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The following is a summary of the major adverse health and psychological effects of acute and chronic cannabis use, grouped according to the degree of confidence in the view that the relationship between cannabis use and the adverse effect is a causal one.
- anxiety, dysphoria, panic and paranoia, especially in naive users;
- cognitive impairment, especially of attention and memory, for the duration of intoxication;
- psychomotor impairment, and probably an increased risk of accident if an intoxicated person attempts to drive a motor vehicle, or operate machinery;
- an increased risk of experiencing psychotic symptoms among those who are vulnerable because of personal or family history of psychosis;
- an increased risk of low birth weight babies if cannabis is used during pregnancy.
The major health and psychological effects of chronic heavy cannabis use, especially daily use over many years, remain uncertain. On the available evidence, the major probable adverse effects appear to be:
- respiratory diseases associated with smoking as the method of administration, such as chronic bronchitis, and the occurrence of histopathological changes that may be precursors to the development of malignancy.
- development of a cannabis dependence syndrome, characterised by an inability to abstain from or to control cannabis use;
- subtle forms of cognitive impairment, most particularly of attention and memory, which persist while the user remains chronically intoxicated, and may or may not be reversible after prolonged abstinence from cannabis.
The following are the major possible adverse effects of chronic, heavy cannabis use which remain to be confirmed by further research:
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- an increased risk of developing cancers of the aerodigestive tract, i.e. oral cavity, pharynx, and oesophagus;
- an increased risk of leukemia among offspring exposed while in utero;
- a decline in occupational performance marked by underachievement in adults in occupations requiring high level cognitive skills, and impaired educational attainment in adolescents;
- birth defects occurring among children of women who used cannabis during their pregnancies.
High risk groups
- Adolescents with a history of poor school performance may have their educational achievement further limited by the cognitive impairments produced by chronic intoxication with cannabis.
- Adolescents who initiate cannabis use in the early teens are at higher risk of progressing to heavy cannabis use and other illicit drug use, and to the development of dependence on cannabis.
Women of childbearing age
- Pregnant women who continue to smoke cannabis are probably at increased risk of giving birth to low birth weight babies, and perhaps of shortening their period of gestation.
- Women of childbearing age who smoke cannabis at the time of conception or while pregnant possibly increase the risk of their children being born with birth defects.
Persons with pre-existing diseases
Persons with a number of pre-existing diseases who smoke cannabis are probably at an increased risk of precipitating or exacerbating symptoms of their diseases. These include:
- individuals with cardiovascular diseases, such as coronary artery disease, cerebrovascular disease and hypertension;
- individuals with respiratory diseases, such as asthma, bronchitis, and emphysema;
- individuals with schizophrenia, who are at increased risk of precipitating or of exacerbating schizophrenic symptoms;
- individuals who are dependent on alcohol and other drugs, who are probably at an increased risk of developing dependence on cannabis.
The health risks of alcohol, tobacco and cannabis use
Alcohol. The major risks of acute cannabis use are similar to the acute risks of alcohol intoxication in a number of respects. First, both drugs produce psychomotor and cognitive impairment. The impairment produced by alcohol increases risks of various kinds of accident. It remains to be determined whether cannabis intoxication produces similar increases in accidental injury and death, although on balance it probably does. Second, substantial doses of alcohol taken during the first trimester of pregnancy can produce a foetal alcohol syndrome. There is suggestive but far from conclusive evidence that cannabis used during pregnancy may have similar adverse effects. Third, there is a major health risk of acute alcohol use that is not shared with cannabis. In large doses alcohol can cause death by asphyxiation, alcohol poisoning, cardiomyopathy and cardiac infarct whereas there are no recorded cases of fatalities attributable to cannabis.
Tobacco. The major acute health risks that cannabis share with tobacco are the irritant effects of smoke upon the respiratory system, and the stimulating effects of both THC and nicotine on the cardiovascular system, both of which can be detrimental to persons with cardiovascular disease.
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Alcohol. Chronic cannabis use may share some of the risks of heavy chronic alcohol use. First, heavy use of either drug increases the risk of developing a dependence syndrome in which users experience difficulty in stopping or controlling their use. There is strong evidence for such a syndrome in the case of alcohol and reasonable evidence in the case of cannabis. Second, there is reasonable clinical evidence that the chronic heavy use of alcohol can produce psychotic symptoms and psychoses in some individuals. There is suggestive evidence that chronic heavy cannabis use may produce a toxic psychosis, precipitate psychotic illnesses in predisposed individuals, and exacerbate psychotic symptoms in individuals with schizophrenia. Third, there is good evidence that chronic heavy alcohol use can indirectly cause brain injury - the Wernicke-Korsakov syndrome - with symptoms of severe memory defect and an impaired ability to plan and organise. Chronic cannabis use does not produce cognitive impairment of comparable severity but there is suggestive evidence that chronic cannabis use may produce subtle defects in cognitive functioning, that may or may not be reversible after abstinence. Fourth, there is reasonable evidence that chronic heavy alcohol use produces impaired occupational performance in adults and lowered educational achievements in adolescents. There is at most suggestive evidence that chronic heavy cannabis use produces similar, albeit more subtle impairments in occupational and educational performance of adults. Fifth, there is good evidence that chronic, heavy alcohol use increases the risk of premature mortality from accidents, suicide and violence. There is no comparable evidence for chronic cannabis use, although it is likely that dependent cannabis users who frequently drive while intoxicated with cannabis increase their risk of accidental injury or death. Sixth, alcohol use has been accepted as a contributory cause of cancer of the oropharangeal organs in men and women. There is suggestive evidence that chronic cannabis smoking may also be a contributory cause of cancers of the aerodigestive tract (i.e. the mouth, tongue, throat, oesophagus, lungs).
Tobacco. The major adverse health effects shared by chronic cannabis and tobacco smokers are chronic respiratory diseases, such as chronic bronchitis, and probably, cancers of the aerodigestive tract. The increased risk of cancer in the respiratory tract is a consequence of the shared route of administration by smoking. It is possible that chronic cannabis smoking also shares the cardiotoxic properties of tobacco smoking, although this possibility remains to be investigated.