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Using Marijuana before the Age of 15 Increases Risk of Psychotic Disorders

posted 07:10 AM EST, Fri March 05, 2010
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Teens that start smoking marijuana before the age of 15 are twice as likely to develop a psychotic disorder by the age of 21 as teens that don’t smoke marijuana.

An Australian research study done on siblings has found that teens that start using marijuana at 15 or younger have a significantly higher risk of psychotic disorders.

Australian scientists enlisted 3801 young adults born in Brisbane Australia between 1981 and 1984 (many were sibling pairs) to participate in the study. Study subjects were asked at the age of 21 about their history of marijuana use and whether they’d ever had any incidences of hallucinations or delusions, or had experienced any psychotic disorders, such as schizophrenia.

Sibling pairs were used when possible to minimize genetic and environmental differences that can otherwise confound results.

The researchers found that:

  • Those that used marijuana for 6 years or longer (initiated use prior to the age of 15) were twice as likely to develop schizophrenia or another delusional disorder as those that had never used marijuana
  • Subjects who had used marijuana for the longest period of time were more likely to score highly on tests for psychotic experiences

Lead researcher John McGrath says that the results of this study confirm other studies which have also found an association between marijuana use and an increased likelihood of a psychotic disorder, saying “The evidence is really mounting up.”

He says that although his study didn’t factor in the amount teens were smoking that earlier studies have shown a link between quantity of use and risk of psychosis. Where his study adds to the knowledge base, says McGrath, is by showing that an earlier age of initiation causes an increased rate of psychosis. He explains, saying, “What we think it suggests is that the adolescent brain is more vulnerable to drug abuse.”

McGrath says that young people need to know that marijuana may not be as harmless as they perceive it to be. He cautions, "Everything we do has risks involved - whether we drive or ride a bike or use alcohol - and I think it's important that young people know that if you use cannabis from an early age you do increase your risk of a psychotic disorder."

The full study results can be read in the journal, Archives of General Psychiatry.

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  • Marijuana: Use before the age of 15 doubles the risks of psychotic disorders
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