Almost as many teenagers and young adults smoke cannabis as cigarettes, a study of the drug’s effects on health has found.
Whereas in the 1990s it was regular smokers who tended to start using cannabis, it is now often the other way round following changing attitudes to tobacco, with weed smokers moving on to regular cigarettes.
Professor Wayne Hall, from the National Addiction Centre at King’s College London, said that its negative effects should not be underestimated after looking at two decades of data.
“Over the past 20 years, we have seen a large increase in the number of people smoking cannabis," he said.
“What’s clear is that cannabis, especially when users smoke it regularly and from a young age, can have a detrimental impact on people’s mental health.”
The study, published in the journal Addiction, found that cannabis is less addictive than nicotine, with approximately 9 per cent of users becoming addicted compared to 32 per cent.
But the risk of addiction is higher with teenagers, with one in six who regularly use it becoming dependent and one in ten adults.
For comparison, the figure for heroin is 23 per cent and 15 per cent for alcohol.
Professor Hall, who is also Director of the Centre for Youth Substance Abuse Research at the University of Queensland, said more people are becoming addicted than before.
“It is now difficult to argue that cannabis dependence does not require professional attention,” he added.
“Whilst the mental and physical impact of cannabis dependence is less severe than alcohol or heroin dependence, the number of people who are able to stop smoking cannabis completely following treatment is still very low.”
Video: New study shows Cannabis is harmful
Daily cannabis users double their risk of experiencing psychotic symptoms and disorders, especially if they have a personal or family history of psychosis and start using cannabis in their teens, the study said.
Teenagers who smoke weed were also found to be more likely to use other illegal drugs but evidence suggests the connection could be because of other risk factors common to all substance misuse rather than cannabis itself.
Driving while high doubles the risk of car crashes, the study found, and smoking cannabis during pregnancy reduces babies’ birth weight.
Other health risks, including heart disease, are also associated with the drug because of the tobacco often smoked with it or alongside it by users.
Campaigners calling for cannabis to be legalised have focused on the fact that no user has ever died from an overdose and argued that the global “war on drugs” is responsible for more deaths than it has ever prevented.
The medical use of cannabis is allowed in some countries and Colorado became the second US state to legalise the drug earlier this year.
CLEAR Cannabis Law Reform, a British political party advocating the end to the criminalisation of cannabis, put forward its own plans last year.
It suggested the creation of a Government “cannabis inspectorate” controlling licensing for domestic and commercial cultivation, importation and retail.
Tony Blair's government relaxed the law on cannabis in 2004, reclassifying it from class B to C in 2004, but the change was reversed after Gordon Brown entered Downing Street in 2007.