcomes. The most rigorous evidence is provided by laboratory investigations using experimental animals or in-vitro preparations of animal cells and micro-organisms in which wellcon- trolled drug doses are administered over a substantial period of the organisms’ lives and related to precisely measured biological outcomes. The relevance of such laboratory research to human disease, however, is often problematic. A great many inferences have to be made in reaoning from the demonstra- tion of health effects in laboratory animals (and even more so in the case o f cell cultures) to the probable health effects of cannabis under existing patterns of human use.

Correspondence t o Profcssor IVavnc Ilall, I)irtwor, National Drug and Ncohol Research Centre, University of S e w Sorith Wales, Sydncy, XSW 2052. Fax (02) 3!)9 71-43. An earlier version of this piper was prcwntrd a s the 19!)4 Caiighry Le(.iurc t o t l i e Anniial Confcrcnce of. t h e Royal Arirtrali;in and New Zcalantl Collqc* of I’liy\ic.ian\. 18 ;\II~II\I

l‘l!l-4, in ~ ~ l ~ i ~ i ~ ~ ~ ~ l i ~ i i ~ ~ - I ~ . Scw %c.;il;inrl.

Epidemiological studies of relationships between cannabis use and human disease have manifestly greater relevance to evaluating the human health effects but this is purchased at the price of reduced rigour in assessing the degree of exposure to cannabis and in excluding alternative explanations of observed associations. A major interpretive problem with positive epidemiologcal findings is that cannabis use is correlated with other types of drug use (for example, alcohol and tobacco use) known to affect health adversely. A different problem in interpretation arises when studies fail to find adverse health effects of chronic cannabis use. Does this mean that cannabis has few, if any, adverse health effects in humans, or have we not looked hard enough for such effects? The answer depends upon the likely magnitude of such effects, their relationships to dose, frequency and duration of use, and the ability of studies with small sample sizes to detect them.2

These technical issues have been compounded by polarised social attitudes towards cannabis use. Appraisals of the health hazards of recreational drug use are unavoidably affected by the societal approval or disapproval of the drug in question. As Room has observed, ethnographers studying the impact of alcohol on nonindustrialised societies have often engaged in ‘problem deflation’ in response to mis sionaries and colonial authorities who used inflated estimates of alcohol’s health and social effects to jus- tify paternalistic alcohol pol i~ies .~ A similar infla- tionary-deflationary dialectic has been at work in the debate about the health effects of recreational cannabis use.

A more rational assessment of the health rish of cannabis requires a number of things. First, appraisals of the health effects of cannabis should be distinguished clearly from the debate about the legal status of cannabis use. Although these issues are con- nected, a failure to separate them means that appraisers’ views o f the legal stiltus of cannabis often p,rejudice their appraisals of its health effects, arid wce versa.

Second, we should stop treating cannabis as a special case, whether that be as an unusually benign ‘mind-expanding’ drug, 01- ;is a ‘dtwq,tivt.ly dmgrr-

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oils' drug which adversely affects the fabric of soci- e ~ . If we adopt the same approach to appraising the health et'kcts of cannabis as we do to the health eftt.cts of alcohol and tobacco then we will begin with an assumption that cannabis may harm health when used at some dose, at some frequency or duration of use, or by some methods of administration.

Third, we need to use reasonable standards of proof in judging evidence of harm. If we insist upon proof beyond reasonable doubt that there are adverse health effects of cannabis or tobacco, very few conclusions can be drawn. Sensible, if fallible, inferences can be drawn if we are prepared to draw provisional conclusions about the pobnbb adverse health effects of cannabis.

Fourth, we should apply our standards of evidence consistently. The best protection against the use of double eiidential standards is to be as explicit as possible about the evidential standards that have been used, and as even-handed as possible in their appli- cation.

The following sections summarise the major adverse health and psychological effects of acute and chronic cannabis use identified in a literature review recently completed by the National Drug and Alcohol Research Centre.4 In this brief account, only a few exemplary references are cited in support of i ts asertions, which are supported in detail elsewhere." In what follows, acute health effects are taken to be those that occur shortly after a single dose or after a small number of occasions of use. Chronic health effects are taken to be those that occur after a period of regular use (for example, daily) over a period of years or decades. Unless otherwise stated, the route of administration of cannabis is assumed to be primarily by smoking.

Cannabis the drug Cannabis is a generic name for a variety of preparations derived from the plant, Cannabis sativa. Laboratory research on animals and humans has demonstrated that the primary psychoactive constituent in cannabis is the cannabinoid, delta-9- tetrahydrocannabinol or THC5 Cannabis resembles the opioid drugs in acting upon specific receptors in the brain. Very recently, an endogenous brain mole- cule ('anandamide') has been discovered; it binds to the cannabinoid receptor and mimics the action of cannabinoid~.~

The concentration of THC varies in the three most common forms of cannabis: marijuana, hashish and hash oil. Marijuana is prepared from the dried flowering tops and leaves of the harvested plant. The concentration of THC in a batch of marijuana containing mostly leaves and stems may range from 0.5 to 5 per cent, while the 'sinsemilla' variety may have 7 to 14 per cent THC. Hashish consists of dried cannabis resin and compressed flowers containing 2 to 8 per cent of THC, although it can be as high as 10 to 20 per cent. Hash oil, which contains between 15 and 50 per cent of THC, is obtained by extracting THC from hashish (or marijuana).

Marijuana is the most widely used form of the plant, which is usually smoked as a hand-rolled 'joint', the size of a cigarette or larger. Tobacco is often added to assist burning, and a filter is some-

times inserted. Hashish may also be mixed with tobacco and smoked as a joint, hut i t is probabljz n1o1-c frequently smoked tlii.oiigh a pipe, with or

popular implement for all cannabis preparations.

Adverse health and psychological effects Aruk ejjerts Anxiety, dysphoria, panic and paranoia may occur during early experiences with the drug, especially in naive users." More experienced users may experience similar effects if receiving doses that are much larger than usual. There is a higher risk of experiencing psychotic symptoms at very high doses of THC6

The inhalation of marijuana smoke o r the inges- tion of THC has a number of bodily effects, the most dependable of which is ;I 'LO per cent to 50 per cent increase in heart rate occurring within a few minutes to a quarter of an hour and lasting up to three hours.' Changes in blood pressure also occur: blood pressure is increased while the person is sitting, and decreases while standingH In healthy young users these cardiovascular effects are unlikely to be of any clinical significance because tolerance develops to the effects of THC, and young, healthy hearts will be only mildly s t r e s~ed .~

The acute toxicity of cannabis, and of cannabinoids more generally, is very low. There have been no confirmed cases of human deaths from cannabis poisoning in the world medical literature, and animal studies have indicated that the dose of THC required to produce 50 per cent mortality in rodents is extremely high by comparison with other com- monly used pharmaceutical and recreational drugs.'" The lethal dose also increases as one moves up the phylogenetic tree, suggesting that a lethal dose could not be very easily achieved by smoking or ingesting the drug.

Cognitive impairment, especially of attention, memory, and psychomotor impairment, are hall- marks of cannabis intoxication; they persist for the duration of intoxication." The acute effects of recreational doses of cannabis on driving performance in laboratory simulators and over standardised driving courses are similar to those of doses of alcohol that achieve blood alcohol concentrations between 0.07 per cent and 0.10 per cent.12 Although cannabis impairs performance in laboratory and simulated driving settings, studies of the effects of cannabis on on-road driving performance have found, at most, modest impairments. Cannabis-in toxicated persons drive more slowly, perhaps because they are more aware of their level of psychomotor impairment than alcohol-intoxicated drinkers, who generally drive at faster speeds.'?

No controlled epidemiologcal studies have estab lished that cannabis users are at higher risk of motor vehicle accidentq. Uncertainty about the role of cannabis in motor vehicle accidents is likely to remain, since the necessary case-control studies are difficult to conduct. Blood levels of cannabinoids do not indicate whether a driver or pedestrian was intoxicated with cannabis at the time of an accident, and many drivers with cannabinoids in their blood were also intoxicated with alcohol at the time ofthe

without tobacco. A w " r pipe known as a ‘ban$,,' r 1s ' a

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accident.” Factors other than psychomotor performance also contribute to the danger of drug use when driving. Foremost among these is the user’s preparedness to take risks when intoxicated, which the available evidence suggests is reduced by cannabis intoxication by contrast with alcohol intoxication, which consistently increases risk-taking.12

Chronic (Iffylts Since smoking is the usual method of administration, chronic cannabis users are at higher risk of respiratory disease^.^^,'^ Chronic heavy cannabis smoking impairs the functioning of the large airways and probably causes symptoms of chronic bronchitis, such ac coughing, sputum and wheezing.’J4J5 Given the documented adverse effects of tobacco smoke (which is qualitatively very similar in composition to cannabis smoke),lb it is likely that chronic cannabis users have a higher risk of developing chronic bron- hiti is.'^ Heavy cannabis smokers are also likely to show histopathological changes in lung tissues of the kind that precede the development of lung cancer in tobacco smokers.

Persons who use cannabis on a daily basis over periods of months or years are at risk of developing a dependence syndrome characterised by difficulty in abstaining from or controlling their cannabis ~ s e . ~ ~ J ~ There is good experimental evidence that chronic heacy cannabis users can develop tolerance to its subjective and cardiovascular effects, and there is suggestive evidence that some users experience withdrawal symptoms on the abrupt cessation of cannabis use.% There is clinical and epidemiological evidence that some heavy cannabis users experience problems in controlling their cannabis use, and con- tinue to use the drug despite experiencing adverse personal consequences.21.22 If the estimates of the community prevalence of drug dependence provided by the Epidemiologic Catchment Area Study are correct, cannabis dependence is the most common form of dependence on illicit drugs.I8

There is suggestive evidence that large doses of THC can produce an acute psychosis in which con- fusion, amnesia, delusions, hallucinations, anxiety, agitation and hypomanic symptoms predomi- nate.2s*24 There is less support for the hypothesis that cannabis use can cause an acute or a chronic psychosis that persists after the excretion of THC.25 There is suggestive evidence that chronic cannabis use may precipitate schizophrenia in vulnerable individual^.^^**^ This claim is still controversial because in the only prospective study conducted to date, the use of cannabis was not documented at the time of diagnosis,26 there was a possibility that cannabis use was confounded by other drug use, and there are doubts about whether the study could reli- ably distinguish between schizophrenia and acute cannabis-induced, or other drug-induced, psychosis. 25*27

The weight of the available evidence suggests that even the long-term heavy use of cannabis does not produce any severe or grossly debilitating impairment of cognitive function like that produced by chronic heavy alcohol use.w5o There is some clinical and experimental evidence, however, that the long- term use of cannabis may produce more subtle cog-

nitive impairment in the higher cognitive functions of memory, attention and the integration o f com- plex inlorination.g1-35 Although they ale subtle, these impairments may affect everyday functioning, par tic u 1 a r 1 y in i n d i vi d u a 1 s who se oc c u p a t i on s require high levels of cognitive capacity. The evidence suggests that the longer the period that cannabis has been used, the more pronounced is the cognitive impairment.$$ I t remains to be seen whether the impairment can be reversed by an extended period of abstinence from cannabis.

There are a number of major possibb adverse effects of chronic, heavy cannabis use which remain to be confirmed by controlled research.

First, recently, concern about the possibility of cancers being induced by chronic cannabis smoking has been increased by case reports of cancers of the upper aerodigestive tract in young adults with a history of heavy cannabis Although these reports fall short of providing convincing evidence, because many of the smokers concurrently used alcohol and tobacco, they are clearly cause for concern, as such cancers are rare in adults under the age of 60, even among those who smoke tobacco and drink alcoh01.l~ The conduct of case-control studies of these cancers should be a high priority for research that aims to identify the possible adverse health effects of chronic cannabis use.

Second, there is a single case-control study that suggests a higher risk of leukemia among offspring exposed to cannabis in utero.$’ The study requires replication because it was not designed to test the hypothesis that cannabis use was a risk factor for childhood leukemia; the finding emerged from a study of the causal role of other factors in which maternal drug use was measured as a covariate, and the rates of reported cannabis use in the control condition were much lower than has been reported in other studies.

Third, regular heavy cannabis users appear to experience a decline in cognitive performance, and possibly motivation, marked by underachievement in adults and by impaired educational attainment in adolescents. These findings remain contentious because children and adults whose cognitive ability and performance are marginal are more likely to become heavily involved in cannabis US^.^-$^

High-risk groups Adolesrmts Cannabis use by adolescents appears to increase the risk of discontinuing a high school education, and of experiencing job instability in young a d ~ l t h o o d . ~ ~ The apparent strength of these relationships in cross-sectional studies (for example, Kandelm) has been exaggerated because those adolescents who are most likely to use cannabis have lower academic aspirations and poorer high school performance prior to using cannabis than their peers who do not.39 It seems a reasonable inference, however, that adolescent$ with a history of poor school performance who use cannabis may have their educational achievement further limited by the cognitive impairments produced by chronic intoxication with can n a b i ~ . ~ . ’ ~

There is strong evidence for a regular sequence of

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initiation into the use of illicit drugs among American adolescents in the 1970s in which cannabis use preceded involvement with harder drugs, such as stimulants and ~p io ids .~ ’ Adolescents who initiated cannabis use in the early teens were at higher risk of progressing to heavy cannabis use and other illicit drug

The causal significance of this sequence of initiation into drug use remains controversial. The hypothesis that it represents a direct effect of cannabis use upon the use of later drugs in the sequence is the least compelling. There is better s u p port for two other hypotheses which are not mutual- ly exclusive: that there is a selective recruitment into cannabis use of nonconforming adolescents who have a propensity to use other illicit drugs, and that once recruited to cannabis use, the social interac- tion with other drug-using peers increases the opportunity to use, and encourages the use of, other illicit dr~gs.‘-l~‘~

Women of childbearing age Chronic cannabis use disrupts the male and female reproductive systems in animals, reducing the secre- tion of testosterone and production, motility, and viability of sperm in males, and disrupting the ovu- latory cycle in fern ale^.^^^^ I t is uncertain whether it has these effects in humans, given that there has been limited research on human males and virtually none on human females4’ There is also uncertainty about the clinical significance of these effects in nor- mal healthy young adults.

Cannabis use during pregnancy probably impairs fetal development, leading to a reduction in birthweight,@ perhaps as a consequence of shorter gesta- tion, and perhaps by the same mechanism as cigarette smoking, namely, fetal hypoxia. There is uncertainty about whether cannabis use during pregnancy produces a small increase in the risk of birth defects as a result of exposure of the fetus in utero. There has been some animal evidence of such effects, although these studies have usually involved very high doses by the oral route.qH The limited studies in humans have generally produced null ~-esul ts .~~-~* There is not a great deal of evidence that cannabis use can produce chromosomal or genetic abnormalities in either parent which could be transmitted to offspring. Such animal and in vitro evidence as exists suggests that the mutagenic capacities of cannabis smoke are greater than those of THC, and are probably of greater relevance to the risk of users developing cancer than to the transmission of genetic defects to ~ h i l d r e n . ~ ~ . ~ ’

There is suggestive evidence that infants exposed in utero to cannabis may experience transient behavioural and developmental effects during the first few months after b i ~ - t h . ~ ~ < ~ ’ Given this finding, the findings on reduced birth weight,50,52 and a possz- blt increased risk of birth a policy of prudence would suggest that women should be advised not to use cannabis during pregnancy or when attempting to ~onceive.~’

Persons ruitti preixisting diseases Some persons with pre-existing diseases who smoke cannabis art‘ protxhly iit an increased risk of precip-

itating or exacerbating symptoms of their diseases. Those with cardiovascular diseases, such as coronary artery disease, cerebrovascular disease and hyper- tension, may exacerbate their symptoms by smoking cannabis because of the cardiovascular effects of THC.56 So too may cannabis smokers with respiratory diseases, such as asthma, bronchitis and emphyse- ma.15 Those with schizophrenia are probably at increased risk of precipitating or of exacerbating schizophrenic symptoms if they use cannabis.6s26 Persons who are or have been dependent upon alcohol and other drugs are probably at a higher risk than others of developing dependence on cannabis.

Public health significance of cannabis use The public health significance of cannabis use is measured in terms of the number of individuals whose health is likely to be adversely affected, and the severity of its health consequences for those indi- Iiduals. I t depends upon the magnitude of the risks associated with specific patterns of use (for example, occasional use over a period of months or daily cannabis smoking over decades) and on the prevalence of these different patterns of use. For all the reasons indicated, the magnitude of many of these risks is difficult to quantify. All that is possible is a qualitative judgment based on estimates of the prevalence of varying types of exposure to cannabis use, and some judgments about the probable severity of health consequences.

Patterns of cannabis use in Australia The most recent 1993 National Campaign Against Drug Abuse household survey (see Table 1) indi- cates a number of things about the predominant pattern of cannabis use in Males are more likely than females to have ever used cannabis, and to have used it in the past year, and in the past week. The most common pattern of involvement with cannabis is experimentation in the late teens and discontinuation in the mid- to late-twenties. This is indicated by the much higher prevalence of ever- use compared to use in the last year and in the last week, and the steeply decreasing percentage with age (in those who have ever used cannabis) of those who have used it in the past week. For example, among males in the younger age groups, whose rates of experimentation with cannabis use are at their

Table 1: Prewlence of having used cannabis ever, in the past year and in the past week, by sex, in Australia, 1993

(n = 3500)

Prevalence of cannabis use (O/O)

Males Females Age group Ever Year Week Ever Year Week

14-19 39 27 6 32 20 3 20-24 a4 57 22 60 26 6 25-29 60 35 12 46 15 4 30-34 53 20 7 47 14 2 35-39 54 1 1 3 39 13 3 40-54 35 9 4 21 5 55 + 4 1 0 1 0 0 Note (a) L e s s than 1% Source: National Drug Strategy Survey”

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highest, only one in four of those who ever used cannabis did so in the previous week. This decreases to 13 per cent in the 30- to 35year age group.

The pattern of use that poses the greatest risk of producing adverse health effects from chronic use (at least weekly use) is relatively rare, and the p r e portion of users who maintain this pattern for a period of years is likely to be very small. It cannot be estimated from Australian data because no prospective studies have been conducted, but American research on cohorts of adolescents who initiated use in the middle 1970s, when the prevalence of cannabis use was as high as in contemporary Australia, has suggested that around one in ten cannabis users continued to use it on a daily basis beyond their mid- to late-twenties4

All the available data on patterns of use suggest that the adverse acute effects of cannabis use are much more likely to be experienced by large numbers of cannabis users than are the adverse effects of long-term use.45 Hence, the most significant public health risks ofcannabis use are likely to be the acute effects, since these may be experienced by the many who use the drug on a small number of occasions before discontinuing their use.

Acute health effects Adverse $19 chologzcal e@erim res The most common adverse effects of cannabis use are likely to be dysphoric and unpleasant psychological reactions. These may occur in as many as a third of those who ever use the drug, and their occurrence may be a major explanation for the high rates of discontinuation of its The public health significance of the majority of these experiences is probably minor, since they are not life-threatening but are self-limited, easily managed by reassurance, and rarely lead to helpseeking.

Motor vehirk, arcidcnts Given the high rates of cannabis use among young adults, who are at highest risk of being involved in motor vehicle accidents, accidental injury and death are clearly a potentially much more serious public health issue than transient dysphoria. However, i t is difficult to assess the public health significance of motor vehicle accidents caused by cannabis because of the strong association between cannabis and alcohol use. The epidemiological studies suggest that in its own right, cannabis makes at most a very small contribution to motor vehicle accidents, and so on the whole it may seem to be a minor road safety problem by comparison with alcohol.IJ Its major public health significance for road safety may be in amplifying the adverse effects of alcohol on the performance of those drivers who combine alcohol and cannabis intoxication; they seem to comprise the majority of cannabis users in fatal accidents' and the majority o f cannabis users in the cornmunity.j!' On the grounds of prudence we should recommend that cannabis users not drive when intoxicated, and we should support the same legal penalties for cannabis users who smoke and drive as we currently do for those who drink alcohol and drive a car.

Imulri?-thwtight babies If we make a worst-case assumption that cannabis and tobacco smoking are equivalent in their effects on fetal development, then cannabis smoking during pregnancy may double the r isks of a woman p v - ing birth to a low-birthweight baby. The public health significance of cannabis use by pregnant women is likely to be much lower than that of tobacco smolung during pregnancy because the prevalence of cannabis use is much lower than that of tobacco. As with alcohol, however, although rates of fetal exposure to cannabis smoke may be relatively low, the risks of a low-birthweight baby will be higher among those cannabis users who also smoke tobacco and have other risk factors for a low-birthweight baby.

Chronic health effects D@ndenrc Cannabis dependence is probably the chronic health effect of cannabis that presents the largest public health problem. On the estimates provided by the Epidemiologc Catchment Area Study in 1982, approximately 3 per cent of the adult population in the United States met diagnostic criteria for cannabis abuse or dependence, compared with 14 per cent who met diagnostic criteria for alcohol abuse and dependence.l8 Similar figures have been reported in more recent Australian and New Zealand studies.60*61 These figures are not reflected in large numbers of cannabisdependent persons seeking treatment for their dependence, probably because there is a high rate of remission of symp toms in the absence of treatment, and there are fewer adverse personal and social consequences of cannabis dependence than of alcohol or opioid dependence. But the risks and consequences of cannabis dependence are issues deserving of further examination.

&spirntury diseases If we make the worst-case assumption that the risks of cancer are comparable among daily tobacco and cannabis smokers, then cannabis smoking will make at most a small contribution to the occurrence of these cancers. This is because only a minority of those who ever use cannabis become daily users, and a much smaller proportion of these daily users persist in smoking cannabis beyond their mid-twenties than the proportion of tobacco smokers who do so. Among this minority, however, concurrent cannabis and tobacco use may ampli9 each other's adverse respiratory effects. Given the widespread initiation of cannabis use among young Australian adults in the middle 197% it may be timely to conduct case-control studies on cannabis use and cancers of the upper respiratory tract in middle-aged adults.

The public health significance of respiratory diseases such as chronic bronchitis is probably greater than that for respiratory cancers. This is s o for two reasons. First, respiratory cancers probably require a greater duration of exposure to smoke (15 to 20 years in the case of cigarette smoke) than is required to develop chronic bronchitis (which has been reported after six months of hrtaw hashish and

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tobacco smoking).69 Second, only about 10 per cent of cannabis users use the drug for more than five years. Nonetheless, the exposure period required to develop chronic bronchitis may be shorter among those cannabis smokers who also smoke tobacco, since there is evidence that concurrent tobacco and cannabis smoking have additive adverse effects on the respiratory system.14

Schizophrenia Even if we assume that the observed association between cannabis use and schizophrenia is a token of a causal re la t ion~hip,~~ its public health significance in initiating cases of schizophrenia that would not otherwise occur is likely to be relatively small. Schizophrenia affects approximately 1 per cent of the adult population, and on the data of Andreasson et al., cannabis use would be a contributory cause of less than 10 per cent of cases of schizophrenia.26 Even this figure seems high, however, since the inci- dence of schizophrenia declined during the period when cannabis use among adolescents and young adults increased.63 The public health significance of cannabis use in schizophrenia may be less in precip itating cases and more in making it harder to control symptoms in affected individuals who use c a n n a b i ~ . ~ ~ . ~ These may be small in number but costly in terms of the suffering of afflicted individuals and their families and health service use. Mental health services should accordingly be alerted to inquire about cannabis use among their schizophrenic patients and to counsel them against its use.

Cognitive impainnent It is difficult to be precise about the public health significance of cognitive impairment produced by chronic cannabis intoxication. First, the effects in even long-term users are very subtle and require sophisticated electrophysiological methods to detect them;” they are not readily apparent in affecting gross cognitive performance in experienced users. They are probably most likely to make a difference to the performance of those who are employed in highly demanding jobs (for example, aeroplane pilot, air-traffic controller, heavy transport driver). Second, the greatest community concern is about the educational consequences of cognitive impairment among adolescent users. This is an age at which cannabis use is widespread and when there is a special vulnerability to the effects of cannabis on educational performance. Here, the task of appraising the seriousness of the risks is complicated because it is precisely those adolescents whose school performance is poorest who are most likely to become involved in heavy cannabis use.4 It is a plau- sible inference, nonetheless, that cannabis use will not help the educational achievement of these adolescents. On the grounds of prudence it would be desirable to develop educational strategies to dis courage or at least delay adolescent cannabis use.66

An approximate comparative appraisal Overall, most of the probable public health risks of cannabis use are small to moderate in size in terms of the number of individuals likely to be affected

and the likely severity of its effects. In aggregate, on current pntlirrns of use the health consequences of cannabis use are unlikely to be comparable to those currently produced by alcohol and tobacco. This is largely because, on current patterns of use, the p r e portion of the population that uses cannabis heavily over a period of years is much smaller than the proportions that use alcohol or tobacco in a comparable way. For example, the proportions of the Australian population who have ever used alcohol, tobacco and cannabis are 95 per cent, 70 per cent, and 33 per cent, respectively, while the proportions who are weekly users are 66 per cent, 29 per cent, and 5 per cent.67

Conclusions There probably are adverse health and psychological effects of cannabis use. The clearest are the acute risks of adverse psychologcal experiences, the possibility of an increased risk of motor vehicle accidents if people drive while intoxicated, and low-birthweight babies for women who smoke cannabis during their pregnancies. The adverse effects of chronic use are less certain but probably include, in order of frequency: dependence, respiratory diseases (including perhaps cancers of the upper respiratory tract) and subtle forms of cognitive impairment. High-risk groups include: adolescents with poor school performance; women of child-bearing age; persons with cardiovascular and respiratory diseases; and persons with a personal or family history of psychotic illness. Given current patterns of cannabis use, the health effects of greatest public health significance are likely to be the acute effects, namely, a higher risk of adverse psychological experiences and probably a higher risk of motor vehicle accidents. The major public health risks of regular cannabis use are, in probable order of magnitude: dependence, respiratory disease, precipitation or exacerbation of psychoses, low-birthweight babies and cognitive impairment.

Acknowledgments I would like to acknowledge the assistance of the following people in the preparation of the review of the health effects of ~ a n n a b i s , ~ on which the analysis in this paper depends: Dr Robert Ali, Chairman of the National Task Force on Cannabis, and the members of the Task Force for their feedback on earlier drafts of the document; Dr Mario Argandano (WHO Program on Substance Abuse), Dr Greg Chesher (National Drug and Alcohol Research Centre), Paul Christie (Project Officer, National Task Force on Cannabis), Dr Bill Corrigal (Senior Scientist, Addiction Research Foundation, Toronto), Emeritus Professor Harold W a n t (Department of Pharmacology, University of Toronto), and Dr Jean- Marie Rue1 (Bureau of Dangerous Drugs, Health and Welfare Canada) for their useful comments on the whole manuscript. Expert comments on specific sections of the manuscript were provided by: Dr Peter Fried (Carleton University, Ottawa, Ontario, on reproductive effects); Dr Richard Mattick (National Drug and Alcohol Research Centre, on the dependence syndrome); Dr Peter Nelson (Southern Cross University, New South M‘ales, on

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psychologcal eftects); Dr Paes (Department of’ Psychiatry, University o f Kabat, Morocco, on psychiatric disorders) ; and Pi-ofessor Donald Tashkin (Division of Pulmonary and Critical Care Medicine, University of‘ California, Los Angeles Medical School, on cardiovascular and respiratory effects).

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27th Annual PHA Conference Participation in Health 24-27 September 1995, Cairns International Hotel, Queensland

The theme for the 1995 Public Health Association annual conference is ‘Participation in Health’. It will focus in particular on

public health interventions resource allocation intersectoral action community involvement and consumers Aboriginal and Torres Strait Islander health

Confirmed speakers include Professor Richard Feachem, Dean, London School of Hygiene and Tropical Medicine, UK Dr Duane Gubler, Director, Division of Vector Borne Diseases, Centers for Disease Control, US Dr Sally Redman, Director, National Breast Cancer Centre, Sydney Dr Bruce Armstrong, Director, Australian Institute of Health and Welfare, Canberra Mr Stephen Duckett, Secretary, Commonwealth Department of Human Services and Health Dr Norman Swan, Radio National Health Report

Contact Margarete Conroy Conference Coordinator, PHA Secretariat Phone (06) 285 2373 or fax (06) 282 5438

242 AUSTRALIAN JOURNAL OF PUBLIC HEALTH 1995 v a . 19 NO 3

the public health significance of cannabis use in australia

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