Cannabis users have a fivefold risk of taking psychosis meds

In the new study, the researchers linked data from the Young in Norway study with data from the Norwegian Prescription Database (NorPD).

"The study suggests that using cannabis increases the likelihood of being prescribed psychoactive drugs later," says Eline Rognli, who is the lead researcher behind the new study and a psychologist and researcher at Oslo University Hospital.

Psychopharmaceuticals are a collective term for medicines used to treat mental illness.

The study shows that cannabis users have more than five times the risk of being prescribed antipsychotics later, compared to non-users. Antipsychotics are used to manage mental illness, with schizophrenia being the most serious psychotic illness.

Bipolar disorder and depression

The researchers also found that the risk of needing mood stabilizing drugs was five times greater if patients had previously used hashish or marijuana. These medications are often used for bipolar disorder.

Cannabis users were twice as likely to use antidepressants as compared to individuals who hadn’t used cannabis. However, users did not have an increased likelihood of using anxiety medications.

The researchers did not know how much cannabis study participants had used or their frequency of use.

In the Young in Norway study, participants are asked whether they have used cannabis during their lifetime but not in the past year, and whether they have used it during the past year.

“Both groups were compared to individuals who had never used cannabis. We only found associations with later psychopharmaceutical use in the group that reported using it during the past year. The participants were well into their 20s when asked about this, so they may have been using cannabis for a while,” says Rognli.

Chicken or egg question

So what comes first – cannabis use or mental illness?

“This study leads us to believe that cannabis use increases the risk of mental illness – or at least the risk of being prescribed medication for mental illness. However, we can’t prove that the mental disorders didn’t exist before cannabis use, even though we tried to control for that in the study,” Rognli says.

The researchers did this by excluding people who had used psychoactive drugs before claiming to have used cannabis.

The analyses also took into account whether individuals reported feeling mentally unbalanced before cannabis use was measured.

In addition, a psychiatrist and a pharmacist assessed whether the medications that patients took could have been prescribed for other disorders, and therefore should be excluded from the study – psychopharmaceuticals can for instance be used to treat epilepsy.

Researchers have studied the link between cannabis use and mental illness in numerous studies.

“Some studies have looked at hospitalization for mental illness after cannabis use, and others have looked at diagnoses. This is the first time a major study is dealing with the prescription of medication for mental disorders,” says Rognli.

The results of this study are in line with several other studies, which also indicate a fivefold risk of psychotic disorders after high levels of cannabis use.

“Schizophrenia is a very serious disorder. At the same time, it's important to note that the risk of getting schizophrenia is low to begin with, so even if it increases fivefold, very few cannabis users will suffer from the illness,” she says.

Risk from cannabis use

Rognli points out that many people have a lot of opinions about mental disorders and cannabis use.

“It’s easy to fall into one of two camps – either to exaggerate how dangerous cannabis use is or to trivialize its use. Most people don’t run into problems, but we have to recognize that there is a risk associated with cannabis use,” says Rognli.

Adolescents who have other risk factors, such as psychosis, should be on the alert. Some people are genetically more susceptible to mental illness.

“Some people may have a relative who has had mental health issues, or themselves experienced some symptoms of psychosis after earlier cannabis use. That should ring the alarm bells, and health professionals and family members should make it clear that cannabis can be harmful to the young person in such cases,” she says.

She believes that the sum of risk factors can be decisive for mental illness – and that cannabis is one of the risk factors.

No definitive causal link

Trine Vik Lagerberg heads the Psychosis Research Unit in the Division of Mental Health and Addiction at Oslo University Hospital.

“This study was conducted well, and it complements previous studies, but despite being prospective it can’t speak with any certainty about the causal relationship,” says Lagerberg.

She says the connection between cannabis and later development of psychosis disorders has long been known, but whether cannabis use actually triggers mental disorders is still an open question.

One possibility is that the association is due to genes.

Lagerberg says that genetic studies have begun to investigate whether the genes that cause the risk of psychosis disorders also increase the risk of developing cannabis use – that there is a genetic overlap, in other words. One finding is that mentally healthy people who use cannabis have a higher genetic risk of schizophrenia than people who do not use cannabis.

Thus, it seems that a genetic vulnerability for the one issue also implies a genetic vulnerability for the other.

Genetic vulnerability

But Lagerberg doesn’t rule out that cannabis may be psychosis-triggering.

“For example, a recent multi-centre study has shown that the higher the THC content of the cannabis you buy on the street, the more people in that city you’ll find with a psychosis disorder. THC is the most psychosis-causing component of the drug. If the relationship was only due to genetics, you wouldn’t expect such a connection,” says Lagerberg.

It’s also not always the case that both identical twins get schizophrenia – the risk is 40 to 50 per cent if you have one identical twin with schizophrenia.

“So there must be environmental factors that also play into it, such as drug use, trauma, infections, birth complications, or growing up in a big city – these are the risk factors that have been found so far. What you inherit is a genetic vulnerability. If drug use triggers psychosis disorders, it’s most likely to happen because you have this kind of genetic vulnerability,” she says.

She adds that the vast majority of people with psychosis disorders develop them without having used drugs to any greater extent than the rest of the population.

Reference:

Eline Rognli et al: Cannabis use in early adulthood is prospectively associated with prescriptions of antipsychotics, mood stabilizers, and antidepressants, Acta Psychiatrica Scandinavia, September 2019.

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Read the Norwegian version of this article at forskning.no