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The long-term effects of cannabis have been the subject of ongoing debate. Due to cannabis being illegal in most countries, research presents a challenge; as such, there remains much to be concluded.

Memory and intelligence
A 2016 review found that chronic use of cannabis during adolescence, a time when the brain is still developing, was correlated in the long term with lower IQ and chronic cognitive deficits, but it was not clear if chronic use caused the problems or if "persons who have poorer cognitive functioning may be more vulnerable to cannabis use and abuse." A 2013 review had similar findings. A recent large longitudinal twin study found little evidence that adolescent cannabis use causes IQ decline. Previous findings of a detrimental effect were likely due to confounding factors that influence both substance initiation and IQ, however the cause of the declined IQ pointed towards the marijuana initiation and low intellectual attainment, rather than the amount and frequency of marijuana consumption. A 2018 review of marijuana on memory found that there is no difference in the encoding process of memory, however there was a delay in the activation of the hippocampus in cannabis consumers. The study showed that marijuana caused a dysfunction in the prefrontal and the hippocampal area of the brain, impairing memory.

Dependency
Cannabis is the most widely used illicit drug in the Western world, and although in the United States 10 to 20% of consumers who use cannabis daily become dependent, it is different from addiction. 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 United States by Danovitch et al said that "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.

Cannabis dependence develops in 9% of users, significantly less than that of heroin, cocaine, alcohol, and prescribed anxiolytics, but slightly higher than that for psilocybin, mescaline, or LSD. Dependence on cannabis tends to be less severe than that observed with cocaine, opiates, and alcohol. A 2018 review of the nature of dependency on marijuana states that the risk of dependence formation among regular marijuana consumers has declined since 2002.

Mental health
Historically, the possible connection between psychosis and cannabis has been seen as controversial. Recent medical evidence strongly suggests that the long-term use of cannabis by people who begin use at an early age display a higher tendency towards mental health problems and other physical and development disorders, although the causal link is not yet definitively established. These risks appear to be most acute in adolescent users.

There have been numerous reports and analyses that suggest that different cannabis strains have consequences that differ due to the numerous agents that underlie the psycho-activity of cannabis. Cannabis can contain over 100 different cannabinoid compounds, many of which have displayed psychoactive effects. The most distinguished cannabinoids are ∆9 – THC (THC) and cannabidiol (CBD), with THC being the primary agent responsible for the psychoactivity of cannabis. The effects of THC and CBD are salient regarding psychosis and anxiety.

Anxiety
Numerous studies have been conducted regarding anxiety and cannabis for therapeutic purposes, and these cross-sectional studies have been mostly consistent regarding the anxiolytic effects. The studies showed relief of anxiety as a result of cannabis consumption.

Depressive disorder
Less attention has been given to the association between cannabis use and depression, though according to the Australian National Drug & Alcohol Research Center, it is possible this is because cannabis users who have depression are less likely to access treatment than those with psychosis. A 2017 review suggest that cannabis has been shown to improve the mood of depression diagnosed patients.

Suicidal behaviour
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.

Numerous studies have investigated the role of marijuana and prevent risk of harm to others, however finding very inconsistent results. Some research suggest that cannabis is positively associated with violence due to the alteration in the cognitive functioning, the consequences of withdrawal , and its associations with deviance and risk-taking behavior , whilst other research suggests that there is no association between cannabis and violence because of the quieting and sedative effects, resulting in a reduction of irrritability, hostility and violent expectancies.

Gateway drug hypothesis
The gateway drug hypothesis asserts that the use of soft drugs such as cannabis, tobacco or alcohol may ultimately lead to the use of harder drugs. This hypothesis is however very controversial, as it is difficult to determine the reasoning behind the consumption of harder drugs. Studies have shown that 45% of cannabis users find that it is too difficult to categorize marijuana as a gateway or non-gateway drug, 35% feel that it is a gateway drug and 20% support the notion.

Large-scale longitudinal studies in the UK and New Zealand from 2015 and 2017 showed an association between cannabis use and an increased probability of later disorders in the use of other drugs.