The long-term effects of cannabis have been the subject of ongoing debate. Given that the use of cannabis is illegal in most countries, clinical research presents a challenge and there is limited evidence from which to draw conclusions. In 2017, the U.S. National Academies of Sciences, Engineering, and Medicine issued a report summarizing much of the published literature on health effects of cannabis, into categories regarded as conclusive, substantial, moderate, limited and of no or insufficient evidence to support an association with a particular outcome.
Dependency
Cannabis is the most widely used illicit drug in the Western world. In the United States, 10-20% of those who begin the use of cannabis daily will later become dependent. In 2007 there were an estimated 143-190 million cannabis users around the world, making cannabis the most used illicit drug in the world. Cannabis use can lead to addiction, which is defined as "when the person cannot stop using the drug even though it interferes with many aspects of his or her life." 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 academic review, published in partnership with Canopy Growth, discussed the limitations of current studies of therapeutic and non-therapeutic cannabis use, and further stated that the nature of dependence formation among regular marijuana consumers has declined since 2002.
Cambridge University published a study in 2015 that showed in England and Wales, the use of cannabis had decreased. Although there was a reported decrease in use, the need for addiction treatment was surging. The study looked more in depth on how the potency of the cannabis affected someone's dependence on the drug. They tested three different levels of potency and found that the most potent cannabis had the highest amount of dependence. Researchers believe that this is because of the high that the participants felt after using. The lower potency strains did not give users the same high, which made them not desire or in turn depend on that strain as much.
Memory and intelligence Acute cannabis intoxication has been shown to negatively affect attention, psychomotor task ability, and short-term memory, impairing motor coordination and operative tasks as well as the capability people have to learn and recall information. Studies of chronic cannabis users have demonstrated, although inconsistently, a long-lasting effect on the attention span, memory function, and cognitive abilities of moderate-dose, long-term users. Once cannabis use is discontinued for several months, these effects disappear, unless the user started consuming during adolescence. It is speculated that this is due to neurotoxic effects of cannabis interfering with critical brain development.
Chronic use of cannabis during adolescence, a time when the brain is still developing, is correlated in the long term with lower IQ and cognitive deficits. It is not clear, though, if cannabis use causes the problems or if the causality is in the reverse. Recent studies have shown that IQ deficits existed in some subjects before chronic cannabis use, suggesting that lower IQ may instead be a risk factor for cannabis addiction.
A prospective cohort study that took place between 1972 and 2012 investigated the association between cannabis use and neuropsychological decline. Subjects were tested at various points in their life administering multiple different neuropsychological tests. The authors concluded that: persistent cannabis users show neuropsychological decline from childhood to midlife. neuropsychological impact of cannabis use is global instead of restricted to specific cognitive domains its effects last for longer than a week the findings cannot be explained by comorbidity with dependence on other drugs. the findings cannot be explained by comorbidity with schizophrenia cannabis use is correlated with lower levels of education the negative effect on intelligence is greater than that attributable to the lack of education. ceasing consumption does not fully restore cognitive function on adolescents. Cannabis intoxication was found to not only affect attention, psychomotor task ability, and short-term memory, but also that intoxicated users were having false memories.
The use of cannabis has been heavily shown to affect the working memory network function. Having recently used a large amount of cannabis is associated with hyperactivity of the network during a working memory task. Most of these findings show that people who use cannabis on a daily basis will need to take additional efforts in order to perform certain tasks.
Mental health
Cannabis contains over 100 different cannabinoid compounds, many of which have displayed psychoactive effects. The most distinguished cannabinoids are tetrahydrocannabinol (THC) and cannabidiol (CBD), with THC being the primary psychoactive agent. The effects of THC and CBD are salient regarding psychosis and anxiety.
Chronic psychosis and schizophrenia spectrum disorders
According to the National Academies of Sciences, Engineering and Medicine, there is substantial evidence of a statistical association between cannabis use and the development of schizophrenia or other chronic psychoses, with the highest risk potentially among the most frequent users. A possible connection between psychosis and cannabis is controversial because observational studies suggest a correlation but do not establish any causative effect of cannabis on long-term psychiatric health. 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 a causal link could not be proven by the available data. The risks appear to be most acute in adolescent users. In one 2013 review, the authors concluded long-term cannabis use "increases the risk of psychosis in people with certain genetic or environmental vulnerabilities", but does not cause psychosis. Important predisposing factors were genetic liability, childhood trauma and urban upbringing. Another review that same year concluded that cannabis use may cause permanent psychological disorders in some users such as cognitive impairment, anxiety, paranoia, and increased risks of psychosis. Key predisposing variables included age of first exposure, frequency of use, the potency of the cannabis used, and individual susceptibility. Nevertheless, some researchers maintain there exists "a strong association" between schizophrenia and cannabis use, while cannabis use alone does not predict the transition to subsequent psychiatric illness. Many factors are involved, including genetics, environment, time period of initiation and duration of cannabis use, underlying psychiatric pathology that preceded drug use, and combined use of other psychoactive drugs.
The temporal relationship between cannabis and psychosis was reviewed in 2014, and the authors proposed that "[b]ecause longitudinal work indicates that cannabis use precedes psychotic symptoms, it seems reasonable to assume a causal relationship" between cannabis and psychosis, but that "more work is needed to address the possibility of gene-environment correlation."
In 2016 a meta-analysis was published on associations studies covering a range of dosing habits, again showing that cannabis use is associated with a significantly increased risk of psychosis, and alleged that a dose–response relationship exists between the level of cannabis use and risk of psychosis. The risk was increased 4-fold with daily use, though the analysis was not adequate to establish a causal link. Another 2016 meta-analysis found that cannabis use only predicted transition to psychosis among those who met the criteria for abuse of or dependence on the drug.
Another 2016 review concluded that the existing evidence did not show that cannabis caused psychosis, but rather that early or heavy cannabis use were among many factors more likely to be found in those at risk of developing psychosis. An opposing view was expressed in 2016, in a study that regarded the epidemiologic evidence on cannabis use and psychosis was strong enough "to warrant a public health message that cannabis use can increase the risk of psychotic disorders," but also cautioning that additional studies are needed to determine the size of the effect. Such a public health message was subsequently issued in August 2019 by the Surgeon General of the United States. The review also stated "If the association between cannabis and schizophrenia is causal and of the magnitude estimated across studies to date, this would equate to a schizophrenia lifetime risk of approximately 2% in regular cannabis users (though risk for broader psychotic outcomes will be greater). This implies that about 98% of regular cannabis users will not develop schizophrenia...[and that] risk could be much greater in those at a higher genetic risk, or in those who use particularly potent strains of cannabis. Expressed in terms of odds ratio, another study found that "Daily cannabis use was associated with increased odds of psychotic disorder compared with never users (adjusted odds ratio [OR] 3.2, 95% CI 2.2–4.1), increasing to nearly five-times increased odds for daily use of high-potency types of cannabis (4.8, 2.5–6.3)." To calculate what the increased odds ratio means for schizophrenia specifically, a 2005 review placed the lifetime morbid risk of narrowly defined schizophrenia at 0.72%. For some locations, this translates into a substantial population attributable risk, such that "assuming causality, if high-potency cannabis types were no longer available, then 12% of cases of first-episode psychosis could be prevented across Europe, rising to 30% in London and 50% in Amsterdam."
A 2019 meta-analysis found that 34% of people with cannabis-induced psychosis transitioned to schizophrenia. This was found to be comparatively higher than hallucinogens (26%) and amphetamines (22%).
However, a 2004 study noted that general population statistics show no increase in psychosis incidence rates in any developed country over the last 50 years, despite a five-fold increase in cannabis use rates. The study noted that "cannabis use appears to have increased substantially amongst young people over the past 30 years, from around 10% reporting ever use in 1969–70, to around 50% reporting ever use in 2001, in Britain and Sweden. If the relation between use and schizophrenia were truly causal and if the relative risk was around five-fold then the incidence of schizophrenia should have more than doubled since 1970. However population trends in schizophrenia incidence suggest that incidence has either been stable or slightly decreased over the relevant time period."