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Pot could boost psychosis risk later in life, study finds

July 27, 2007
Courtesy The Lancet
and World Science staff

There’s now enough ev­i­dence to warn young peo­ple that ma­ri­jua­na use could in­crease their risk of de­vel­op­ing psy­cho­sis lat­er in life, a group of re­search­ers say. Psy­cho­sis is gen­er­ally de­fined as men­tal ill­ness in­volv­ing loss of con­tact with real­ity.

The sci­en­tists re­ported their find­ings in this week’s edi­tion of the med­i­cal jour­nal The Lan­cet, based on a re­view of past stud­ies.

The re­search­ers ac­knowl­edged the da­ta was­n’t yet con­clu­sive. Yet “de­spite the in­ev­i­ta­ble un­cer­tain­ty, pol­i­cy­makers need to pro­vide the pub­lic with ad­vice about this widely used drug,” they wrote. “We be­lieve that there is now enough ev­i­dence to in­form peo­ple that us­ing can­na­bis,” or ma­ri­jua­na, “could in­crease their risk of de­vel­op­ing a psy­chot­ic ill­ness lat­er in life.”

“Gov­ern­ments would do well to in­vest in sus­tained and ef­fec­tive educa­t­ion cam­paigns on the risks” of the drug, an ac­com­pa­nying ed­i­to­ri­al in the jour­nal said. Ma­ri­jua­na, al­so called can­na­bis, is the most com­monly used il­le­gal sub­stance in most coun­tries. Up to one in five young peo­ple re­port us­ing it at least once week­ly. 

In the new re­search, The­re­sa Moore of the Un­ivers­ity of Bris­tol, U.K., and Stan­ley Za­m­mit of Car­diff Un­ivers­ity, Wales, an­a­lyzed 35 past stud­ies on ma­ri­jua­na up to last year. They found that peo­ple who had used can­na­bis were 41 per­cent more likely than those who had nev­er used it to have any form of psy­cho­sis. The risk rose with dos­age, the re­search­ers added. They cal­cu­lat­ed that about 14 per­cent of psy­chot­ic episodes in young adults in the UK would not oc­cur if can­na­bis weren’t con­sumed.


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There’s now enough evidence to warn young people that marijuana use could increase their risk of developing psychosis later in life, a group of researchers say. Psychosis is generally defined as mental illness involving loss of contact with reality. The scientists reported their findings in this week’s edition of the medical journal The Lancet, based on a review of past studies. The researchers acknowledged the data wasn’t yet conclusive. Yet “despite the inevitable uncertainty, policymakers need to provide the public with advice about this widely used drug,” they wrote. “We believe that there is now enough evidence to inform people that using cannabis,” or marijuana, “could increase their risk of developing a psychotic illness later in life.” “Governments would do well to invest in sustained and effective education campaigns on the risks to health” of the drug, an accompanying editorial in the journal said. Marijuana, also called cannabis, is the most commonly used illegal substance in most countries. Up to one in five young people report using it at least once weekly. In the new research, Theresa Moore of the University of Bristol, U.K., and Stanley Zammit of Cardiff University, Wales, analyzed 35 past studies on marijuana up to last year. They found that people who had used cannabis were 41% more likely than those who had never used it to have any form of psychosis. The risk rose with dosage, the researchers added. They calculated that about 14% of psychotic episodes in young adults in the UK would not occur if cannabis were not consumed.