Overview

In the UK, cannabis is a Class B drug. By the end of 2023, 31.1% of 16-59-year-olds in England and Wales reported having used it at least once in their lifetime, according to the Office for National Statistics. 

The cannabis plant is thought to have originated in central or southeast Asia approximately 11, 700 years ago. The first reports of its medicinal use are from China in 2000 BC.¹ 

Cannabis contains hundreds of compounds, including a type of compound known as a cannabinoid. The most prominent cannabinoids are delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD). THC is the main psychoactive component and is most heavily associated with cannabis addiction.² CBD is not psychoactive and is the main component of medical cannabis due to its anti-inflammatory and pain-relieving effects.³ 

Defining psychosis

What is psychosis?

The term ‘psychosis’ refers to a group of symptoms that alter the mind. Common symptoms include:

• Delusions - false beliefs, e.g., thinking that somebody is trying to hurt them
• Hallucinations - hearing or seeing things that aren’t there
• Sleep disruption
• Incoherent speech, trouble communicating

Psychosis can be a symptom of a mental illness or psychotic disorder.⁴

Types of psychotic disorders

Schizophrenia

Schizophrenia is a psychotic disorder which is thought to affect about 1% of the population, mainly characterised by positive symptoms (symptoms which are an excess of normal function) such as hallucinations and paranoid delusions, and negative symptoms (symptoms which are absent from normal functioning) such as a lack of motivation or personal hygiene. It can be very debilitating, with less than 14% of patients recovering within the first five years after experiencing a psychotic episode. In Europe, less than 20% of people with schizophrenia are employed. The causes of schizophrenia still are not fully understood, making it difficult to treat, but some drug treatments are available.⁵

One of the main theories about the cause of schizophrenia is that it results from excessive dopamine in the brain (also known as the Dopamine Hypothesis of Schizophrenia). Dopamine is a chemical messenger, or neurotransmitter, that is heavily associated with pleasure and motivation, components of the reward pathway. The Dopamine Hypothesis is supported by the fact that drugs which increase the amount of dopamine in the brain worsen symptoms such as hallucinations in schizophrenia patients, while drugs that inhibit dopamine have antipsychotic properties.⁶ 

Another theory behind the cause of schizophrenia is the Glutamate Hypothesis of Schizophrenia. Glutamate is a neurotransmitter which binds to a type of receptor known as an N-methyl-D-aspartate receptor (NMDAR). Studies have shown that antagonists (drugs which block receptors) of NMDARs induce schizophrenia-like symptoms in people who do not have a schizophrenia diagnosis. Therefore, it is thought that schizophrenia could be a result of there not being enough glutamate in the brain binding to NMDA receptors.⁷ 

Schizoaffective disorder

Schizoaffective disorder is different to schizophrenia in that patients with schizoaffective disorder have symptoms of schizophrenia, as well as emotional symptoms such as depression or mania. Its causes are unknown, but studies have suggested that, similarly to schizophrenia, it could be the result of an excess of dopamine in the brain. Other neurotransmitters, such as serotonin and noradrenaline may also play a role.⁸

The science behind cannabis and psychosis

There is a strong link between patients with psychotic disorders and regular cannabis use. The exact mechanisms through which cannabis use increases the risk of psychosis are unknown, but there are some theories. Results from a Swedish study showed that people who had tried cannabis by the age of 18 were 2.4 times more likely to be diagnosed with schizophrenia, and the risk of developing schizophrenia increase was directly proportional to the frequency of cannabis use.⁹ 

The use of cannabis alone is unlikely to cause schizophrenia but could contribute to the onset of schizophrenia when combined with other factors such as genetics and environment. A large amount of data suggests that cannabis worsens psychosis as opposed to causing it.¹⁰

The endocannabinoid system

The endocannabinoid system consists of cannabinoid receptors 1 and 2 (CB1 and CB2). Endocannabinoids are molecules similar to cannabinoids but are made by the body. They bind to CB receptors and are important in short-term memory and neural (nervous system) development.¹⁰

The role of THC

When cannabis is ingested or smoked, the cannabinoid THC is released and binds to CB receptors, producing psychotomimetic effects (symptoms similar to psychosis), impaired memory, and altered perception of events. Studies have also suggested that THC increases the amount of dopamine in the brain as well.¹⁰ 

Psychosis warning signs

According to the NHS, some early warning signs that you or somebody you know are experiencing symptoms of psychosis include:

• Experiencing hallucinations or delusions
• Confused or disrupted thought patterns
• Disturbed speech, e.g., suddenly changing topics mid-sentence

UK support hotlines and websites

The NHS

If you or somebody you know needs support for a drug addiction or symptoms of psychosis, you can seek help from a GP, who will be able to offer treatment or refer you to a local drug service.

Frank

You can access the Frank website to find a local drug treatment service yourself, or you can ring their helpline on 0300 123 660.

Adfam

Adfam is a charity in England who supporta families and provides support to those struggling with drug addiction.

Samaritans

Samaritans are available 24/7 to provide support for anybody experiencing mental health problems. Access their website or ring them at 116 123. They also have a Welsh language line on 0808 163 0123.

Rethink mental illness

Rethink Mental Illness provides information and support for anybody affected by mental health problems.

Hearing voices network

Hearing Voices provides information and support for those experiencing hallucinations or altered perceptions.

Summary

There is a positive correlation between regular cannabis users and the diagnosis of psychotic disorders such as schizophrenia and schizoaffective disorder, especially in people who have been using cannabis since adolescence. One of the main hypotheses behind the cause of psychosis is that it is due to an excess of dopamine in the brain. Cannabis contains a cannabinoid called THC which increases the amount of dopamine and binds to cannabinoid receptors in the brain, bringing about symptoms of psychosis in otherwise healthy patients.