Marijuana is a mixture of dried, shredded leaves, stems, seeds and flowers of the hemp plant. People usually smoke it in a cigarette or in a pipe.

It is one the most commonly used illegal substances in many countries, with up to 20 percent of young people in places such as Britain reporting either some use or heavy use, British researchers said, citing government statistics.

A news study has found that using marijuana increases the risk of one day developing a psychotic illness-a mental state in which the perception of reality is distorted, reported the online edition of the New Scientist.

The study provides some of the strongest evidence yet linking the drug to a mental disorder.

The researchers did not look directly at people who used marijuana but instead reviewed 35 studies in search of a potential connection between psychotic illness and marijuana use.

They reviewed evidence from studies ranging from one year to 27 years and only looked at research that did not include people already showing signs of psychotic illness.

The researchers also adjusted for factors - such as depression or a susceptibility to harder drugs - that could one day lead to a mental disorder to focus more directly on the links between marijuana and psychosis.

"We have described a consistent association between cannabis use and psychotic symptoms, including disabling psychotic disorders," the team said.

"If you compare other substances like alcohol or tobacco, it may not be as harmful, but what we are saying is neither is it completely safe," Stanley Zammit, a study author and a psychiatrist at Cardiff University and the University of Bristol said.