Weed is one of the most widely used drugs, with 7.6% of adults reporting having used the drug in the last year, 30% saying they had used it at least once in their lifetime, and likely more who do not admit it (ONS, 2022). For perspective, in the last twelve months, 3.3% report using a Class A drug, and 9.5% report using any drug. The ongoing political debate about weed legalisation has encouraged more research into its potential harms. However, its status as class B drug slows the progress of research, leaving the long-term effects of cannabis consumption somewhat unclear.

From a neuroscientific perspective, concerns revolve around the high distribution of cannabinoid receptors in the brain, which are important neurotransmitters. These receptors develop more intensely during adolescence. Regular use of weed, and the psychoactive effects caused by tetrahydrocannabinol (THC), interact with these cannabinoid receptors, and as such may significantly impact neurodevelopment, particularly during adolescence.

It is important to mention that empirical studies into the consequences of drug use use group means and other such group-based statistical measures to draw conclusions. As a result, the evidence below is applicable on a broad level, but not on an individual level, and each person will experience their own specific interactions with weed depending on all sorts of variables. This evidence is also largely focussed on the direct neurological impacts of cannabis use, and their are other important elements in its consumption, ranging from the social to the economic to the cultural.

            The Evidence

Prenatal Exposure

Robust evidence suggests that children exposed to cannabis in the womb have significantly impaired neurodevelopmental and behavioural outcomes that persist into adulthood when compared to controls that are not exposed (Badowski, 2020).

Cognitive Impairment

A large body of research links cannabis use to cognitive impairment. Meta-analyses reveal that frequent users perform worse on cognitive tasks than infrequent or non-users, especially in learning, working memory, attention, executive functioning, inhibition, and decision making (Meier, 2024). Frequent use generally refers to consuming cannabis more than once a week over periods of months or years. Twin studies, which control for genetic differences, often, but not always, show that frequent and long-term cannabis use is associated with cognitive impairments (Jackson, 2016). However, evidence also suggest that long-term abstinence can lead to the recovery of these cognitive functions due to neuroplasticity, although this depends on the frequency and severity of use, as well as the age of onset.

Age-Related Effects

The impact of cannabis use varies with age. Data on adolescents is limited, but daily use has been shown to pose risks to full psychosocial development. There are increased risks of psychoses, depression and anxiety disorders, cognitive impairments, early school leaving, other drug use, and in conjunction with mental illnesses, violence, with the rate of risk roughly correlating to frequency/severity of use, as well as age of onset. Interestingly, the largest risk of cannabis use amongst adolescents are car crashes when driving while intoxicated (NASEM, 2017).

Older adult users perform better on cognitive tasks than younger adult users, although it is unclear whether this is due to higher tolerance developed over longer histories of use, or a more inherent decreased impact of THC on cognition. Significant IQ decline is mostly seen in those who start using cannabis during adolescence and continue long-term frequent use (Meier, 2012).

White Matter

Research indicates that cannabis users experience poorer integrity of  white matter fibre tracts compared to non-users. (Robinson, 2023). White matter damage, also a major harm of alcohol, affects the connectivity between brain regions, although current research is unclear about the specific causal impact of cannabis.

Animal Studies

Studies involving THC exposure in rats show a slight negative impact on cognitive and affective behaviour regulation (Silveira, 2017).

Weed and Mental Illnesses

The perception of cannabis as a tool for self-medication is common amongst those with mental illnesses. Many users describe using weed ‘medically’ rather than ‘recreationally’, with pain (61.2%), anxiety (58.1%), and depression (50.3%) quoted as the most commonly stated reasons (Sexton, 2016). However, although cannabis has been shown to alleviate the subjective experience of certain symptoms, particularly regarding pain and anxiety, the effects are modest.

A common reason for cannabis use is as a sleep aid, which is likely correlated with mental illness. However, while cannabis users do have increased expectations that it serves as a sleep aid, there is limited evidence supporting a positive relationship, and if anything, regular use predicts worse subjective sleep quality, both regarding smoking cannabis and eating edibles (Winiger, 2021).

Those with mental illnesses are at higher risk of developing cannabis use disorder (CUD). Similar to other drug use disorders, withdrawal from cannabis encourages further use following the positive sensations from alleviating the symptoms of withdrawal, otherwise known as the ‘rebound effect’. Over time, the body adapts to maintain allostasis, leading to more negative effects during withdrawal, the increased perception of a necessity of use, which may progress to full addiction.

A Gateway Drug?

A classic concerns regarding cannabis is that it is a ‘gateway drug’. A recent review found that there is moderate evidence of an association between cannabis use and the development of substance dependence, but little to no evidence of an association between cannabis use and changes in use patterns of other illicit substances (National Academies of Science, 2017). Again, this depends on the dose and frequency of use, as well as the age of starting. Interestingly, it has been shown that nicotine users are more likely to progress to other drug use, suggesting cigarettes might be a greater gateway drug than cannabis.

            Limitations and Summary

The fundamental flaw of human drug studies is isolating the specific effects of cannabis. Correlating marijuana use with negative outcomes involves comparing users with non-users, who may differ in other traits (e.g., other drug use, varying lifestyles, mental illness, neurobiological differences). If someone is predisposed to impulsive behaviours or anxiety, both of which correlate with weed use, is it the predispositions or the weed that result in later negative outcomes? While the science suggests various negative effects of cannabis use, the direct causal connection is not as clear-cut as these correlations may seem.

In summary, the majority of the data suggests that the regular use of marijuana is linked to cognitive impairments, particularly in attention and memory, respiratory issues if smoked, the exacerbation of mental illnesses, particularly regarding psychosis, the development of CUD and substance addiction, and a few others. However, the extent of these links depends on consumption frequency and quantity, prior psychological issues, and the age of onset – particularly detrimental if regular use starts before age 17. Encouragingly, cognitive impairments may resolve to normal levels following long-term abstinence.