Last year, a personal injury lawyer attacked a family of immigrants in the parking lot of an Ontario mall because they were speaking Spanish. He accused one man of being a terrorist and broke his ribs with a baseball bat.

There is disagreement about whether cannabis can cause transient psychotic episodes in otherwise healthy people.

The Toronto-based lawyer, Mark Phillips, pled guilty to assault but claimed he was suffering from cannabis-induced psychosis. The court gave him a conditional discharge. After serving three years’ probation and doing some community service, he won’t have a criminal record.

Critics denounced the sentence as too lenient and were quick to note that Phillips is the great-grandson of Nathan Phillips, the late Toronto mayor after whom a public square is named. (Nathan Phillips Square forms an expansive courtyard in front of Toronto City Hall.)

The incident also renewed debate about cannabis and psychosis.

Cannabis consumption has been associated with psychotic behaviour since the 1930s, when audiences were introduced to Reefer Madness. In the 68-minute film, high school students descend into madness after smoking cannabis.

In Quebec, the perceived threat of cannabis psychosis has been presented as evidence to support its prohibition.

More recently, the perceived threat of cannabis psychosis has been presented as evidence to support its prohibition: In an interview with CBC/Radio-Canada last year, Quebec’s health minister Gaétan Barrette noted that cannabis “can induce a deterioration of mental state all the way to causing psychosis” in people who are biologically predisposed, adding that “nobody has ever [experienced] psychosis on alcohol.” That elicited groans across the province and beyond; the link between alcohol and psychosis has been well-documented.

Is there a comparable link between cannabis and psychosis? Some experts say cannabis consumption can cause mental illnesses that feature psychosis but others disagree. There is also disagreement about whether cannabis can cause transient psychotic episodes in otherwise healthy people.

In a 1987 study, researchers found that Swedish conscripts had an increased risk of developing schizophrenia if they had consumed cannabis more than 50 times in their lives. Since then, other researchers have also found an intricate relationship between cannabis and mental illness — and concluded that cannabis use increases the risk of developing mental disorders.

Evidence indicates cannabis can promote the onset of schizophrenia in people with a biological predisposition to it— individuals who would probably develop the mental disorder anyway.

But other experts take issue with that hypothesis. There may be an intricate relationship between cannabis and mental illness, they say, but that is not proof that cannabis causes mental illness.

In 2015, Matthew Hill of the Hotchkiss Brain Institute at the University of Calgary wrote an article for the journal Nature, in which he said there is little evidence of a causal relationship. He went further, saying there is evidence that cannabis use does not cause schizophrenia.

However, he added that evidence indicates cannabis can promote the onset of schizophrenia in people with a biological predisposition to it— individuals who would probably develop the mental disorder anyway.

“Imagine that [schizophrenia] is like a campfire,” he wrote. “Adding fuel to a pile of sticks has little effect, but throwing fuel on a weakly burning fire will increase its strength. Regardless of whether fuel is added, the embers will continue to burn.”

Dr. Romina Mizrahi, director of the Focus on Youth Psychosis Prevention Clinic at The Centre for Addiction and Mental Health in Toronto told Leafly that “cannabis could be a trigger of an underlying condition.” She added that, although “there is a proven link between cannabis and psychotic experience,” the link is only proven in young people — whose brains are still developing — and it remains unclear “who will experience those symptoms or when.”

“The argument that cannabis can cause psychosis is not based on sound scientific evidence. Causation has not been proven.” Paul Armentano, deputy director of NORML

Many experts speculate that cannabis use is not the cause of mental illness but rather the byproduct of it — that people who have psychotic episodes use cannabis to self-medicate before they are ever diagnosed with mental illness.

Hill and other experts say clinical studies have shown that pure THC can produce a temporary psychotic state in people who don’t have mental illness — but not everyone is convinced.

“The argument that cannabis can cause psychosis is not based on sound scientific evidence. Causation has not been proven,” Paul Armentano, deputy director of the National Organization for the Reform of Marijuana Laws (NORML), told Leafly. “It is our position that the relationship between cannabis and psychosis is not well-defined or well-understood.”

There is a lot of debate about whether Mark Phillips was in a cannabis-induced psychotic state when he swung his bat at Sergio Estepa yelling, “ISIS!” but there is much less debate about whether that was an adequate defence.

“My presumption is that if someone knowingly engages in substance that alters mood or behaviour, how can it be excusable?” said Armentano. “That is like saying that, if I hadn’t consumed so much alcohol, I wouldn’t have done this — so my actions were excusable.”

Added Dr. Mizrahi, “How could a man of Mr. Phillips’s education and training as a lawyer not have been aware of the effects of cannabis before using it?”

Randi Druzin Randi Druzin is an author and journalist in Toronto. She has worked at several major media outlets, including the National Post and the CBC, and has written for dozens of publications, such as The New York Times, Time magazine, ESPN The Magazine, and The Globe and Mail. View Randi Druzin's articles