Unpacking the complex relationship between cannabis and mental health

Cannabis has long occupied a contentious space in conversations about health and society.

Once dismissed solely as an illegal recreational drug¹, cannabis is now increasingly recognised for its nuanced impact on the brain and body – although, both clinically and legally, the position on cannabis use is still an area of contention²⁻³.

In mental health, however, cannabis presents a paradox. Some individuals might report short-term relief from symptoms such as anxiety, low mood, or insomnia, while others can experience negative effects such as heightened vulnerability to conditions like psychosis, depression, or cognitive decline⁴⁻⁵.

Understanding this complexity requires a move beyond the polarising narratives of “harmless herb” versus “dangerous drug.”⁶ Instead, the clinical community is now focused on examining how cannabis interacts with neurobiology, psychology, and social context. Patterns of use, potency, and individual predispositions can dramatically alter outcomes – and so, recognising both the risks and the potential therapeutic applications is the best platform from which to explore this evolving field.

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The neurobiology and chemistry of cannabis

Cannabis exerts its effects primarily through the endocannabinoid system (ECS), a widespread network of receptors, neurotransmitters, and enzymes that helps regulate⁷: 

• Mood
• Memory
• Appetite
• Sleep
• Stress responses

Two main compounds in cannabis – tetrahydrocannabinol (THC) and cannabidiol (CBD) – interact with this system in distinct ways, shaping both its potential benefits and its risks⁸.

THC and the brain’s reward pathways

THC is the psychoactive component most responsible for the ‘high’ associated with cannabis use.⁹ 

It binds to CB1 receptors in the brain, particularly in regions such as the prefrontal cortex, hippocampus, and basal ganglia¹⁰. This activation influences dopamine release, which explains the short-term sense of euphoria or relaxation10. 

However, repeated exposure can disrupt natural dopamine regulation, increasing vulnerability to dependence, mood instability, and cognitive impairments¹¹.

CBD and modulation of effects

In contrast, CBD does not produce intoxication – and may even counteract some of THC’s effects¹²⁻¹³. Research suggests CBD interacts indirectly with serotonin receptors (5-HT1A) and influences GABA and glutamate signalling, contributing to potential anxiolytic and antipsychotic properties¹⁴. 

This difference explains why cannabis strains with higher CBD-to-THC ratios may produce a milder psychological impact and are being investigated for potential therapeutic use in conditions such as anxiety disorders and epilepsy¹⁵.

Furthermore, it’s worth noting that the developing brain appears especially sensitive to cannabis exposure.16-17Adolescents that regularly use high-potency cannabis  may experience altered synaptic pruning and reduced white matter integrity, increasing risks for long-term cognitive decline and psychotic disorders¹⁶⁻¹⁷. Neurobiological vulnerability is also shaped by genetics, with certain gene variants (e.g., COMT, AKT1) linked to a heightened risk of psychosis following cannabis use¹⁸.

Negative mental health effects of cannabis

Clinical and scientific communities agree that cannabis use is far from risk-free. 

While its cultural image often leans toward relaxation or harmless recreation, research consistently highlights a spectrum of potential harms to mental wellbeing¹⁹⁻²⁰. These risks vary depending on potency, frequency, age of first use, and genetic vulnerability, but they can be profound, ranging from transient mood disturbance to the onset of severe psychiatric illness.

Psychotic disorders and psychosis risk

One of the most consistent findings in cannabis research is its association with psychosis²¹. High-potency cannabis, particularly when used frequently and from a young age, has been shown to increase the risk of developing psychotic disorders such as schizophrenia²². 

This is since THC interacts with dopamine pathways that are already implicated in psychosis, and in vulnerable individuals this can sometimes trigger or exacerbate symptoms like hallucinations, delusions, and disordered thinking²³. 

While cannabis does not cause psychosis in every user, genetic predisposition and early-life factors appear to magnify risk²⁴⁻²⁵. For those with a family history of schizophrenia or related disorders, cannabis can act as a potent trigger (or a “mediator” between early trauma and first-episode psychosis) that accelerates the onset or worsens its trajectory.

Mood, anxiety, and cognitive vulnerability

Beyond psychosis, cannabis use has complex and often detrimental effects on mood and anxiety regulation. While some individuals report short-term relief from stress or depressive symptoms²⁶ – which is supported consistently by research findings²⁷ – chronic or heavy use is strongly associated (especially at higher doses of THC) with increased rates of depression, generalised anxiety, and panic disorders.²⁸

The neurochemical disruption caused by prolonged THC exposure can undermine the brain’s natural balance of serotonin and dopamine, creating cycles of dependency and emotional instability²⁹. 

Cognitive domains such as attention, memory, and executive function are also at risk, with research suggesting that persistent cannabis use may impair learning and decision-making over time³⁰. 

The potential therapeutic mental health effects of cannabis

Alongside concerns about harm, cannabis and its active compounds have attracted increasing interest in psychiatric research for their possible therapeutic benefits.³¹ Much of this focus centres on cannabinoids such as cannabidiol (CBD), which appears to act differently from tetrahydrocannabinol (THC) and may carry protective or stabilising effects³².

• Some studies suggest that CBD may help reduce anxiety symptoms, dampen overactive stress responses, and even hold promise in managing certain forms of post-traumatic stress disorder (PTSD)³³⁻³⁵. 
• Emerging evidence also points to a potential role in sleep regulation and as an adjunct in addiction treatment, where CBD could help reduce cravings and withdrawal severity³⁶⁻³⁷.
• There is also interest in whether cannabinoids might ease depressive symptoms or modulate psychotic processes, though findings are mixed and heavily dependent on dose, composition, and patient profile³⁸⁻³⁹.

Despite encouraging signals, the research base remains nascent and uneven, with many studies limited by small samples, short duration, or methodological variability⁴⁰. 

For now, cannabis-based interventions in mental health should be seen as experimental adjuncts rather than established treatments, warranting careful clinical oversight and rigorous further investigation⁴¹.

Clinical landscape: use patterns and vulnerability

In the UK, cannabis remains the most widely used illicit drug⁴², with around one in eleven adults reporting use in the year ending 2020⁴³. While occasional or recreational consumption is relatively common among younger adults, a smaller subset of people develop patterns of frequent or heavy use that carry greater psychiatric risks. 

Daily or near-daily consumption, high-potency cannabis (sometimes referred to as “skunk”), and early initiation in adolescence are consistently linked with poorer mental health outcomes.⁴⁴

Vulnerability is not uniform. Individuals with a family history of psychosis, pre-existing anxiety or mood disorders, or those exposed to social stressors and deprivation appear more susceptible to harm⁴⁵. 

These risks are compounded by a shifting cannabis market in the UK, where higher-THC products are now more prevalent than they were a decade ago.⁴⁶

Risks, potential, and final reflections

The relationship between cannabis and mental health resists binary categorisation. On one side, the evidence linking frequent, high-potency use with increased vulnerability to psychosis, cognitive decline, and mood disturbances is compelling; on the other, carefully controlled research continues to explore potential therapeutic benefits, from alleviating anxiety and PTSD symptoms to supporting treatment-resistant depression.

What emerges is a picture of risk and potential coexisting, shaped by variables such as age of initiation, genetic predisposition, frequency of use, and product composition⁴⁷. 

Cannabis can be a source of harm for some and a possible avenue of relief for others, depending on the context.

As the clinical landscape evolves, with more studies probing mechanisms and outcomes, nuance remains essential⁴⁸. Neither demonisation nor uncritical optimism serves patients or practitioners well. 

Instead, recognising complexity, fostering open dialogue, and grounding decisions in the best available evidence allows for a more balanced engagement with cannabis and mental health.

Do you know someone showing signs of cannabis addiction? 

Harbor London can help individuals reclaim a life of fulfilment and wellbeing.

Make a referral today → 

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