Experimenting with cannabis on a casual basis damages the brain permanently, research has found.
It is far from being a "safe" drug and no one under the age of 30 should ever use it, experts said.
People who had only used cannabis once or twice a week for a matter of months were found to have changes in the brain that govern emotion, motivation and addiction.
Researchers from Harvard Medical School in America carried out detailed 3-D scans on the brains of students who used cannabis casually and were not addicted and compared them with those who had never used it.
Two major sections of the brain were found to be affected.
The scientists found that the more cannabis the 40 subjects had used, the greater the abnormalities.
Around 10 million people in Britain, almost a third of the population, have used illegal drugs, with cannabis the most popular. Research author, Dr Hans Breiter, professor of psychiatry and behavioural sciences at Northwestern University Feinberg School of Medicine, said: "This study raises a strong challenge to the idea that casual marijuana use isn't associated with bad consequences. Some people only used marijuana to get high once or twice a week.
"People think a little recreational use shouldn't cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case.
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"I've developed a severe worry about whether we should be allowing anybody under age 30 to use pot unless they have a terminal illness and need it for pain."
The team examined sections of the brain involved in emotion, motivation and addiction in 20 students who had used cannabis and 20 who had not. Anne Blood, assistant professor of psychiatry at Harvard Medical School, said: "These are core, fundamental structures of the brain. They form the basis for how you assess positive and negative features about things in the environment and make decisions about them."
The changes are thought to be the first steps towards addiction as the brain alters the way it perceives reward and pleasure, making ordinary experiences seem less fulfilling compared with drug use.
Jodi Gilman, a researcher in the Massachusetts General Center for Addiction Medicine, said: "It may be that we're seeing a type of drug learning in the brain. We think when people are in the process of becoming addicted, their brains form these new connections.
"Drugs of abuse can cause more dopamine release than natural rewards like food, sex and social interaction. That is why drugs take on so much salience, and everything else loses its importance."
The study is published in the Journal of Neurosciences.
Mark Winstanley, chief executive of Rethink Mental Illness, said: "For too long cannabis has been seen as a safe drug, but as this study suggests, it can have a really serious impact on your mental health.
"Research also shows that when people smoke cannabis before the age of 15, it quadruples their chance of developing psychosis. But very few people are aware of the risks involved."
Prof David Nutt, from Imperial College, London, said a sample of 40 was not big enough to draw conclusions.
Prof Nutt, who was sacked as a government drugs adviser for his views, added: "Whatever cannabis does to the brain its not in the same league as alcohol which is a proven neurotoxin."