Scientists in the UK say the heavy consumption of the drug cannabis may trigger psychotic illnesses such as schizophrenia.
The revelation is particularly relevant as there is increasing concern amongst experts about the mental health impact of smoking large amounts of modern super-strength cannabis, or skunk, particularly among young people.
Professor Philip McGuire and Zerrin Atakan of London's Institute of Psychiatry used magnetic resonance imaging, or MRI, to record the activity in the inferior frontal cortex brain region which is responsible for controlling inappropriate emotional and behavioural responses to situations.
The researchers gave Tetrahydrocannabinol (THC), anti-psychotic medicine, Amisulpride (CBD) or placebo capsules to healthy adult male volunteers who had not abused cannabis and then carried out the brain scans along with a battery of tests.
Professor McGuire says the THC appeared to switch off the part of the brain that is associated with how paranoid people become.
Although the effects were short-lived, some people appeared more vulnerable than others.
How cannabis reacts with the brain has until now been a puzzle but the use of MRI scanning techniques means that experts can now glean a clearer picture of the drugs' impact on brain activity.
Experts say in recent years, the THC levels in cannabis available in Britain has doubled to 12 percent from around 6 percent, while in the Netherlands it is about 18 percent; while the majority of users do not have a problem with the drug, for a minority there exists the possibly of long-term damage from modern skunk.
The research, due to be presented this week at a two-day International Cannabis and Mental Health Conference at the Institute of Psychiatry, will echo similar findings from other teams who have also seen the link between THC dose and the risk of schizophrenia-like symptoms.
Conference organiser Professor Robin Murray says experts on the whole accept that cannabis contributes to the onset of psychotic symptoms in general and schizophrenia, and it is no longer a contentious issue.
Murray says if something has an active effect in inducing the symptoms of psychosis after one dose, then it would not be at all surprising if repeated use induced a chronic condition.
Researchers estimate that as many as 500,000 people in the UK may be dependent on cannabis and they say increasing numbers of people are seeking help for cannabis problems at specialist clinics and were only superceded by heroin users in 2005.
Another study by a team from Yale University administered THC intravenously and found that even at relatively low doses, 50% of healthy volunteers began to show symptoms of psychosis; while volunteers with a history of psychotic symptoms appeared to be particularly vulnerable.
Professor Murray who is a consultant psychiatrist at the Institute, says the research provides the strongest evidence that cannabis had a significant impact on the brain and warns that the high potency cannabis now widely available was likely to pose a much bigger risk to health than the significantly weaker formulations of previous years.