Cannabis does pose serious health risk, says major review

Regular cannabis use increases the risk of mental and physical health problems, and may cause intellectual impairment, according to a review of studies over 20 years by a leading professor.

Research published yesterday in the scientific journal ‘Addiction’ examined evidence on the adverse health effects of the drug since 1993.

The review concluded that:

  • One in six regular cannabis users who start in their teens risk becoming addicted to the drug.
  • Regular cannabis users double their risk of experiencing psychotic symptoms and disorders.
  • Teenagers who regularly take cannabis double the risk of schizophrenia or other psychotic symptoms in adulthood.
  • Regular cannabis use that begins in teenage years and continues throughout young adulthood appears to impair intelligence.
  • The study reported that the number of cannabis users wanting help to stop taking the drug has increased over the last 20 years in the United States, Europe and Australia.

    The same rise was seen in the Netherlands, where cannabis use has been decriminalised for nearly 40 years.


    The research was led by Professor Wayne Hall, a visiting professor at King’s College London’s Addictions Department, and advisor to the World Health Organisation.

    He said: “The important point I am trying to make is that people can get into difficulties with cannabis use, particularly if they get into daily use over a long period”.

    “There is no doubt that heavy users experience a withdrawal syndrome as with alcohol and heroin”, he warned.


    “If cannabis is not addictive then neither is heroin or alcohol”, Hall added.

    Hall noted that only a minority of cannabis users who had cognitive behaviour therapy for addiction managed to stay off the drug six and twelve months after treatment.

    Cannabis was downgraded from a Class B drug to a Class C drug in 2004, but in the three years that followed the number of cannabis addicts receiving NHS treatment doubled.

    Mental illness

    The reclassification was also accompanied by a surge in the number of children aged 15 and under being treated for mental illness.

    Faced with such overwhelming evidence, the Labour Government restored cannabis to a Class B drug in January 2009.

    Earlier this year, the Deputy Prime Minister Nick Clegg endorsed a controversial report calling for governments around the world to consider legalising cannabis.

    But a spokesman from No 10 said in response that David Cameron thinks the current approach to drugs laws is right.