Scientists in Queensland, Australia, on Monday suggested that young adults who smoke cannabis or marijuana for six years or more are twice as vulnerable to suffer from psychotic episodes, hallucinations or delusions compared to people who have never used the drug.

In addition, the researchers also checked out pot use and psychosis symptoms among siblings with an aim to make sure they haven't been missing some genetic or environmental factor.

However, the researchers write in their conclusion, “The nature of the relationship between psychosis and cannabis use is by no means simple".

The recent figures from United Nations claim that up to 190 million people worldwide use cannabis, equating to about 4 percent of the adult population.

John McGrath of the Queensland Brain Institute in Australia analyzed more than 3,801 men and women born between 1981 and 1984 and followed them up after 21 years to ask about their cannabis use and assessed them for psychotic episodes.

It reported that nearly 18 percent using cannabis for three or fewer years, 16 percent for four to five years and 14 percent for six or more years.