An aggressive cannabis lobby is fuelling a dangerous complacency about the risks the drug poses to teenagers’ mental health, Britain’s top psychiatrist said.

Dr Lade Smith, president of the Royal College of Psychiatrists, said young people were damaging their brains by smoking high-strength cannabis and risking psychotic episodes that could ruin their lives.

“When you start smoking with your mates at 14 or 15, you are literally growing your brain in a cannabis soup,” she said. “There’s no doubt at all. Cannabis is a cause of psychosis.”

Cannabis is consistently the most consumed illegal drug in England and Wales. In the year to March last year, 2.3 million people were estimated to have used it. Yet it was too often viewed as relatively safe by parents, Smith said, pointing to research last year showing adolescents who used it were 11 times more likely to have a psychotic episode later in life.

She called on the government to do more to educate parents and young people, adding: “We’ve not got the public health message right. We know that cannabis is not a safe option.”

“People’s brains don’t stop growing until you’re an adult in your early to mid-twenties. The reality is — and this is evidence-based, therefore it’s the truth — cannabis is associated with a higher risk of anxiety, depression, and, unfortunately, a higher risk of psychosis. Anyone who smokes cannabis regularly will admit that they’ve had a ‘para’, and what they mean by that is that they’ve had a paranoid fit. People laugh about it.”

Psychosis is a severe mental illness where a person loses touch with reality and can experience hallucinations and delusions. While in this state they can be a danger to themselves and others.

Smith, who runs forensic mental health services in south London as a consultant psychiatrist with the South London and Maudsley NHS Trust, said: “People think ‘it’s never gonna happen to me’ but in my work I’ve seen patients who are from very, very, well-to-do families who have developed psychotic illnesses because of their cannabis use.

“They have ended up falling foul of the police as a result, doing some really quite wild and, at times, very dangerous things. If you do have a problem with cannabis, the kind of problem that you have is so profound, it potentially could completely ruin your life. And the life of others.”

Rates of first episode psychosis are higher in southeast London, where she works, than anywhere else in Europe, second only to rates found in Trinidad, according to a study published in the JAMA Psychiatry journal.

She said that 97 per cent of patients in her service had a substance abuse problem, with high-strength cannabis the main drug. “Over the last 10-15 years, the cannabis that’s been available in the UK has been basically high-strength cannabis,” she said. “From police seizures, something like 94 per cent of cannabis that’s available is high strength. We’re talking 20, 30 or 40 per cent THC content.”

THC, or tetrahydrocannabinol, is the active compound in cannabis and affects the brain by binding to and activating cannabinoid receptors. These are found in areas of the brain including the cerebral cortex, cerebellum and the hippocampus and involved in brain functions such as cognition, memory, reward, mood and stress responses.

Crucially it affects the release of neurotransmitter chemical signals such as dopamine in the brain’s reward centres, which helps produce the “high” feeling. Studies have shown that, over time, persistent users of cannabis can see a reduction in the amount of grey matter cells in their brain.

Smoking cannabis, like tobacco which it is often mixed with, irritates the lining of the lungs and is known to cause chronic bronchitis symptoms including coughing, phlegm and wheezing.

Alcohol affects the brain in similar ways to cannabis, disrupting chemical signals known as GABA and glutamate that stimulate the brain. It reduces inhibitions and reasoning, and chronic long-term alcohol abuse permanently affects chemical levels in the brain.

“The problem is that if you smoke regularly, and regularly is one spliff of 10 per cent-plus THC cannabis per week, then your risk of developing a psychotic illness by the time you are 25 is 11 times higher,” said Smith.

Raising concerns about this has brought her abuse online from people she suspects were linked to the cannabis industry. “When I’ve said this before to people I was trolled and attacked on social media and it became clear they were linked to some of the lobbyists with vested interests.”

She said one of the attacks claimed her view was a racist trope because black people use cannabis. “It was a way of trying to shut me up,” she said: “There is a very strong lobby.”

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In 2018, the law was changed to allow the prescriptions of cannabis-based medicines, which has also encouraged a burgeoning industry in over-the-counter products.

While THC remains illegal for recreational use, other products using cannabidiol, or CBD, which does not give the same high but can still alter consciousness and perception of pain, are widely available.

Smith believes the use of cannabis as a medicine, and the wider industry that has developed around products containing CBD should face further scrutiny. In the UK, products containing CBD include vitamin supplements, sprays and gels from high-street stores promising to help with everything from chronic pain to anxiety and sleep issues. She said such claims should be tested through properly run clinical trials.

“It’s not as simple as saying that all THC is bad and CBD is good,” she added. “There’s evidence that medicinal cannabis is good for certain types of epilepsy, but unfortunately, there are claims that it does everything.”

Sir Chris Whitty, the chief medical officer, called for this in 2019, warning MPs against what he called “cannabis exceptionalism”.

Smith, a mother-of-two, goes into schools in south London to warn teenagers about the risks of cannabis. She is also leading efforts to get ministers focused on investing in mental health treatment generally as a way of reducing demand on the health service and improving the economy.

Smith believes the government could do more to help young people struggling with mental illness and said ministers needed to understand the difference between mental health and mental illness. Mental illness, she said, can be successfully treated.

She said: “Three-quarters of mental health problems develop before the age of 24, and half before the age of 14. Yet we still don’t prioritise it. If you invest in assertive treatment for children and young people, you can stop them from becoming adults with chronic relapsing problems. You can significantly increase economic activity and growth.”

She said social media was partly to blame for people confusing mental health and mental illness but she added the rising problem of mental illness was predicted after the 2008 recession and was only going to worsen after austerity and the pandemic.

She said NHS mental health services were at breaking point: “You don’t get into hospital now unless you’re extremely unwell. The thresholds are much higher because we just don’t have the capacity. If we prioritised substance abuse services and primary prevention, getting the right public health messages out there and ensuring sufficient care, we would save so much money.”