More than two-fifths of teenagers (42 per cent) know someone who has suffered mental health harm from using cannabis, according to a survey published today.
The study by drug information service FRANK found that 74 per cent of young people are aware that cannabis can harm mental health, while 56 per cent associate cannabis use with losing motivation and doing badly at school or college.
A spokesman for the service said that mental health harm refers to paranoia, panic attacks and memory loss.
The study found that despite being aware of the risks, some teenagers still admit to feeling under pressure to try cannabis (18 per cent), with 11 per cent saying it makes them look cool.
Others said they use it to help them to cope with life (14 per cent), while a quarter (26 per cent) see it as a "natural" drug, despite the risks.
Chris Hudson from FRANK said: "The majority of teenagers (55 per cent) 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.
"For free, confidential advice about drugs talk to FRANK at any time of the day or night on 0800 77 66 00 or visit www.talktofrank.com ."
The organisation is to re-run an advertising campaign called "Braincrashers" on TV, radio and online from next Monday.
Nearly three-quarters (74 per cent) of 11-18-year-olds said they were less likely to try cannabis in the future as a result of seeing its last airing, the service said.
Almost one third (29.1 per cent) of calls to the FRANK helpline in 2008/09 were about cannabis - making it the most asked-about topic.
* 27,838 11-18-year-olds were surveyed using the social networking site Habbo Hotel.
* FRANK is provided by the Department of Health, the Home Office and the Department for Children, Schools and Families.