Decoding Cannabis: Why schizophrenia is a red flag for medical cannabis

Most of the time, our Decoding Cannabis blog series focuses on the potential of medical cannabis by breaking down new research into accessible, easy-to-digest articles. The therapeutic efficacy that cannabinoids offer, the conditions they are being shown to help with, and how that help is being brought to light through emerging research. 

We have already focused on how cannabinoids help treat mental health conditions more generally, but any honest look at medical cannabis also needs to accept and discuss where the risks are too great.

Schizophrenia is one of those places.

To mark National Schizophrenia Awareness Day, we’re turning toward the evidence that warns us of real and measurable harm. Let’s be very clear from the very beginning - while medical cannabis has been shown to help with some of the symptoms of schizophrenia (anxiety, depression, poor sleep), the bigger picture shows that it is simply not a suitable treatment option for people with a history of, or potential genetic leaning towards, psychosis.

A landmark Danish study, titled “Uncovering metabolic dysregulation in schizophrenia and cannabis use disorder through untargeted plasma lipidomics”  and published in 2023, tracked more than 7 million people across five decades. It found that people with cannabis use disorder (CUD) significantly increased their risk of developing schizophrenia, with the strongest associations in young males.

This isn’t a grey area. There are no ifs or buts. Medical cannabis can not be prescribed to people with schizophrenia in the UK, period. 

Here’s the best available case study on why. 

But first…

What is cannabis use disorder (CUD), and is it something that Releaf patients need to worry about? 

Cannabis use disorder, or CUD, is the clinical term for what happens when cannabis use slips beyond one’s control. Far more common in recreational settings than when medical cannabis is prescribed by a specialist and with ongoing clinical oversight (and support), and especially where high-THC products are used frequently and without any kind of structure or clinical guidance. 

Markers of a potential CUD diagnosis are when people find themselves using cannabis more than they meant to, struggling to cut down, or continuing to use cannabis even when it begins to have a measurable effect on their health, relationships, or day-to-day life.

It’s not especially common, but it is real, and it’s worth being mindful of, particularly for those who may already be vulnerable.

At Releaf, every person who comes to us is screened carefully from the outset and offered a high level of clinical oversight throughout their treatment journey.

It starts with our medical cannabis eligibility checker, then moves into an initial consultation with a doctor who is listed on the GMC Specialist Register. This consultation lasts at least 30 minutes, and an extremely detailed conversation led by one of the doctors from Releaf’s world-class clinical team takes place. 

During this appointment, the specialist will review your medical history to understand your symptoms, goals, and anything in your background that might need extra care. That includes things like a family history of schizophrenia or of psychosis, where we take particular caution.

From there, everything is guided by clinical need. Releaf’s team of over 27 doctors work alongside more than 30 UK-based patient support specialists to make sure treatment is appropriate, regularly reviewed, and never left to drift. Prescriptions are tailored, tracked, and adjusted when needed, and Releaf patients always able to check in or ask for extra support, or advice. 

Now let’s move on to the study and its findings.

What did the study focus on?

Drawing on five decades of health data from over 6.9 million people in Denmark, the study set out to answer a very specific question:

“How much does cannabis use disorder contribute to schizophrenia in the general population?”

That’s an important distinction to point out - the study did not look for participants who indulged in occasional recreational weed use. It focused on people who have used cannabis in what is seen to be a clinically worrying way and have developed cannabis use disorder.

What was the scope of the study?

Along with wanting to know just how much heavy, unregulated cannabis use added to the chances of someone developing schizophrenia, the researchers looked to find out how that risk changed across different groups. 

• Was it more pronounced in younger people? 
• Did it taper off with age? 
• Was one gender more likely to develop schizophrenia after heavy cannabis use? 
• Had the chances increased as cannabis potency has risen across the last five decades?
• And what about the role of broader life factors, like income or family history?

By including variables like psychiatric history, parental education, and even region of residence, they were able to focus on cannabis use disorder itself and isolate its connection with future schizophrenia diagnoses.

But the study didn’t stop there. 

Beyond the broad patterns seen in national health records, the researchers wanted to know what might be happening at the molecular level, in the bloodstream, not just in the data.

They invited a much smaller group of volunteers into a clinical trial (107 participants). Some had schizophrenia. Others had been diagnosed with cannabis use disorder. A few had both. And a fourth group, the controls, had neither.

Each participant gave a small blood sample. What the researchers were looking for were lipids. Lipids are fat-like molecules that make up much of our cell membranes and help our neurons talk to one another, especially in the complex circuitry of the brain. Without them, our nervous system would struggle to function at all.

In this study, the team wanted to see whether these lipids followed different patterns depending on the diagnosis. 

• Did the blood of someone with schizophrenia tell a different story than that of someone with cannabis use disorder? 
• And what about people living with both conditions at once?

What did the study reveal about schizophrenia risk and cannabis use?

The findings were as striking as they were sobering. While there has long been a suspected link between unregulated and non-clinically guided usage of recreational marijuana, this is the first time we are seeing population-wide evidence paired with biological data that shows how deep and measurable that connection may run.

By combining 50 years of population data with new clinical insights, the researchers were able to draw a much clearer line between cannabis use disorder and schizophrenia than had previously been possible.

And they did not stop at surface-level patterns.

How strongly is cannabis use disorder linked to schizophrenia?

According to the study, the association between cannabis use disorder and schizophrenia was particularly strong in young men. In fact, the authors estimate that up to 30% of schizophrenia diagnoses in men aged 16 to 25 could be attributed to CUD.

The data also showed that:

• The younger the cannabis use began, the greater the risk

• The higher the frequency and potency of use, the more pronounced the association

• The risk persisted even after accounting for other factors, such as family history of psychiatric illness, and socio-economic background. 

Importantly, the authors were clear that cannabis use disorder was not simply a symptom of schizophrenia developing under the surface. They concluded that CUD was likely contributing to the onset of schizophrenia in a significant number of cases, not just happening alongside it.

What did the blood samples reveal about the brain?

Of the 107 participants from the smaller focus group (which included people with schizophrenia, people with cannabis use disorder, and those with both conditions), the results were, again, very striking…

• The levels of Glycerophospholipids, which help build healthy cell membranes, showed the largest change
• Sphingolipids, important for protecting nerves and transmitting signals between them, showed unusual patterns
• Acylcarnitines, which help manage energy and support mitochondrial health, were out of balance

The most severe lipid disruptions were seen in people who had both schizophrenia and cannabis use disorder. This pointed toward a compounding effect on brain chemistry, where the presence of both conditions may heighten the biological strain.

From these results, the researchers concluded that cannabis may be imitating or worsening some of the same metabolic disturbances seen in schizophrenia - even potentially accelerating or intensifying its effects in people who are already at risk.

So, to answer the pressing questions above…

How much does cannabis use disorder contribute to schizophrenia in the general population?

Across the board, cannabis use disorder was linked to around 15% of schizophrenia diagnoses in younger people. But in young men aged 16 to 25, the figure rose sharply, with up to 30% of cases potentially attributed to CUD.

Was it more pronounced in younger people? 

Yes, very much so. The link between cannabis use disorder and schizophrenia was strongest in the youngest age group. The earlier the heavy, high-THC recreational weed usage began, the higher the risk.

Did it taper off with age? 

Yes. After the age of 35, the connection between cannabis use disorder and later schizophrenia became much weaker. Risk was still present, but the pattern flattened out.

Was one gender more likely to develop schizophrenia after heavy cannabis use? 

In this study yes. Men were at much higher risk, especially younger males.

Have the chances increased as cannabis potency has risen across the last five decades?

The study did not measure THC levels directly, but it does suggest that the growing strength of cannabis products in recent decades may explain why risk has gone up over time.

And what about the role of broader life factors, like family history?

The study took these into account and acknowledged the higher risk factors for participants with a family history of schizophrenia or psychosis. Even after adjusting for background, family mental health history, and education, the link between CUD and schizophrenia still held up. 

Why Releaf does not prescribe cannabis to patients with a history of schizophrenia or psychosis risk

Everything we do at Releaf starts with one goal: to make sure that cannabis-based treatment is safe, appropriate, and truly in the best interest of the patient. 

That means knowing when it may offer therapeutic effectiveness, and when it is simply not an appropriate treatment option.

Schizophrenia and psychosis fall into the latter 

While there is evidence that cannabis might help manage symptoms like anxiety or poor sleep, the wider research tells us something else. In people with a history of psychosis, cannabis can increase the risk of relapse or even trigger new episodes. 

That is not a risk we are willing to take

This is why we do not offer prescriptions to anyone who has been diagnosed with schizophrenia or psychosis in the past, regardless of how long ago. These conditions are complex, and cannabis is not a suitable option in these cases.

The same level of caution applies to age 

Releaf does not treat patients under 18. For people aged between 18 and 25, lower-THC prescribing limits are in place, and our clinicians take additional steps to assess and monitor safety. 

Patients under the age of 25 will typically begin treatment with:

• A 10:10 flower and/or oil (10% THC, 10% CBD), which allows our clinicians to monitor how the patient responds in the early stages of treatment while minimising any risk.
• If the patient is responding well to treatment and is finding stable symptom relief, during their first follow-up consultation, they will be able to be considered for an increase to a 20% THC flower option or a higher strength medical cannabis oil, but only if this is clinically appropriate. 
• For THC vape prescriptions, we offer a balanced formulation containing 200 mg of both THC and CBD to suitable patients under the age of 25.

This approach helps ensure that younger patients have a gentle and gradual introduction to cannabinoid treatment while they are still in a stage of neurodevelopmental sensitivity. 

It’s how we uphold our commitment to evidence-based, patient-first care.

We asked Releaf’s resident psychiatrist, Dr Imran Ali, for his thoughts on medical cannabis for schizophrenia:

“People with schizophrenia may look to cannabis as a treatment to relieve their symptoms, or share their experience of taking cannabis for several years without adverse effects. These perceptions can create a smokescreen of confusion between cannabis use and its relationship with schizophrenia. 

This is where the research, which is often complex and evolving, must be understood by clinical experts in order to make decisions that place patients at the centre of care.”

What does this mean for safer medical cannabis prescribing in the UK?

The emerging evidence is quite direct, the biology behind the results is becoming better understood, and all of that points to the risk posed by medical cannabis for schizophrenia/psychosis being too great. 

But this does not mean medical cannabis isn’t an appropriate option for everyone. With the right guardrails in place, it can be part of a responsible, effective treatment plan for many patients.

That is why every journey with Releaf starts with a careful clinical screen. 

If you are wondering whether medical cannabis is right for you, the first step is simple - head to our medical cannabis eligibility checker. It only takes a few minutes, and it helps our team understand whether treatment might be appropriate in your case.

Safe access starts with the right questions. We are here to help you ask them and to provide the most up-to-date, unbiased answers.

Releaf - Let’s rethink healthcare.