Award-winning researchers have claimed that smoking cannabis trebles the risk of becoming schizophrenic.
The study lasted 27 years and examined 50,000 people
Scientists from Cardiff University studied the life patterns of 50,000 people who carried out national service in Sweden over a 27-year period.
The study found that people who had used cannabis more than 50 times before the age of 18 were three times more likely to develop schizophrenia.
The team has won an award from health charity Bupa Foundation for its work.
Project leader Stanley Zammit, a clinical lecturer in psychological medicine at the University, said the findings could be "hugely important" for public health.
He said: "If cannabis does indeed increase the risk of developing schizophrenia, then you could potentially prevent hundreds of people getting the mental illness if cannabis was not available in the population."
Dr Zammit and his team won the Bupa Foundation Epidemiology Award and have presented with a cheque for £10,000 at a ceremony at Lincoln's Inn in London.
The vice-chairman of the foundation, which funds medical research, Andrew Vallance-Owen, said: "This is a key piece of research that sheds light on cannabis and a mental illness that blights the lives of one in one hundred of us.
"The findings are thought provoking not only for those people who suffer from schizophrenia, but for everyone, and will spark widespread debate."
The UK government downgraded cannabis from a Class B to a Class C drug in January 2004 after advice from the Advisory Council on the Misuse of Drugs.
But ministers said earlier this year the decision may be reviewed after studies linked the drug to mental illness.
In March 2005 researchers in New Zealand released the results of a study which claimed smoking cannabis virtually doubled the risk of developing mental illnesses such as schizophrenia.