The research is led by Theresa Moore of the University of Bristol, western England, and Stanley Zammit of Cardiff University, Wales.
The risk increased with use. The study found the increased risk for psychotic illness was relative to the dose. Those who smoked cannabis regularly were at an increased risk of between 50 per cent and 200 per cent of developing schizophrenia and disorders with symptoms including hallucinations or delusions.
The researchers call on health supremos to warn young people about the risk to their mind from a drug that many today may dismiss as harmless and recreational.
"We conclude that there is now sufficient evidence to warn young people that using cannabis could increase their risk of developing a psychotic illness later in life," concluded Stanley Zammit, Ph.D., of Cardiff University, and colleagues.
Stopping cannabis use would decrease the risk, said the lead author, Dr Stanley Zammit, a psychiatrist from Cardiff University and Bristol University in Britain.
Marijuana, or cannabis -- the parent plant -- is the most commonly used illegal substance in most countries, the authors noted. In 2006, about 42% of America's high school seniors reported having tried marijuana at least once, according to an annual report funded by the National Institute on Drug Abuse. More than one third of Australians over 14 years of age have smoked cannabis, or marijuana, at least once in their life and one in 20 have used the drug in the past week, according to figures on drug use from the Australian Institute of Health and Welfare. In the case of Britain, about 40 per cent of young adults and adolescents have used cannabis, according to figures cited in the study. By extrapolation, around 14 per cent of cases of psychotic episodes among young British adults would be avoided if cannabis were not consumed, the paper contends.
Marijuana can cause psychiatric problems because it throws off the balance of neurotransmitters in the brain.
About 20% of young people report using the hallucinogen at least once a week or heavy use, defined as using on more than 100 occasions. The development of chronic symptoms that persist beyond or independently of intoxication provides reason for greater concern.
Previous studies have had difficulty untangling marijuana's role in psychiatric disorders. Smoking the drug could be a symptom of a disorder rather than a cause.
One in 100 people had a chance of developing severe psychotic illness. That risk increased to 1.4 in 100 if they had ever smoked cannabis.
In an accompanying comment, Merete Nordentoft and Carsten Hjorthoej, of the department of Psychiatry at the Copenhagen University Hospital, said cannabis had long been considered a harmless drug and its potential long-term effects on psychosis had been overlooked. "There is a need to warn the public of these dangers, as well as establish treatment to help young, frequent cannabis users," they wrote.
The researchers also studied the relationship between marijuana use and mood disorders, such as depression and bipolar disorder. They analyzed 22 studies involving 52,000 participants.
The researchers found that any marijuana use increased the lifetime risk for mood disorders by about 40%, and weekly or daily use increased the risk by about 50%.
Cannabis accounted for 45 per cent of hospital admissions due to drug-induced psychosis in 2003-04, according to a study published in the Australian Medical Journal. John Saunders, Professor of Alcohol and Drug studies at the University of Queensland, said the latest research strengthened the need for increased education on the dangers of cannabis.