A new study says people who toke marijuana every day or every week double their chances of developing a mental illness over their lifetime. And if you've ever smoked a joint, you increased your lifetime risk of developing psychotic illness by 40 per cent.
Digital Journal - The study published in the Lancet medical journal (subscription required) is being touted as "one of the most comprehensive, thorough and reliable reviews of its kind."
The study found a strong relationship between heavy marijuana use and schizophrenia, paranoia, hearing voices and hallucinations.
"It's not as if you smoke a joint and you're going to go crazy," Richard Rawson told the L.A. Times. Rawson directs the Integrated Substance Abuse Program at UCLA and was not part of the study. However, he said, "It's definitely not a good idea to use heavy amounts of marijuana."
According to the Lancent, Cannabis is the most commonly used illegal substance in most countries and up to 20 per cent of young people smoke it at least once per week.
The study's co-author, Dr. Stanley Zammit of Cadiff University and the University of Bristol in Britain, said risk of developing psychotic illnesses rises with dosage, suggesting that cutting back on pot smoking or stopping altogether would lower risks.
"People who have ever used cannabis, on average, have about a 40 per cent increased risk of developing psychotic illness later in life compared with people who have never used cannabis," Zammit told WebMD. "People who used it on a weekly or daily basis had about a 100 per cent increased risk, or twofold."
While not likely to change laws on medicinal marijuana usage, the study's authors believe there is solid evidence linking psychosis with pot smoking, and the public needs to know about it.
"If you compare other substances like alcohol or tobacco it may not be as harmful, but what we are saying is neither is it completely safe," Zammit told Reuters.
The results of the study say up to 800 schizophrenia cases could be prevented in the U.K. each year by stopping marijuana usage.
The study did not look directly at pot smokers, but instead a "meta-analysis" of 35 other studies. Researchers looked for a connection between psychotic illness and marijuana consumption.
The research focused directly on the links between marijuana and psychosis, rather that elements that could one day lead to psychosis such as depression or harder drug usage. Researchers looked at evidence from studies that ranged from one year, to 27 years.
"We have described a consistent association between cannabis use and psychotic symptoms, including disabling psychotic disorders," the team wrote.
The authors said ultimate proof to confirm the study's findings should come from a randomized trial of healthy young people with long-term follow-ups.