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In many western societies, cannabis has been used by a substantial minority, and in some a majority, of young adults, even though its use is prohibited by law. Debate about the justification for continuing to prohibit cannabis use has polarised opinion about the seriousness of its adverse health effects. In addition, the possible therapeutic effects of cannabinoids have become entangled in the debate about prohibition of recreational cannabis use (see Further reading). The health effects of cannabis use, especially of long-term use, remain uncertain because there is very little epidemiological research and because of disagreements about the interpretation of the limited epidemiological and laboratory evidence. Here we summarise the evidence on the most probable adverse health effects of cannabis use acknowledging where appropriate the uncertainty that remains.
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Acute effects of cannabis
Cannabis produces euphoria and relaxation, perceptual alterations, time distortion, and the intensification of ordinary sensory experiences, such as eating, watching films, and listening to music.2 When used in a social setting it may produce infectious laughter and talkativeness. Short-term memory and attention, motor skills, reaction time, and skilled activities are impaired while a person is intoxicated.2
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Summary of adverse effects of cannabis
Acute effects
* Anxiety and panic, especially in naïve users.
* Impaired attention, memory, and psychomotor performance while intoxicated.
* Possibly an increased risk of accident if a person drives a motor vehicle while intoxicated with cannabis, especially if cannabis is used with alcohol.
* Increased risk of psychotic symptoms among those who are vulnerable because of personal or family history of psychosis.
Chronic effects (uncertain but most probable)
* Chronic bronchitis and histopathological changes that may be precursors to the developmentof malignant disease.
* A cannabis dependence syndrome characterised by an inability to abstain from or to control cannabis use.
* Subtle impairments of attention and memory that persist while the user remains chronically intoxicated, and that may or may not be reversible after prolonged abstinence.
Possible adverse effects (to be confirmed)
* Increased risk of cancers of the oral cavity, pharynx, and oesophagus; leukaemia among offspring exposed in utero.
* Impaired educational attainment in adolescents and underachievement in adults in occupations requiring high-level cognitive skills.
Groups at higher risk of experiencing these adverse effects
* Adolescents with a history of poor school performance, who initiate cannabis use in the early teens, are at increased risk of using other illicit drugs and of becoming dependent on cannabis.
* Women who continue to smoke cannabis during pregnancy may increase their risk of having a low-birthweight baby.
* People with asthma, bronchitis, emphysema, schizophrenia, and alcohol and other drug dependence, whose illnesses may be exacerbated by cannabis use.
Less harmful than alcohol. Do you support banning alcohol? If not, why not?