A major review into cannabis use has found that although the risk of fatal overdoses is extremely small, the potential risks of long-term use remain high.
Professor Wayne Hall from the National Addiction Centre at King’s College London, working also similar centres in Australia, went through available evidence on the health effects of cannabis since 1993.
He found that there are many adverse effects of chronic cannabis use. One in ten regular users can develop dependency, and one in six of those who start to use cannabis in adolescence go on to develop dependency.
Regular cannabis users also double their risk of experiencing psychotic symptoms and disorders. This risk is heightened if they have a personal or family history of psychotic disorder, or if they start to use cannabis in their mid-teens.
People who used cannabis regularly in their teens are also twice as likely to be diagnosed with schizophrenia or to report psychotic symptoms in adulthood.
Middle-aged adults who smoke marijuana could have a higher risk of myocardial infarction, Professor Hall found.
However, Hall’s report does acknowledge the difficulty in separating concerns about the risks of lung damage from cannabis to the risks from associated use of tobacco.
Driving while cannabis-intoxicated also doubles the risk of a car crash.
Professor Hall concluded that: "The epidemiological literature in the past 20 years shows that cannabis use increases the risk of accidents and can produce dependence, and that there are consistent associations between regular cannabis use and poor psychosocial outcomes and mental health in adulthood."
The study, which was published in the journal Addition, will be available to download in full for free until November 2014.