Long-term cannabis use can double risk of psychosis

LONDON Mon Mar 1, 2010 7:50am EST

A young woman smokes marijuana before the 10th annual Marijuana March in downtown Toronto May 3, 2008. REUTERS/Mark Blinch

A young woman smokes marijuana before the 10th annual Marijuana March in downtown Toronto May 3, 2008.

Credit: Reuters/Mark Blinch

Related Topics

LONDON (Reuters) - Young people who smoke cannabis or marijuana for six years or more are twice as likely to have psychotic episodes, hallucinations or delusions than people who have never used the drug, scientists said on Monday.

The findings adds weight to previous research which linked psychosis with the drug -- particularly in its most potent form as "skunk" -- and will feed the debate about the level of controls over its use.

Despite laws against it, up to 190 million people around the world use cannabis, according to United Nations estimates, equating to about 4 percent of the adult population.

John McGrath of the Queensland Brain Institute in Australia studied more than 3,801 men and women born between 1981 and 1984 and followed them up after 21 years to ask about their cannabis use and assessed them for psychotic episodes. Around 18 percent reported using cannabis for three or fewer years, 16 percent for four to five years and 14 percent for six or more years.

"Compared with those who had never used cannabis, young adults who had six or more years since first use of cannabis were twice as likely to develop a non-affective psychosis (such as schizophrenia)," McGrath wrote in a study published in the Archives of General Psychiatry journal.

They were also four times as likely to have high scores in clinical tests of delusion, he wrote, and a so-called "dose-response" relationship showed that the longer the duration since first cannabis use, the higher the risk of psychosis-related symptoms.

A study by British scientists last year suggested that people who smoke skunk, a potent form of cannabis, are almost seven times more likely to develop psychotic illnesses such as schizophrenia than those who smoke "hash" or cannabis resin.

Previous studies had also suggested smoking cannabis can double the risk of psychosis, but the British study was the first to look specifically at skunk. Skunk has higher amounts of the psychoactive ingredient THC which can produce psychotic symptoms such as hallucinations, delusions and paranoia.

McGrath said, however, that "the nature of the relationship between psychosis and cannabis use is by no means simple" and more research was needed to examine the mechanisms at work.

As part of his study, McGrath and his team looked at links between cannabis use and psychotic symptoms among a group of 228 sibling pairs and found the association still held. This suggests other influences like genes or the environment were less likely to be responsible for the psychosis, they said.

A international group of drug policy experts published a book earlier this year arguing that laws against cannabis have failed to cut its use but instead led to vast numbers of arrests for drug possession in countries like Britain, Switzerland and the United States, which cause social division and pointless government expense.

(Editing by Myra MacDonald)

FILED UNDER:
We welcome comments that advance the story through relevant opinion, anecdotes, links and data. If you see a comment that you believe is irrelevant or inappropriate, you can flag it to our editors by using the report abuse links. Views expressed in the comments do not represent those of Reuters. For more information on our comment policy, see http://blogs.reuters.com/fulldisclosure/2010/09/27/toward-a-more-thoughtful-conversation-on-stories/
Comments (117)
phil9x wrote:
If this were true there would be millions of people that would need to check into the mental institutions. These so called scientists should look into more dangerous drugs such as alcohol, tobacco or any number of anti depressants being pushed by big pharma!!!

Mar 01, 2010 6:37am EST  --  Report as abuse
fx4nwoo wrote:
John McGrath of the Queensland Brain Institute in Australia studied more than “3,801 men and women born between 1981 and 1984 and followed them up after 21 years to ask about their cannabis use and assessed them for psychotic episodes.”

“1981-1984″ and then the specific time frame of smoking for 6 years?? Hardly scientific!

Mar 01, 2010 7:17am EST  --  Report as abuse
RJD wrote:
Where is the source??? What study did these findings come from??
I can also say “Findings say all cats are actually dogs.” and it has as much credence. What shoddy journalism.

Mar 01, 2010 7:18am EST  --  Report as abuse
This discussion is now closed. We welcome comments on our articles for a limited period after their publication.

Pictures