Smoking cannabis daily triples the risk of developing serious mental illness

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Daily users of high potency cannabis had a four times elevated risk of psychosis, the researchers said.

A box of cannabis cigarette is seen in a coffee shop in Amsterdam October 6, 2011.

They found daily marijuana use was more common among people with a new psychosis diagnosis than among those who did not smoke marijuana.

Cannabis is defined as being strong when it has a THC level of more than 10 per cent.

High-potency cannabis was classified as having THC content over 10 percent. Still, the research suggests that cannabis users should think twice before using the strong stuff, especially if they use the drug on a regular basis.

"As the legal status of cannabis changes in many countries and states, and as we consider the medicinal properties of some types of cannabis, it is of vital public health importance that we also consider the potential adverse effects", said Marta di Forti, who co-led the work at King's College London's Institute of Psychiatry, Psychology and Neuroscience (IoPPN).

The study found that those who used pot daily were three times more likely to have a psychotic episode compared with someone who never used the drug. This, they said, would rise to 30.3%, a drop from 45.7 to 31.9 per 100,000 people per year in London, and 50.3%, a drop of 37.9 to 18.8 per 100,000 people per year, in Amsterdam.

And they said that even medicinal cannabis oil - available in the United Kingdom for a very limited number of people - should come with a warning of psychosis as a possible side effect. "But now the evidence is pretty clear". Using it a few times a week was less risky, but still increased the risk 1.6-fold. The research comes after a Lancet study said cannabis is responsible for 60,000 cases of depression in young people in Britain. She said, "That's a really interesting finding and that's not something anyone has done before".

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However, the study doesn't prove causality, cautions Dr. Diana Martinez, a psychiatrist and addiction researcher at Columbia University.

The study published in the Lancet Psychiatry Journal probed the patterns of cannabis or marijuana use across Europe to show its impact on mental health, particularly on the incidence of psychotic disorder.

Psychotic disorders such as schizophrenia and bipolar are complicated, "multifaceted disorders", Gage notes.

Genetics is known to play a major role, as are a host of environmental factors.

In the meantime, the new findings should be of interest to anyone using cannabis, says study author Di Forti.

Josie Laurent, 65, said her son Henri, 25, created an imaginary girlfriend, heard voices, pushed his mother and fought with his father and brother after becoming addicted to the drug.

[3] Data derive from the Population Attributable Fraction (PAF) which measures the population effect of an exposure by providing an estimate of the proportion of cases that would be prevented if the exposure (ie, daily/high potency cannabis use) were removed.

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