Norwegian researchers claim cannabis abuse can triple risk of psychosis

Norwegian research results suggest that cannabis abuse triggers psychosis and not the other way round, Norwegian media reported on Wednesday.

Cannabis abuse can triple the risk of psychosis, according to a report by researchers from the Norwegian Institute of Public Health (FHI) who, together with U.S. colleagues, studied the abuse of cannabis and symptoms of psychosis among nearly 1,400 pairs of twins.

They have found a significant connection between the abuse of cannabis and symptoms of psychosis in general, something that had been proved before. The question was: what was causing what.

"The new discovery is that, among a pair of twins where only one smokes cannabis, that twin has a 3.5 times higher risk of developing psychosis," Eivind Ystrom, senior researcher from FHI, told Norwegian news agency NTB.

"We have also tested the hypothesis that symptoms of psychosis cause cannabis abuse, but that hypothesis was less suited to the data. It appears, therefore, that cannabis abuse is a cause of psychosis," he said.

According to Ystrom, the survey was done by direct interviews and diagnosing, not by filling in a questionnaire, which made it altogether "far more than a suggestion."

Both psychosis and cannabis use and abuse have high heritability. Therefore, it can happen that people who have developed psychosis would have done so anyway, only at a somewhat later stage of life. Previous research has shown that cannabis abuse can accelerate a hereditary psychosis for almost three years.

Ystrom thinks that these factors should be taken into consideration when evaluating the consequences of decriminalization or legalization of personal use of cannabis products.

"A basic question is about determining in what grade psychotic symptoms should be regarded as triggers of addiction," said Cecilie Hillestad Hoff, a specialist in clinical psychology who has worked with psychotic patients for nine years.

"An important question then becomes whether the individual patient has ingested cannabis to reduce internal, psychological pain, or whether cannabis smoking precedes the development of psychosis. Research that helps to point out the direction of causality is very helpful," she said.

Hillestad Hoff also emphasized the complexity of determining causality. "We still know very little about the reason some people develop a psychotic disorder," she said. Endit