Brendan Kelly says the majority of research shows an association between cannabis use and a range of psychological problems, including anxiety, depression, and psychosis.


Gone to pot: Leading psychiatrist on the cannabis debate

Psychiatrist Brendan Kelly says the majority of research shows an association between cannabis use and a range of psychological problems, including anxiety, depression, and psychosis.

Gone to pot: Leading psychiatrist on the cannabis debate

Psychiatrist Brendan Kelly says the majority of research shows an association between cannabis use and a range of psychological problems, including anxiety, depression, and psychosis.

Richard was not happy to see me. He was ambivalent about speaking with anyone, let alone a doctor, so why should he have to see a psychiatrist anyway?

Richard was a 20-year-old engineering student, living at home.

Over the previous three months, he had become increasingly withdrawn, skipping lectures, and spending most of his free time in his bedroom.

For the past month, he had been making unusual comments about the television, the internet and his neighbours.

He felt he was being watched and he advised his parents to take extra care outside the house: There was vague, unspoken danger everywhere.

“You know what I’m talking about,” he told his baffled father.

Eventually, Richard’s life became so restricted, and his paranoia so obvious, that his parents persuaded him to see his GP, who referred him to me, a psychiatrist.

Reading the GP’s referral letter, and having read hundreds of similar letters in the past, it was immediately clear to me that Richard was likely smoking cannabis and had developed psychosis as a result.

The term ‘psychosis’ refers to a mental state in which a person has lost touch with reality in at least one important respect, while not acutely intoxicated (for example, with alcohol).

Common features include delusions, which are fixed false beliefs (such as Richard’s paranoia) and hallucinations (such as hearing voices when there is no one there).

Not all psychotic symptoms are necessarily problematic, many people hear voices and live with them without much difficulty, but many other people with psychosis experience significant problems with mental wellbeing, education, occupation, relationships and so forth.

People with schizophrenia, depression or bipolar disorder can develop psychosis when symptoms are severe, but it is now exceedingly rare to see a young man with new-onset psychosis who did not start smoking cannabis prior to the development of symptoms.

Stopping cannabis helps greatly.

There has been much discussion about cannabis in Ireland in recent times.

The vast majority of research studies show clear associations between cannabis and a range of psychological problems, including lack of motivation, difficulties with thinking and learning, anxiety, depression, and psychosis.

In 2017, the US National Academy of Medicine reviewed the evidence linking cannabis with psychosis and concluded that “the higher the use, the greater the risk”.

Like all drugs, cannabis does not affect everyone equally, but this does not alter the systematic risks it poses, just as the fact that someone who smokes 40 cigarettes per day might live to age 100 doesn’t change the fact that tobacco systematically increases risk of cancer.

Also, not only do I see the harm wrought by cannabis every week in my clinical work, but whenever I ask a cannabis-user if they know people who are adversely affected by the drug, they nearly always answer ‘yes’ and look at me strangely, wondering why I ask such a question when the answer seems so blindingly obvious.

Following a long discussion, Richard agreed to stop smoking cannabis, improve his diet and lifestyle, and take some anti-psychotic medication.

He also told the student advisor in his college that he wanted to take three weeks off.

The medication helped settle Richard’s acute paranoia, and he was still off cannabis two months later, with the help of drug counselling.

If he remains free of cannabis there is every chance he will never have another episode of mental illness again.

Though Minister for Health Simon Harris signed legislation this week to allow a medical cannabis programme to run on a pilot basis, the current debate about cannabis in Ireland routinely confuses several important issues.

The first area of confusion concerns illegal cannabis use on the one hand and medical use on the other. These are separate issues.

The evidence base for medical cannabis is thin and does not justify routine prescription, but for a small number of treatment-resistant cases of specific conditions, there is evidence that individual patients benefit.

For these patients, medical cannabis should be accessible. (The website of the Department of Health provides further information about the Ministerial Licence Application Process.)

The second area of confusion arises when the issues of cannabis-related harm and decriminalisation are merged together.

These, too, are somewhat separate issues.

The National Academy of Medicine and HSE are clear (and I agree) that cannabis poses a risk to mental health, but that does not necessarily mean that cannabis should be illegal.

Some very harmful substances are illegal (cocaine) but some others are legal and regulated (tobacco and alcohol).

We need to set aside ideological positions and focus on reducing cannabis-related harm.

Reducing harm involves decreasing cannabis use by either reducing demand for the drug or reducing supply.

Clear public education is important in reducing demand. Reducing supply is considerably more difficult and may be impossible.

The current approach — prohibition — has clearly failed: I have never met anyone who had difficulty accessing cannabis, and the strength of cannabis in Ireland has increased in recent years.

Legalisation would, in theory, permit better regulation of supply, quality control and taxation, but, in practice, a black market for high-strength products would likely continue to operate alongside any legal market for cannabis.

As a result, legalisation of cannabis would be a step too far, but a degree of decriminalisation would likely prove helpful in re-directing users from the criminal justice system to the health system, thus better meeting both their needs and those of society.

Regardless of the legal position, the public health message should be clear: Cannabis poses a risk to mental health.

If you are affected by cannabis use, information is available from, which has a helpful leaflet titled Cannabis and You.

The HSE Drugs and Alcohol Helpline offers free confidential support and information at 1800 459 459, or email

Richard’s story is an anonymised, typical case-history that reflects a common presentation with cannabis-related mental health problems, but does not refer to any specific person.

Brendan Kelly is professor of psychiatry at Trinity College Dublin and author of The Doctor Who Sat For A Year (Gill, 2019).

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