Even short term use of cannabis can damage the lungs of young people, say researchers.
Cannabis is a controversial drug
A team from Queen Elizabeth Hospital, Birmingham, conducted tests on smokers, non-smokers and cannabis users.
Non-smokers had the healthiest lungs. But cannabis users showed more signs of damage than those who stuck to cigarettes.
A British Thoracic Society meeting heard less than six years cannabis use was enough to cause significant damage.
Dr Sarah Nuttall, a research fellow from the Department of Clinical Pharmacology at Queen Elizabeth Hospital, said: "The consensus among many young people who use cannabis seems to be that they will not suffer any long-term effects as long as they stop smoking it early enough.
"However this is clearly not the case - our study shows that even short-term
use of cannabis in addition to tobacco use does have an impact and makes a
serious difference to lung function."
Dr John Harvey, chairman of the BTS communications committee, said: "It is
vital that young people understand the dangers of both cigarette and cannabis
smoking since these habits can start having a serious impact on their lungs at
an early stage."
A British Medical Journal study published earlier this year warned that regular cannabis use may be as dangerous as smoking in the long term.
Professor John Henry, a toxicologist at Imperial College London, feared that deaths attributable to cannabis could soar.
There are currently an estimated 3.2 million people in the Britain who smoke cannabis regularly, compared with 13 million tobacco smokers.