Cannabis use can become addictive and cause mental health problems, according to a 20-year study.
The research, carried out over 20 years by Wayne Hall, a British expert on addiction, found that one in six teenagers who regularly smoke the drug become dependent on it.
It concluded that using the drug while pregnant reduces birth weight, while teenage users do worse at school and are twice as likely to be diagnosed with schizophrenia or report psychotic symptoms in adulthood.
Prof Hall, of King’s College, London, found that one in 10 of all regular users develop a dependence.
His study, published in the journal Addiction, also found that cannabis smokers put themselves at a higher risk of developing chronic bronchitis, while use by middle-aged people were “probably” at higher risk of a heart attack.
Evidence has strengthened over the past 20 years that users who stop smoking the drug can suffer withdrawal symptoms which are “often of sufficient severity to impair everyday functioning”. The most common conditions include anxiety, insomnia, appetite disturbance, and depression.
It was noted that the number of users seeking help to quit or control their habit has increased during the past two decades in Europe, the US, and Australia.
Prof Hall said the risk of dependence was “under-appreciated by lots of users”.’
“People get caught up, particularly in daily patterns of use, find it difficult to stop or control their use or cease, and continually use the drug despite problems caused by its use,” he said.
The expert said cannabis was at the lower scale of risk for addiction to drugs, but warned that the danger was higher for young people.
“For teenagers, the risk of developing dependence goes up,” he said. “American research suggests it might be as high as one in six amongst young people who might start using cannabis in their mid-teens.
“The particular concern in adolescence are people who get involved in that very heavy pattern of use are much more likely to disengage from school, not complete their education, not engage in further training and drift into affiliating with other people who are similar heavy cannabis users.”
Niall Campbell, consultant psychiatrist at the mental health hospital, Priory, said: “At Priory we are seeing increasing numbers of patients presenting directly with cannabis addiction and also patients who on further questioning have had considerable difficulties over the years from cannabis.
“There is still a perceived popular wisdom that cannabis is not addictive. This is not our experience.”
Danny Kushlick of Transform Drug Policy Foundation said: “Like all drugs, cannabis does carry risks and some people will become dependent. This is precisely why it needs to be strictly legally regulated by governments, rather than left in the hands of organised criminals and unregulated dealers.
“Far from undermining the call for legalisation and regulation of cannabis, this study adds weight to calls for alternatives to the failed policy of prohibition.”
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