WASHINGTON, USA (AFP) -- Young adults who started using cannabis at an early age and used it for several years are at increased risk of being diagnosed with a form of psychosis, of hallucinating or having delusions, a study published Monday found.

The study conducted at the Queensland Brain Institute at the University of Queensland in Australia asked more than 3,800 young adults, aged around 20 years, about their cannabis use, and assessed them for psychoses, delusional behaviour and to see if they had hallucinations.

Just over 14 per cent reported using cannabis for six or more years. The study found that they were twice as likely as those who had never used cannabis to be diagnosed with nonaffective psychoses, such as schizophrenia.

These longtime users were also twice as likely to report having hallucinations, and four times as likely to show delusional behaviour as non-users, the study shows.

"There were significant linear trends between the exposure variable and all three psychosis-related measures: the longer the duration since first cannabis use, the higher the risk of the adverse outcomes," the study says.

But which came first -- cannabis-use or psychosis -- remains unclear, the researchers said.

Study participants who reported having hallucinations early in life were more likely to have used cannabis longer and to use it more frequently, highlighting the "complexity of the relationship," the study says.

"Those individuals who ... had isolated psychotic symptoms were more likely to commence cannabis use, which could then contribute to an increased risk of conversion to a non-affective psychotic disorder," it says.

"Apart from the implications for policy-makers and health-planners, we hope our findings will encourage further clinical and animal model-based research to unravel the mechanisms linking cannabis use and psychosis," the study says