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The long term effects of cannabis have been the subject of ongoing debate. Because cannabis is illegal in most countries, research presents a challenge; as such there remains much to be concluded. Studies have investigated both the detrimental and beneficial effects of long-term use of cannabis.

A 2013 literature review said that exposure to marijuana had biologically-based physical, mental, behavioral and social health consequences and was "associated with diseases of the liver (particularly with co-existing hepatitis C), lungs, heart, and vasculature". The authors cautioned that "evidence is needed, and further research should be considered, to prove causal associations of marijuana with many physical health conditions".

Memory and intelligence

Long term exposure to cannabis poses a risk of irreversible cognitive impairment in children and adolescents; other than for the very highest of doses, no similar risk has been established for adults. Changes in attention, psychomotor task ability, and short-term memory are associated with very recent (12 to 24 hours) marijuana use. Any long-term central nervous system effects of the residual drug are indistinguishable from variations in the user's susceptibility, or any pre-existing psychiatric disorder.

Dependency

Cannabis is the most widely used illicit drug in the Western world, and in the US 10 to 20% of consumers who use cannabis daily become dependent. Cannabis use disorder is defined in the fifth revision of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) as a condition requiring treatment. A 2012 review of cannabis use and dependency in the US by Danovitch et al said that in the US, "42% of persons over age 12 have used cannabis at least once in their lifetime, 11.5% have used within the past year, and 1.8% have met diagnostic criteria for cannabis abuse or dependence within the past year. Among individuals who have ever used cannabis, conditional dependence (the proportion who go on to develop dependence) is 9%." Although no medication is known to be effective in combating dependency, combinations of psychotherapy such as cognitive behavioural therapy and motivational enhancement therapy have achieved some success.

Mental health

Cannabis use is not free of risk. Although cannabis is not believed to cause psychosis alone, it may be a contributory factor, particularly when combined with an existing susceptibility. Higher levels and frequencies of use, and exposure from an early age carry the highest risk of developing or exacerbating psychiatric disorders. It is likely that the two principal active ingredients of cannabis, delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) have opposing effects, with CBD counteracting the adverse psychological effects of THC.

Acute psychosis

Although there has been an association noted between cases of acute psychosis and long-term cannabis use, the precise nature of the relationship is controversial; evidence suggests that cannabis use may worsen psychotic symptoms and increase the risk of relapse.

Chronic psychosis

There is no support for the hypothesis that cannabis causes cases of psychosis which would not have occurred otherwise. A risk does exist in some individuals with a predisposition to mental illness to develop symptoms of psychosis; the predisposing factors identified include genetic liability, childhood trauma and urban upbringing. Although most do not experience such problems, some cannabis users develop chronic psychosis with a risk directly related to high dosage, frequency of use and early age of introduction to the drug.

Schizophrenia

Among people with schizophrenia there is insufficient evidence to determine whether cannabis use leads to improvement or deterioration of the condition.

Use of cannabis in adolescence or earlier increases the risk of developing schizoaffective disorders in adult life, although the proportion of these cases is small. Susceptibility is most often found in users with at least one copy of the polymorphic COMT gene.

Cannabis with a high THC to CBD ratio produces a higher incidence of psychological effects, and CBD shows antipsychotic and neuroprotective properties, acting as an antagonist to some of the effects of THC.

Depressive disorder

Less attention has been given to the association between cannabis use and depression, though according to the Australian National Drug & Alcohol Research Centre, it is possible this is because cannabis users who have depression are less likely to access treatment than those with psychosis.

Teenage cannabis users show no difference from the general population in incidence of major depressive disorder (MDD), but an association exists between early exposure coupled with continued use into adult life and increased incidence of MDD in adulthood. Among cannabis users of all ages, there may be an increased risk of developing depression, with heavy users seemingly having a higher risk.

Suicidality

Adolescent cannabis users show no difference from their peers in suicidal ideation or rate of suicide attempts, but those who continue to use cannabis into adult life exhibit an increased incidence of both, although multiple other contributory factors are also implicated.

In the general population a weak (indirect) association appears to exist between suicidal behaviour and cannabis consumption in both psychotic and non-psychotic users, although it remains unclear whether regular cannabis use increases the risk of suicide. Cannabis use is a risk factor in suicidality, but suicide attempts are characterized by many additional risk factors including mood disorders, stress, personal problems and poor support.

Cognitive decline

Cannabinoids are strong antioxidants and therefore defend cells from β-amyloid, the peptide that causes Alzheimer's disease. Some studies have found that cannabis has no effect on ageing-related cognitive decline

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while others suggest that it slows cognitive decline through its antioxidant effect. The cannabinoids present in cannabis lessen cell damage and death from ischemia, likely due to their antioxidant properties.

Behavioral effects

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Government studies often point to statistical data accumulated by methods like the National Household Survey on Drug Abuse (NHSDA), the Monitoring the Future (MTF) study, and the Arrestee Drug Abuse Monitoring (ADAM) program, which claim lower school averages and higher dropout rates among users than non-users. However, these surveys are usually self-administered and may be anonymous, which greatly reduces their reliability. Additionally, while they establish a relationship between cannabis use and academic underperformance they do not determine whether the former causes the latter. The ADAM study is conducted anonymously, but only seeks information from a sample of people who have been arrested for drug-related offenses. Socially deviant behavior may be found more frequently in individuals of the criminal justice system compared to those in the general population, including non-users. In response, independent studies of college students have shown that there was no difference in grade point average, and achievement, between cannabis users and non-users. However, the users surveyed had slightly more difficulty deciding on career goals, and a smaller number were seeking advanced professional degrees.

A longitudinal study of heavy cannabis users from ages 14 to 25 in a Christchurch, New Zealand birth cohort concluded, "The results of the present study suggest that increasing cannabis use in late adolescence and early adulthood is associated with a range of adverse outcomes in later life. High levels of cannabis use are related to poorer educational outcomes, lower income, greater welfare dependence and unemployment and lower relationship and life satisfaction. The findings add to a growing body of knowledge regarding the adverse consequences of heavy cannabis use." However, this study primarily established correlation rather than causality.

A study published in the American Journal of Epidemiology in 2011, concluded that the prevalence of obesity is lower in cannabis users than in nonusers. A 2013 study confirmed this correlation and also found that cannabis users had better insulin resistance, lower insulin levels, and higher high-density lipoprotein ("good cholesterol") levels.

A 2008 study published in the British Journal of Psychiatry showed significant differences in Oxford-Liverpool Inventory of Feelings and Experiences scores between three groups: The first consisted of non-cannabis users, the second of users who tested positive for THC only, and the third consisted of users who tested positive for both THC and CBD. The Δ9-THC only subset scored significantly higher for unusual experiences, while users of both THC and CBD had much lower introvertive anhedonia scores. This suggests that CBD prevents some of the negative behavioral effects of THC.

Gateway drug hypothesis

The gateway drug hypothesis asserts that the use of cannabis may ultimately lead to the use of harder drugs.

Cancer

According to a 2013 literature review, marijuana could be carcinogenic, but there are methodological limitations in studies making it difficult to establish a link between marijuana use and cancer risk. The authors say that bladder cancer does seem to be linked to habitual marijuana use, and that there may be a risk for cancers of the head and neck among long-term (more than 20 years) users. Gordon and colleagues said, "there does appear to be an increased risk of cancer (particularly head and neck, lung, and bladder cancer) for those who use marijuana over a period of time, although what length of time that this risk increases is uncertain."

Glioma

A 2003 review of potential cannabinoid use to treat glioma said the "possible antitumor effects in patients have not been well established".

Testicular

In 2012 WebMD said that a number of studies had suggested a link between cannabis use and an increased risk of testicular cancer, but that the overall risk remained small and that more research is needed to confirm the findings. According to Gordon and colleagues, "several recent studies suggest an association between marijuana use and testicular germ cell tumors".

Lung

Gordon and colleagues in a 2013 literature review said: "Unfortunately, methodological limitations in many of the reviewed studies, including selection bias, small sample size, limited generalizability, and lack of adjustment for tobacco smoking, may limit the ability to attribute cancer risk solely to marijuana use." Reviewing studies adjusted for age and tobacco use, they said there was a risk of lung cancer even after adjusting for tobacco use, but that the period of time over which the risk increases is uncertain.

Cannabis smoke contains thousands of organic and inorganic chemicals, including many of the same carcinogens as tobacco smoke. A 2012 literature review by the British Lung Foundation suggested that the risk of developing lung cancer is nearly 20 times higher from smoking typical cannabis cigarettes than from smoking tobacco cigarettes, due to deeper, longer inhalation and the lack of filters. They identified cannabis smoke as a carcinogen and also said awareness of the danger was low compared with the high awareness of the dangers of smoking tobacco particularly among younger users. They said there was an increased risk from each cannabis cigarette due to drawing in large puffs of smoke and holding them.

In 2013 the International Lung Cancer Consortium found no significant additional lung cancer risk in tobacco users who also smoked cannabis. Nor did they find an increased risk in cannabis smokers who did not use tobacco. They concluded that "Our pooled results showed no significant association between the intensity, duration, or cumulative consumption of cannabis smoke and the risk of lung cancer overall or in never smokers." They cautioned that "Our results cannot preclude the possibility that cannabis may exhibit an association with lung cancer risk at extremely high dosage."

Head and neck cancer

At this point the majority of studies do not support the hypothesis that smoked cannabis is strongly associated with an increased risk of head and neck cancer once tobacco and alcohol intake are adjusted for. Gordon and colleagues (2013) said there was a risk of these cancers associated with marijuana use over a long period of time.

Respiratory effects

According to a 2013 literature review by Gordon and colleagues, inhaled marijuana is associated with lung disease.

Of the various methods of cannabis consumption, smoking is considered the most harmful; the inhalation of smoke from organic materials can cause various health problems (e.g., coughing and sputum). Isoprenes help to modulate and slow down reaction rates, contributing to the significantly differing qualities of partial combustion products from various sources.

A 2012 literature review by the British Lung Foundation found lack of research on the effect of cannabis smoke alone due to common mixing of cannabis and tobacco and frequent cigarette smoking by cannabis users; a low rate of addiction compared to tobacco; and an episodic nature of cannabis use compared to steady frequent smoking of tobacco.

Reproductive and endocrine effects

Cannabis consumption in pregnancy is associated with restrictions in growth of the fetus, miscarriage, and cognitive deficits in offspring. Although the majority of research has concentrated on the adverse effects of alcohol, there is now evidence that prenatal exposure to cannabis has serious effects on the developing brain and is associated with "deficits in language, attention, areas of cognitive performance, and delinquent behavior in adolescence". A report prepared for the Australian National Council on Drugs concluded cannabis and other cannabinoids are contraindicated in pregnancy as it may interact with the endocannabinoid system.

Mortality

No fatal overdoses associated with cannabis use have been reported. The evidence is insufficient to show an elevated risk of mortality from all causes (including suicide) among cannabis users, although some studies imply that motor vehicle fatalities, and possibly respiratory and brain cancers, may have a higher risk among heavy users.

See also

• Medical cannabis
• Effects of cannabis
• Cannabinoid hyperemesis syndrome

References

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• Cannabis