IMPORTANT NOTE

  Any review of the possible health implications of cannabis needs to be placed in the comparative perspective of what is known about alcohol and tobacco, two other widely used psychoactive drugs. Cannabis shares with tobacco, smoking as the usual route of administration and resembles alcohol in being used for intoxicating and euphoric effects. The comparison serves the useful purpose of reminding us of the risks we currently tolerate with our most widely used psychoactive drugs (Hall, W et al, 1996.).

HEADLINES:

—  Cannabis is not a harm-free drug.

—  It is important to look at cannabis in the context of the risks society already accepts.

—  The primary health problems associated with the drug are the possibility of respiratory disorders similar to those experienced by tobacco smokers.

—  Cannabis use may also exacerbate existing mental health problems.

—  Many of the other possible risks from using cannabis remain unproven or inconclusive.

—  Due to the nature of the evidence it is important that policy makers assess the likely health impact on the greatest number, rather than extrapolating from individual case studies or those involving surveys with only small samples.

—  In relation to the millions of individuals who have been exposed to the drug in this country since the late 1960s, cannabis compares favourably (in terms of health implications) with legal drugs widely used such as alcohol and tobacco.

INTRODUCTION

  1.  This paper summarises the best currently available evidence for the adverse effects of cannabis use. The evidence is predominantly drawn from studies involving humans. There is inconclusive evidence available in a number of areas and this will be indicated where appropriate.

THE SOURCES

  2.  This paper is based in a number of authoritative major reviews of the cannabis research literature conducted in the last decade and supplemented by more recent research reports where appropriate.

INTERPRETING THE EVIDENCE BASE

  3.  Much of the research evidence reviewed derives from American studies, although there is increasing evidence emerging from Australian research. There are a number of difficulties in assessing the value of this evidence:

—  the main psychoactive component of cannabis is tetrahydrocannabinol (THC) and apart from laboratory studies, it is difficult to ascertain the amount of THC consumed by cannabis users.

—  cannabis is often used with other substances, most commonly alcohol and tobacco, which may also have adverse effects on health and make it difficult to distinguish the effects of cannabis from those of these substances

—  the prohibited status of cannabis has made the collection of epidemiological evidence difficult

—  there is a lack of controlled long-term studies

—  most cannabis use is intermittent and time-limited, with most users stopping in their mid to late twenties.

SHORT TERM EFFECTS OF CANNABIS

  4.  The reported effects of cannabis use are a sense of euphoria and relaxation, perceptual distortions, time distortion and the intensification of sensory experiences such as listening to music. Cannabis use in social settings can lead to increased talkativeness and infectious laughter followed by states of introspection and dreaminess. The user typically has a feeling of greater emotional and physical sensitivity that can include greater interpersonal empathy. Short-term memory and attention are also impaired. (Hall 1998; Joy et al. 1999).

  5.  Cannabis use can increase the heart rate by 20-100 per cent above baseline. This increase is greatest in the first 10 to 20 minutes then decreases rapidly thereafter. The rate of decrease depends on whether smoked or oral cannabis is used, lasting three hours in the former and five hours in the latter (Graham 1986; Hall et al. 1994; Joy et al. 1999). Blood pressure is increased while the person is sitting and decreased while standing. The change from sitting to standing can cause faintness and dizziness due to the change in blood pressure. These cardiovascular effects are of negligible clinical significance because most cannabis users are young and healthy and develop tolerance to these effects (Hall 1994; 1998; Joy et al. 1999).

MOOD EFFECTS

  6.  Adverse mood effects can occur, particularly in inexperienced users, after large doses of cannabis. Anxiety and paranoia are the most common of these effects and others include panic, depression, delusions and hallucinations. These effects normally disappear after a few hours after cessation of use and are responsive to reassurance and a supportive environment (Adams and Martin 1996; Joy et al. 1999).

TOXICITY

  7.  The acute toxicity of cannabis is very low and there is no overdose risk from cannabis. There are no confirmed published cases of human deaths related to cannabis poisoning (Hall 1998).

PSYCHOMOTOR EFFECTS AND DRIVING

  8.  Cannabis produces dose-related impairments in a wide range of behavioural and cognitive functions. These include slowing reaction time and information processing, and impairing perceptual and motor performance, tracking behaviour and time perception. These effects can increase with the dose of THC and are larger and more persistent with tasks that require sustained attention (Chait and Pierri 1992). These effects may have implications for accidents if the users drive or operate machinery while intoxicated.

  9.  Laboratory driving simulator and standardised road studies have found impairments in driving skills after cannabis use, similar to those effects when blood alcohol levels are between 0.07 per cent and 0.10 per cent. However more realistic on-road and simulator studies have found that cannabis drivers tend to be more cautious and drive more slowly, compared to alcohol-intoxicated drivers. This may be because they are aware of their intoxication and take appropriate precautions (Hall et al 1994; Smiley 1998).

  10.  Epidemiological evidence for the role of cannabis in road accidents is equivocal. UK studies have found traces of illicit drugs in 18 per cent of those killed in fatal accidents, with cannabis constituting around two thirds of the drugs found (Sexton et al 2001). However, because traces of cannabis can remain in the body system for up to 28 days the presence of cannabinoids in the blood of accident victims cannot be taken to indicate that the driver was intoxicated at the time of the accident. Many drivers in accidents also have a high blood alcohol level at the time of their accident. Two studies with drivers who had only used cannabis found that there was no increased culpability of accidents amongst this group (Chesher 1995).

LONG-TERM EFFECTS OF CANNABIS

Respiratory system

  11.  Cannabis smoke contains many of the same components as tobacco smoke. As much as four times the amount of tar can be deposited on the lungs of cannabis smokers as cigarette smokers if a cigarette of comparable weight is smoked. This difference is probably the result of differences in administration. Cannabis cigarettes usually do not have filters and cannabis smokers usually develop a larger puff volume, inhale more deeply and hold their breath several times longer than tobacco smokers (Joy et al 1999).

  12.  Chronic smoking effects are similar to those of tobacco smoking. Chronic heavy use of cannabis is associated with increased symptoms of chronic bronchitis such as coughing and wheezing. Lung function is impaired and there are greater abnormalities in the large airways of cannabis smokers than non-smokers. Cannabis smoking is associated with changes in bronchial tissue. Many cannabis smokers have erythema (increased redness of airway tissues) and edema (swelling of the airway tissues). (Joy et al 1999). Studies have shown that people who are regular users of cannabis but not tobacco have more symptoms of chronic bronchitis than non-smokers (Hall 2001).

CARCINOGENICITY

  13.  There is no conclusive evidence that cannabis causes cancer in humans including those cancers associated with tobacco use. However, cellular, genetic and human studies suggest that cannabis smoke may be an important risk factor for the development of respiratory cancer. There is not yet any evidence from controlled studies showing a higher rate of respiratory cancers among chronic cannabis smokers. However there is evidence of an additive effect of cannabis and tobacco smoking on abnormalities in lung tissue, similar to those that precede lung cancer in tobacco smokers (Joy et al, 1999; Tashkin, 1999; Hall, 2001). These effects are related to the amount of cannabis smoked and it has been argued that cannabis smokers will not smoke as much as tobacco smokers or smoke as long in their lives. Most cannabis users have stopped using cannabis by their mid- to late-twenties.

  14.  There have been case reports of cancers in the digestive tract of young adults with a history of heavy cannabis use. These findings are significant because these kinds of cancers are rarely found in the adults under the age of 60, even among those who smoke tobacco and drink alcohol. This suggests that cannabis smoking may potentate the effects of other risk factors such as tobacco smoking and is a more important risk factor than tobacco and alcohol use in the early development of respiratory cancers (Sridhar 1994; Joy et al 1999; Hall 2001).

REPRODUCTIVE SYSTEM

  15.  THC has been found to inhibit reproductive function in the few human studies reported although these studies have yielded inconsistent evidence. On the basis of research on animals it has been argued that cannabis would probably decrease fertility for both men and women in the short term (Hall et al 1994; Joy et al 1999). It has been suggested that in this respect the possible effects of cannabis use may be most significant for those males whose fertility is already impaired, for example those with a low sperm count (Hall et al 1994).

  16.  The results of research studies on the effects of prenatal cannabis use and birth outcome have been small and inconsistent. Some studies have suggested that cannabis smoking in pregnancy may reduce birthweight. A controlled study has found this relation has remained after controlling for any confounding variables but this relation has not been found in other studies (Zuckerman et al 1989; Hall and Solowij 1998; Joy et al 1999). The effects of cannabis smoking where the study has found an association has been small compared to tobacco (Fried 1998). There is little evidence that gestation is shorter except for adolescent mothers (Cornelius et al 1995). Large well-controlled epidemiological studies have found no evidence that cannabis causes birth defects (Zuckerman et al 1989).

  17.  Cannabis may have behavioural and developmental effects on infants exposed in utero during the first few months after birth. Between the ages of four and nine children who have been exposed to cannabis in utero have shown deficits in sustained attention, memory and cognitive functioning. However the effects were small compared to tobacco and their clinical significant is unclear. The underlying causes might be the cannabis exposure or might be more closely related to the reasons underlying the mothers' use of cannabis during pregnancy (Fried 1998; Hall and Solowij 1998; Joy et al 1999).

  18.  Recent case studies have found an increased risk of child cancers in children born to mothers who reported using cannabis during their pregnancies. However cannabis was one amongst several factors considered in the analysis of the data from these studies and this area requires further study (Hall and Solowij 1998).

IMMUNE SYSTEM

  19.  There is no conclusive evidence that cannabis impairs immune function to any significant extent. The few studies that have suggested that cannabis has an adverse effect on the immune system have not been replicated.

NEUROLOGICAL AND MENTAL HEALTH ISSUES

  20.  There is evidence that large doses of THC can produce an acute psychosis marked by confusion, amnesia, delusions, hallucinations, anxiety, and agitation. Such reactions are rare and occur usually after heavy cannabis use, or in some instances, after acute cannabis use by sensitive/vulnerable individuals. These effects abate rapidly after discontinuing cannabis use. There is little evidence that cannabis alone produces a psychosis that persists after the period of intoxication (Hollister 1986; Hall 1998; Joy et al 1999).

  21.  A Swedish study found an association between cannabis use and schizophrenia. In this prospective study of 50,000 Swedish conscripts a dose response relation was found between the frequency of cannabis use and the risk of a diagnosis of schizophrenia over the next 15 years (Andreasson et al. 1987). Although the value of this study has been debated (Negrete 1989; Hall 1998), it has been suggested that cannabis use may exacerbate the symptoms of schizophrenia and a prospective study has found that continued cannabis use predicts more psychotic symptoms in people with schizophrenia (Linszen et al. 1990). However, the incidence of schizophrenia has decreased in recent years whilst the use of cannabis has increased. Hall suggests that, this may indicate that cannabis use is unlikely to have caused cases of schizophrenia that might not have otherwise occurred. Chronic cannabis use may precipitate cannabis in vulnerable individuals but not cause the underlying psychotic disorder, an effect that would not change reported incidence (Hall and Solowij 1998). Overall, those people with schizophrenia or a family history of schizophrenia are at a greater risk for adverse effects from the use of cannabis (McGuire 1995).

DEPENDENCE

  22.  Heavy smokers of cannabis develop tolerance to the subjective and cardiovascular effects of cannabis. Some users report a withdrawal syndrome on cessation of use with symptoms that may include restlessness, irritability, mild agitation, insomnia, sleep disturbance, nausea and cramping. Controlled laboratory studies have observed withdrawal symptoms which were short lived and abated after four days. However, there are still methodological problems to be addressed in measuring the severity of these withdrawal symptoms (Smith in press).

  23.  There is some evidence that a cannabis dependence syndrome occurs with heavy cannabis use marked by difficulty in controlling use and continued use despite experiencing adverse personal consequences (Stephens et al 1993; Swift et al 1999). American studies have found that about one in 10 of those who ever use cannabis become dependent on it during the four-five years of heaviest use (Anthony et al 1994). However, the risk of dependence is more like that for alcohol, than tobacco and opioids (Hall and Solowij 1998; Joy et al 1999). This may be due to differences in drug effects, its availability or the penalties associated with the use of the drug or some combination of these.

COGNITIVE EFFECTS

  24.  Studies have shown that cannabis can produce a subtle impairment of attention, memory and the organisation and integration of complex information (Block and Ghoneim 1993; Joy et al 1999). The longer the cannabis use the more pronounced the impairment. These impairments are subtle and it remains unclear whether these effects are reversed after an extended period of abstinence or what implications they may have for everyday functioning (Hall and Solowij 1998; Solowij 1998). In addition, researching this complex area is difficult: for example there has been criticism of the adequacy of matching cannabis uses with control subjects in those studies on cognitive deficits in heavy cannabis users (Joy et al 1999). There is no evidence for the scale of severe or debilitating impairment of memory, cognitive function and attention found with chronic heavy alcohol use. There is no evidence that cannabis causes structural brain damage in humans.

CANNABIS AND ADOLESCENT DEVELOPMENT

  25.  There is no evidence to support a causal relationship between cannabis use and those behavioural characteristics which have been described as an amotivational syndrome (Joy et al 1999). Studies have shown an association between heavy cannabis use in adolescence and the risk of leaving school early and of experiencing job instability in young adulthood. However, the strength of these associations are reduced in longitudinal studies when the low educational aspirations and poorer school performance of heavy cannabis users before their cannabis use is taken into account (Fergusson and Horwood 1997; Hall 1998). On balance the evidence available suggests that daily or near daily cannabis use does not improve the educational performance of those who were performing poorly already (Lynskey and Hall 2000).

IMPACT OF INCREASED POTENCY OF CANNABIS

  26.  High THC-containing cannabis seems to have become increasingly available although the published evidence for this is scant (Hall and Swift 1999; Ashton 2001). This may reflect an increased market for more potent cannabis amongst regular users and improved methods of growing high potency cannabis. The health implications of this development are unclear. Those who use these high potency products may increase their risks of developing dependence, having accidents while driving or experiencing psychotic symptoms (Hall 1998). However, regular users may be able to titrate their dose and decrease the risks of respiratory disease and naive users who experience adverse effects may be deterred from further cannabis use (Hall 1998).

CONCLUSION

  27.  Cannabis is not a harm-free drug. The primary problems with the drug focus mainly on the possibility of respiratory disorders similar to those experienced by tobacco smokers and the risk of exacerbating existing mental health problems. Many of the other possible risks from using cannabis remain unproven or inconclusive. In using this data to help formulate policy that will impact on many individuals, it is helpful to think of the likely health impact on the greatest number, rather than extrapolating from individual case studies or those involving surveys with only small samples. In relation to the millions of individuals who have been exposed to the drug in this country since the late 1960s, cannabis compares favourably (in terms of health implications) with legal drugs widely used such as alcohol and tobacco.

REFERENCES

  The major reviews used for this summary are:

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