The longer people use cannabis or marijuana, the more likely they are to experience hallucinations or delusions or to suffer psychosis, according to a study released Saturday.
The study found that people who first used cannabis when they were aged 15 or younger were twice as likely to develop a "non-affective psychosis" — which can include schizophrenia — than those who had never used the drug.
The research led by John McGrath from the University of Queensland in Australia was based on a survey of 3,801 people with an average age of 20.1 years, the United States' Archives of General Psychiatry reported.
"Among all the participants, a longer duration since the first time they used cannabis was associated with multiple psychosis-related outcomes," the study said.
Of the group, 17.7 percent reported using cannabis for three or fewer years, 16.2 percent for four to five years, and 14.3 percent for six or more years.
Sixty-five were diagnosed with "non-affective psychosis", such as schizophrenia, and 233 had at least one hallucination, the study said.
"Individuals who had experienced hallucinations early in life were more likely to have used cannabis longer and to use it more frequently," it said.
But the relationship between psychosis and cannabis use was complex, it said.
People who were vulnerable to psychosis, in other words had isolated psychotic symptoms, "were more likely to commence cannabis use, which could then subsequently contribute to an increased risk of conversion to a non-affective psychotic disorder," the research said.
The article said previous studies had also identified an association between cannabis use and psychosis but there were concerns that research had not adequately accounted for confounding variables.