Weed and psychosis: A closer look at the link to schizophrenia

During recent decades, countries have approached cannabis regulation in different ways. Some, such as the US, have relaxed their laws. In the UK, the government upgraded marijuana from a class C drug to class B in 2009.

Many in the UK believe that laws should be relaxed, arguing that cannabis is relatively harmless.

However, many people disagree. They maintain that cannabis addiction is real and debilitating and point to the existence of mental health issues and withdrawal symptoms associated with cannabis addiction as evidence. These withdrawal symptoms include anxiety, irritability, anger or aggression, disturbed sleep, depression and loss of appetite.

The substance at the centre of this controversy is THC, the main psychoactive component of marijuana. Exposure to THC is the primary subject of study when linking cannabis use to mental health problems such as schizophrenia.



Cannabis psychosis and schizophrenia

The relationship between THC, psychosis and schizophrenia is extremely complex.

This complexity is compounded by the fact that cannabis psychosis, psychosis in general and schizophrenia are actually three separate but overlapping things.


Psychosis is a collection of symptoms that affect the mind. They are:

Hallucinations: Seeing and hearing things that aren’t real
Delusions: Strong beliefs that aren’t shared by others
Disordered thinking and speaking: Thoughts and ideas come thick and fast, making it difficult for a person to be understood.

The element that ties these symptoms together is a loss of contact with reality.

Psychosis is associated with several mental health conditions, including major depressive disorder, bipolar disorder and schizophrenia – and also with substance use, including cannabis.

Cannabis psychosis

Cannabis psychosis falls under the umbrella of Substance Induced Psychotic Disorders – disorders caused by taking a drug – in the DSM. It shares all of its symptoms with the broader category of psychosis.

Cannabis psychosis differs from the broader category of psychosis in two ways. It is caused by a substance and not a pre-existing condition, and it has a shorter duration. If cannabis psychosis lasts for more than one month, then mental health professionals are advised to reevaluate its cause and consider an independent psychotic disorder, such as pre-existing schizophrenia.

In other words, once a person has addressed their cannabis addiction or gone through detox, psychosis should cease. This can take a few hours or a few weeks, but the withdrawal symptoms should stop when the detox process is over. If it doesn’t, another cause may be at play.


Schizophrenia is a psychotic disorder. Psychosis is one of its major symptoms, but it’s not the only one. Others include diminished emotional expression, catatonia and disorganised behaviour. It also differs in duration – signs of disturbance need to be present for at least 6 months for a diagnosis. Crucially, the DSM makes this distinction:

‘The disturbance is not due to the direct physiological effects of a substance (e.g., a drug of abuse, a medication) or a general medical condition.’

Schizophrenia and marijuana – who’s right?

There are two schools of thought surrounding cannabis and schizophrenia.

One argues that cannabis psychosis is a separate issue from schizophrenia, and people who develop schizophrenia after marijuana usage have a genetic predisposition, meaning they would have developed the condition anyway but later in life.

The other argues that these people may never have developed schizophrenia if they hadn’t been exposed to marijuana, that cannabis products, particularly high-strength ones, are associated with people developing schizophrenia at a younger age, and that cannabis addiction can cause schizophrenia.

The implications for public health on both sides of the argument are huge, which is why it’s crucial that we follow the science on this issue.

The link between cannabis and psychosis

A study by The Lancet examined whether consuming cannabis products higher in THC and at higher frequencies was more likely to cause psychosis.

High-strength THC products, such as extracts, tinctures and gummies that far exceed the strength that plant-based marijuana can reach, have never been easier to access, particularly in the US. If consuming these high-strength products more often is associated with a greater incidence of psychosis, then it’s a strong argument in favour of cannabis addiction having negative effects on mental health.

The study’s results are clear. ‘Daily cannabis use was associated with increased odds of psychotic disorder compared with never users, increasing to nearly five-times increased odds for daily use of high-potency types of cannabis.’

In other words, the more frequently THC is used and the higher the strength, the more likely it is that the user develops psychosis.

Crucially, this study distinguishes between temporary cannabis psychosis and the development of psychotic disorders that affect users over a longer period of time and found a positive link with longer-term psychosis.

However, the study did not examine whether cannabis causes schizophrenia, which is one further step along the journey.



So, does cannabis cause schizophrenia?

America began to relax its cannabis laws in 2012. This means that cannabis is now more easily available and at a much higher strength.

We’ve already seen evidence that higher-strength cannabis products, consumed more frequently, increase incidences of psychosis. So, it would be extremely useful to know if incidences of schizophrenia, not just psychosis, have been increasing in America since 2012. While it would not prove that cannabis addiction causes schizophrenia, it would be a huge clue.

Unfortunately, this line of inquiry is currently a dead end. America’s National Institute for Mental Health ‘has not undertaken a serious study of the incidence or prevalence of schizophrenia or other major mental illnesses in 40 years.’

A Danish study provides another piece of evidence. By studying health data between 1972-2021, they found that people with a history of cannabis use disorder had a higher likelihood of developing schizophrenia, and this was more pronounced in men. They also found that age mattered – young men aged 16-20 with cannabis use disorder were four times more likely to develop schizophrenia, and young women were twice as likely.

Strong evidence, but no smoking gun

This is strong correlative evidence – but it does not definitively prove causation. Proving the link between cannabis and schizophrenia is made difficult by ethical considerations. Scientists can’t simply give people marijuana in a controlled environment and measure whether they develop schizophrenia – that would be dangerous and grossly unethical.

Schizophrenia has many potential causes, both environmental and genetic, and studying these in isolation is not possible. However, we do have strong evidence that cannabis usage is associated with an increased risk of psychotic disorders in general and evidence that on a population level, the incidence of schizophrenia is positively correlated with cannabis addiction.

This alone means that for many, particularly people with a family history of mental health problems, people consuming stronger THC products, people consuming cannabis very frequently, or people who started consuming cannabis at a young age, cannabis addiction is risky. For those struggling with cannabis addiction, getting help is crucial.

Detox and the road ahead

If you or a loved one are struggling with cannabis addiction or experiencing withdrawal symptoms, help is available. Supported recovery increases your odds of success in tackling problem cannabis usage.

A supportive, safe and constructive recovery is within reach. Contact UKAT today for help.

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