When he killed his wife at their home in Finglas, Dublin in November 2021, Diego Costa Silva held several delusional beliefs.
He believed Fabiola De Campos Silva was possessed by a serpent, and that she was going to kill him. He told gardaí in interviews after the killing that he had to behead her because she was possessed. He complained of voices in his head.
Earlier this month, a jury found him not guilty of murder by reason of insanity.
The delusional beliefs Mr Costa Silva (35) held at the time he killed his wife were symptoms of a cannabis-induced psychotic disorder, his trial heard.
“A delusion is a false belief, unshakeable [even when] held against evidence of the contrary,” says Dr Marta Di Forti, a psychosis researcher at King’s College London, speaking generally about the disorder.
Sometimes, these delusions can result in violence. “If you are absolutely certain somebody’s out to get you, to harm you, or to harm a loved one ... for the individual perpetrating, it is self-defence, and sometimes, depending on how intricate the delusional system is, there are no alternatives.”
Mr Costa Silva’s trial was the latest in a string of murder cases that have touched on the link between cannabis usage and mental illness.
According to research, long-term, heavy use of cannabis can occasionally lead to serious mental health problems in some people, including the onset of a psychotic disorder. There are instances of acute cannabis use causing psychosis, but these are much rarer.
By ways of definition, a cannabis-induced psychotic disorder is diagnosed when the patient displays persistent symptoms – delusions, hallucinations – a week after intoxication with the cannabis product.
According to Prof Matthew Sadlier, a consultant psychiatrist at the Mater hospital, Dublin, cannabis use can cause someone to develop a psychotic disorder who otherwise would not have, had they abstained from the drug.
“You can say the same thing about type two diabetes,” Prof Sadlier says. “There’s plenty of people out there who, for whatever reason, don’t eat a high-sugar, high-fat diet who have a genetic predisposition for type two diabetes who possibly could go through life without ever developing type two diabetes.”
How many people develop psychosis on account of their cannabis use is more difficult to pin down.
Trials in which cannabis use has been raised as an issue in the context of mental health include the conviction last year of Christina Anderson for stabbing a stranger to death during a psychotic episode outside her home in Dublin, and the conviction of Stephen Silver for the capital murder of Garda Colm Horkan.
Anderson’s trial heard she first used cannabis as a teenager and by her late 20s was using it nearly daily, smoking one or two joints a day. Silver told the court that cannabis use impacted on his mental health as it would bring on episodes of paranoia. In their separate trials, Anderson and Silver pleaded not guilty by reason of insanity but these pleas were rejected by the courts.
In another case last year, a man who strangled and beat his on-off partner to death during a psychotic episode that may have been triggered or exacerbated by drug use was jailed for 14 years. Richard Burke had a cannabis-induced psychotic disorder when he killed Jasmine McMonagle in Killygordon, Co Donegal, a consultant psychiatrist told the trial.
Dr Di Forti is the author of several studies on cannabis use and its link to psychosis.
In the average person, the lifetime prevalence of psychotic disorder is 1 per cent, she says. For someone who smokes THC-heavy cannabis regularly – THC is the component of cannabis that causes the “high” – the risk of developing a psychotic disorder becomes five times greater.
This, Dr Di Forti qualifies, is an imperfect statistic – other factors can change the level of risk. “If you are somebody with a family history of psychosis, your chances of developing psychosis, if you use cannabis, is going to be greater than one of the general population,” she says.
Psychotic disorders are a multifactorial condition, and certain people can be more predisposed because of genetics and history. Those who are affected are predominantly male, spanning all socioeconomic groups.
And while the absolute risk of developing a psychotic disorder remains in the low percentiles, a change in the potency of cannabis products available on the streets over recent years has increased the number of people presenting with mental health issues relating to the drug, says Prof Bobby Smyth.
“15 years ago, it was all hash,” says Prof Smyth, a consultant psychiatrist based in Dublin, currently working with children and adolescents.
Hashish is a dry, soft product made from the resin of cannabis plants. “Hash was pretty mild, and didn’t seem to cause huge mental health issues among the kids I was seeing at that time,” Prof Smyth says.
Hash has been replaced by weed, “which seems to be a stronger substance, and it’s also smoked more intensively”, he adds.
Prof Smyth doesn’t see too many younger people with psychosis induced by cannabis. More often, this cohort present with anxiety and anger – in extreme cases self-harm and suicidality – linked to cannabis use.
In most cases, cannabis-induced psychotic disorder develops gradually, over a longer period of time, according to Dr Niall Campbell, lead addiction consultant at Priory Hospital Roehampton in London.
Compared to other forms, psychosis in the context of heavy cannabis consumption is often characterised by a clinical paranoia, says Dr Campbell.
“In relationships, you’re suspicious, you’re questioning people’s motives, you’re isolating yourself, you’re getting angry, aggressive ... getting into fights and things. It can affect work,” he says.
Cannabis shapes the psychosis, says Dr Di Forti. “Psychosis has a cluster of symptoms, it’s not just one symptom. And when you develop psychosis in the context of cannabis use, it tends to be predominantly [an] over-paranoid type.”
It is a paranoia that is intrusive and pervasive, Dr Di Forti says: “It becomes very difficult to go out – to take public transport, because the perception of people looking at you, and potentially representing a threat, is constant.”
Contrary to public perception, people who are psychotic are less likely to commit a crime – especially a homicide – compared with the general population, according to Dr Di Forti.
However, when considering crimes perpetrated by people with a psychotic illness, “they’re much more likely to be perpetrated by people who are using a drug of abuse, and in particular cannabis or, for instance, methamphetamine,” she says.
The paranoia aspect of cannabis-induced psychosis might explain this, Dr Di Forti adds.
“If you feel like the world is against you, you might defend yourself.”
Those with psychotic illnesses who commit crimes are part of a minority, Dr Di Forti stresses.
“I spent my life ... working with young people who have psychosis and they’re using cannabis, and they’re absolutely delightful and peaceful.
“We’re talking about a minority ... It’s a minority that could be prevented.”
[ Cannabis use in Ireland is ‘of great concern’, chief medical officer warnsOpens in new window ]
For someone who has developed a psychotic disorder in the context of cannabis use, it is possible – with abstinence from cannabis and medication – to manage and overcome the clinical symptoms of a psychotic disorder.
“[One can] go back to a good level of functioning compared to other types of psychosis,” she says.
On the flip side, she says, continued use of cannabis in this context can cause symptoms to recur, and become more severe over time.
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