Cannabis and Mental Illness

Written by Professor Brendan Kelly, Professor of Psychiatry, Trinity College Dublin

Cannabis is the most common illegal drug that I come across in clinical practice, but public discussion about cannabis is often very confused. The problem is that two questions about the drug get mixed up together. First, is cannabis bad for mental health? Second, if cannabis is bad for mental health, what should we do about it? These two questions are linked, but are also separate in several important respects.

Cannabis is the most common illegal drug that I come across in clinical practice, but public discussion about cannabis is often very confused. The problem is that two questions about the drug get mixed up together. First, is cannabis bad for mental health? Second, if cannabis is bad for mental health, what should we do about it? These two questions are linked, but are also separate in several important respects.

With regard to the first question, overwhelming evidence now confirms that cannabis is bad for mental health.

In 2017, the US National Academies of Sciences, Engineering and Medicine reviewed all of the evidence (positive and negative) pertaining to likely links between cannabis and psychosis, and concluded that the higher the cannabis use, the greater the risk of schizophrenia and other psychoses.

Looking at the effects of cannabis more broadly, the College of Psychiatrists of Ireland summarised the evidence in 2021 in a (freely available) paper titled “Cannabis and Your Mental Health.” The College points out that “cannabis can make existing mental illness get worse. Cannabis use can trigger new mental illness, that is, sometimes people become mentally ill for the first time very soon after using cannabis. The number of people admitted to psychiatric hospitals in Ireland with a cannabis related diagnosis has trebled since 2002.”

The College adds that “cannabis is linked with psychosis. Cannabis users are three to four times more likely to develop psychosis than those who never use it”. In addition, “cannabis can make depression worse. It can also bring on depression.”

Early use is especially troubling, because “the younger a person begins using cannabis, the more at risk they are of developing a major depressive disorder. Cannabis use disorder has been associated with a greater risk of developing bipolar affective disorder and having more frequent relapses.

Cannabis can make anxiety disorders worse.

Adolescent cannabis users are at higher risk of developing anxiety disorders, particularly those people who use high-potency cannabis.”

Finally, “cannabis use is also associated with self-harm and suicidal behaviour” because “young people who smoke cannabis regularly are three times more likely to attempt suicide than people who never use cannabis. Long-term cannabis users are more likely to report thoughts of suicide than non-users. In 2018, cannabis was the most common street drug used among men aged 15-24 who had self-harmed in Ireland.”

I can confirm all of this from clinical practice: cannabis presents a systematic risk to mental health.

The situation improves if the person stops smoking cannabis, but sometimes symptoms return.

If this happens, is their ongoing illness due to cannabis alone or would it have happened anyway, with cannabis acting as a trigger? Was schizophrenia precipitated or caused by cannabis? These are difficult questions in individual cases, but the overall research is clear: cannabis is strongly associated with psychosis and other mental illnesses. Cannabis is bad for mental health.

The second question about cannabis is a little different: Given that cannabis is bad for mental health, what should we do in order to reduce the harm caused by cannabis in society? Should cannabis be illegal, decriminalised or legalised? This is a topic of continual public debate and there are many issues to be balanced: medical evidence, the need to protect the vulnerable, and the value of civil liberties.

In the end, we should do whatever reduces cannabis use and minimises the harm from cannabis that I see every week in my clinical work. The first priority must be public education. Regardless of whether it is legal or illegal, cannabis is bad for mental health.

If you smoke cannabis, please seek assistance stopping. Your mental health will thank you.

Brendan Kelly is Professor of Psychiatry at Trinity College Dublin and author of “In Search of Madness: A Psychiatrist’s Travels Through the History of Mental Illness” (Gill Books, 2022).

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