The unmistakable scent of cannabis is no longer confined to specific areas in the UK; it now drifts through serene rural landscapes and bustling city streets alike. This pervasive presence reflects a shifting attitude toward the drug, as surveys reveal that nearly half of urban dwellers and about a third of rural residents report frequent encounters with its distinct smell during their everyday activities.

Experts suggest that this widespread exposure signals the incremental acceptance of cannabis use in Britain. Despite its classification as a Class B drug, carrying a potential five-year prison sentence for possession, cannabis appears to be weaving itself into the fabric of daily life.

Contributing to this shift is the burgeoning industry surrounding ‘medical’ cannabis. Across the nation, numerous private clinics have emerged, offering cannabis prescriptions for a range of conditions, including attention deficit hyperactivity disorder (ADHD), anorexia, Parkinson’s disease, and Tourette’s syndrome, a disorder known for involuntary tics and sometimes swearing.

This medical framework has arguably lent a veneer of legitimacy to cannabis, painting it as a largely ‘harmless’ substance, possibly even beneficial to health. However, there is growing concern that such perceptions could lead the UK towards a significant mental health crisis.

Renowned psychiatrists across the country express alarm, pointing to data from regions where cannabis has been legalized or approved for medical use. These statistics indicate a rise in individuals requiring emergency care for severe psychotic episodes — characterized by delusions, auditory hallucinations, crippling paranoia, and vivid hallucinations — after consuming cannabis.

That’s the worry among some of the country’s leading psychiatrists, who warn the evidence from countries where cannabis has been legalised or sanctioned for medical use shows a spike in the number of people needing emergency treatment for acute psychotic episodes – where they suffer delusions, hear voices, develop extreme paranoia and experience hallucinations – after smoking cannabis.

‘We have more and more people turning up at our clinics with cannabis-induced psychosis, and I know from colleagues in the NHS that it accounts for a huge part of their workload,’ says Dr Niall Campbell, a consultant psychiatrist with the Priory Group, who specialises in treating drug addiction.

‘Research shows the earlier you start on the drug and the more often you smoke it the higher your chances of developing psychosis.’

Evidence from countries where cannabis has been legalised or sanctioned for medical use shows a spike in the number of people needing emergency treatment for acute psychotic episodes 

Last week, scientists from Spain told the European Congress of Psychiatry in Prague that their research showed regular use of cannabis permanently ‘thins’ the frontal cortex, the part of the brain involved in planning, decision making and memory. The frontal cortex is also an area of the brain associated with psychosis.

This follows research involving 464,000 adolescents, published in February in the journal JAMA Health Forum, which found that teenage cannabis users double their risk of developing psychotic and bipolar disorders.

Another concern, experts warn, is that often the effects are lifelong, as cannabis-induced psychosis does not always respond well to current medications.

And there is no way of knowing who might be at risk. As Dr Campbell explains, people’s reaction to the drug ‘is extremely variable. Some people smoke it every day and never get problems, while others can quite quickly develop terrible paranoid psychosis’. According to the Office for National Statistics, around 2.3 million adults in the UK frequently use recreational cannabis, with peak consumption among those aged 16 to 25.

While the numbers of people using cannabis have actually changed very little over the past decade, what has changed is the strength of the drug.

Levels of tetrahydrocannabinol – or THC, the psychoactive element of cannabis that gives users the ‘high’ – have risen from around 2 per cent (of the chemical composition of the drug) in the 1960s to as much as 20 per cent today in the form of skunk, the potent strain that now accounts for almost all of the street cannabis sold in the UK.

At the same time, there has been a drop in levels of cannabidiol (CBD), another chemical in the cannabis plant which is thought to protect the brain against the psychosis-inducing effects of THC and which is also sold as a health-boosting supplement in wellness food shops.

Skunk has almost half the CBD content of cannabis that was used in the 1970s.

Dr Campbell says that in south London, ‘where cannabis use is particularly prevalent, there has been a steady increase in psychosis linked to the increase in cannabis potency in the last 20 years’.

Such is the concern around this that in 2023, the incoming president of the Royal College of Psychiatrists, Dr Shubulade Smith, warned the country was facing a ‘ticking time bomb’ of psychosis brought on by cannabis use.

Evan Yiangou started smoking cannabis as a teenager and, aged 19, began to experience the first stages of psychosis. Soon he was having ‘conversations with myself out loud’

And research suggests it’s not only heavy daily users who are at risk. In 2019, a study in the journal Lancet Psychiatry by scientists from King’s College London warned the chances of experiencing a psychotic disorder were 40 per cent greater among those who smoked cannabis more than once a week than in those who rarely or never used it. 

Evan Yiangou, 46, a father of two from north London started smoking cannabis as a teenager and, aged 19, began to experience the first stages of psychosis.

‘I was on a bus and all of a sudden, I felt like everyone around me could see my thoughts, my insecurities and read my feelings,’ he says. ‘It was terrifying.’

Soon he was having ‘conversations with myself out loud’.

Now drug-free, he runs a private practice that specialises in recovery coaching and acknowledges that without the intervention of his family, who encouraged him to quit drugs through the Perry Clayman Project, a rehab centre in Luton, his life could have taken a very different direction.

Recent tragic cases illustrate the appalling damage cannabis-related mental breakdowns can cause. In April 2025, Kara Alexander, 47, from Dagenham, east London, was jailed for life for drowning her two young sons in the bath at home in December 2022 while in a psychotic state brought on by smoking skunk nightly for several weeks.

And in 2021, 23-year-old Emily Head, from Long Eaton in Derbyshire, jumped to her death when she was in a paranoid state brought on by cannabis.

Why cannabis induces psychosis in some and not others is not clear, but one theory is that THC binds to receptors in the brain and triggers a surge in levels of the ‘pleasure’ chemical dopamine that overloads the brain, leading to runaway thoughts.

Dr Campbell says cannabis-related psychosis can be resistant to anti-psychotic medicine.

‘I have a male patient who stopped smoking the drug 20 years ago, but his mental health problems never went away – he is still plagued by paranoia.’

Experts think those most at risk have a genetic predisposition to psychosis – and using cannabis with high amounts of THC tips them over the edge. Some fear the surge in private clinics prescribing cannabis could lead to more psychosis cases.

The Government legalised the use of medical cannabis in 2018 in cases where the patient has ‘a clinical need that cannot be met by existing licensed products’ and where a specialist doctor thinks it is medically justified.

A number of commercial clinics soon opened their doors to prescribe medical cannabis as a result. Clinics must be registered with the Care Quality Commission and abide by its rules on standards of care – and their numbers are growing. There are now around 25 private clinics supplying cannabis products to some 80,000 people across Britain, according to Releaf, a licensed London-based medical cannabis provider.

Typically, after an initial online assessment, there is a video consultation with the clinic’s doctor, who decides whether or not cannabis is a suitable treatment. If so, the drug is despatched for home delivery. 

There are three cannabis-based medical drugs licensed for use in the UK: Sativex (a spray to ease muscle spasms in multiple sclerosis) that is an equal mix of THC and CBD; Nabilone, a synthetic form which mimics the action of THC, for nausea and vomiting triggered by chemotherapy; and Epidyolex, which contains solely CBD for hard-to-treat forms of epilepsy.

However, private clinics may also prescribe other formulations tailored to the patient’s needs, and with a THC content that can be up to 25 per cent – on a par with skunk.

These types of medical cannabis are usually vaped (using devices similar to e-cigarettes) or taken as drops of oil under the tongue. But it’s not cheap.

The consultation fee is usually around £99, with a £7.99 per gram charge for the cannabis and a £39.99 monthly subscription fee (or £479.88 a year).

Professor Sir Robin Murray, a psychiatrist from the Department of Psychosis Studies at King’s College London, who first warned of the mental health dangers of cannabis over a decade ago, fears the rise in private clinics spreads the misconception that street cannabis may be medically beneficial.

‘The medicalisation of cannabis is most definitely a damaging development,’ he told Good Health. ‘Cannabis is mildly useful for pain, and trials show it may be akin to taking a couple of paracetamol. But just like alcohol, it might help for a couple of weeks but the longer you take it the more you have to increase the dose to get the same effect and eventually it makes people worse, not better.’

What’s more, he fears that private cannabis clinics may be tricked into prescribing the drug to those who want cannabis for recreational use. ‘If people were going to these private clinics for things like chronic pain, you would expect them to be in their 50s/60s – but from what I’ve seen, anecdotally, it’s all men in their 20s and 30s.

‘These are people who, instead of going to their dealer, now realise they can just register with one of these clinics. They see a doctor online once and don’t see them again. It’s just like a pharmacy.’

Another long-standing fear is that the market for medical cannabis may be used to piggyback the case to legalise cannabis for recreational purposes in Britain, a move many doctors oppose, given the evidence from countries which have done it.

In Germany, where it became legal to possess and use cannabis in April 2024, the number of people needing emergency treatment for cannabis-related psychosis doubled over the following 12 months, according to a report last December in the journal of the German Medical Association.

Dr Niall Campbell, a consultant psychiatrist with the Priory Group, who specialises in treating drug addiction, says: ‘Research shows the earlier you start on the drug and the more often you smoke it the higher your chances of developing psychosis’

In Portugal, where cannabis was decriminalised in 2001, the proportion of schizophrenia cases (thought to be linked to the impact of THC) attributed to cannabis use increased tenfold by 2015.

Even sanctioning the drug’s use as medicine could backfire.

In Canada, which approved medicinal use in the early 2000s, the proportion of schizophrenia cases linked to cannabis soared from just 1.6 per cent in 2006 to 9.6 per cent in 2022.

Closer to home, a July 2024 report on mental health on the island of Guernsey revealed that during 2023, some 25 per cent of admissions to mental health wards were linked to cannabis use. That was up from just 4 per cent in 2019 when medical cannabis prescriptions were first introduced on the island.

The risks aren’t just limited to mental ill health either. Studies have found regular cannabis use can increase the risk of a stroke (the drug can cause arteries to constrict, reducing blood flow to the brain), foetal abnormalities in pregnant women, and testicular cancer (by up to 70 per cent – possibly by blocking naturally- occurring anti-cancer chemicals in a man’s body). Even dementia and erectile dysfunction have been shown to occur more frequently in chronic cannabis users.

However, David Nutt, a professor of neuropsychopharmacology at Imperial College London and a long-time advocate of the potential benefits of cannabis, believes the risks are overstated, saying: ‘The risk of psychosis has been exaggerated for high strength THC products, as many also contain cannabidiol as a protection.’

Yet even he warns there is a danger the public will read any endorsement of medical cannabis as the go-ahead to use illegal cannabis for self-diagnosed problems.

Terry Hammond, 79, a retired charity worker from Packington in Leicestershire, wishes the dangers of cannabis could be amplified. His son Steven, once a promising young footballer, was in his early 20s when he experienced his first episode of cannabis-induced psychosis brought on by smoking skunk.

‘I came home from work one day and he was sitting there staring into space,’ recalls Terry. ‘He said, ‘Why did you ring the BBC? They’ve been talking about me all day on the radio and TV’.’

Steven went on to have a ‘full blown psychotic breakdown’, says Terry, and was in a semi-delusional state for the next five years. ‘He was hearing voices and said he felt like aliens had taken over his body. At one point he asked me, ‘Are you my dad or are you an alien?’.’

At times, Terry had to wipe blood off the walls from where Steven had bashed his head to try to banish the voices he was hearing. Eventually, Steven responded to an anti-psychotic drug called olanzapine and, together with cognitive behavioural therapy, began to regain some control over his mental state.

Steven, now 48, hasn’t touched skunk in years but ‘he’s still very paranoid about people so won’t, for example, get on a bus’, says Terry, who has written a book, Gone To Pot – Cannabis: What Every Parent Needs To Know, about the family’s experience.

A recluse who only socialises with his family, Steven ‘lives in the annex and now does three days a week voluntary work at a charity-run farm’, adds Terry. ‘I think the medicalisation of cannabis into a multi-million-pound business is a really big problem.

‘I really do fear that we are sleepwalking into a perfect storm with this mind-altering drug.’

A spokesman for CuraLeaf, one of the UK’s leading suppliers of medical cannabis, said it operates ‘robust prescribing processes’ where a patient’s suitability for cannabis is closely scrutinised.

They added: ‘Prescribed products are pharmaceutical grade and doctors carefully assess the potential benefits and risks for each individual patient before prescribing.

‘Patients are then monitored through regular follow- up consultations.’

• Evan Yiangou interview by JULIE COOK