Although most teenagers accept that cannabis can cause mental health problems, some still take it to "cope with life", according to the FRANK drug information service.
It reports that two-fifths know someone who has had problems, including paranoia, panic attacks and memory loss, and half associate it with doing badly at school or college.
But despite that, a fifth of teenagers still admit to feeling under pressure to try the drug, while a tenth say it makes them "look cool".
Says FRANK spokesman Chris Hudson: "The majority of teenagers don't want to risk their health by using cannabis. However, some people choose to take the risk, while others wrongly believe cannabis is harmless because it is a plant.
"Cannabis messes with your mind - and reactions can be more powerful with stronger strains such as skunk, which is around twice as potent."
Three-quarters of young people aged between 11 and 18 have said that they are less likely to try cannabis in the future after being made aware of the dangers.
FRANK is a service provided by the Department of Health, the Home Office and the Department for Children, Schools and Families.