Smoking cannabis is more harmful than cigarettes and more likely to trigger cancer, according to a report.
Just three cannabis 'joints' a day can cause the same amount of damage to the lungs as an entire packet of 20 cigarettes.
The British Lung Foundation says that when cannabis and tobacco are smoked together, the harmful effects are significantly worse.
Its research suggests young cannabis smokers may also be at greater risk of throat and gullet cancers.
The foundation found that tar from cannabis joints contains 50 per cent more cancer-causing toxins than cigarettes made from tobacco alone.
Eight million Britons are thought to smoke cannabis, which some experts believe is a 'gateway' to harder drugs such as heroin and cocaine.
Earlier this year, researchers found that 79 per cent of children thought cannabis was safe while only 2 per cent recognised there are health risks from smoking the drug.
Dame Helena Shovelton, chief executive of the British Lung Foundation, said the harmful effects of cannabis had been swept under the carpet.
'People are under the illusion it is safe to smoke cannabis. Our report shows it is very dangerous to lung health, at least as dangerous as tobacco.
'It seems society is in the same position as when research first showed the harm caused by tobacco. It took 15 years for the Government to take notice but we don't want to repeat the mistakes of the past.'
Dame Helena said cannabis available today is 15 times stronger than the drug smoked in the 1960s. 'This means studies carried out at that time will probably have underestimated the effects of cannabis smoking,' she explained.
'Puff and inhalation volume with cannabis is up to four times higher than with tobacco - in other words you inhale deeper and hold your breath with the smoke for longer before exhaling.
'This results in more poisonous carbon monoxide and tar entering into the lungs,' Dame Helena said.
The foundation's report - A Smoking Gun? - analyses research from around the world.
It found cannabis smokers have a higher level of chronic and acute respiratory-conditions such as coughingwheezing and bronchitis. 'When cannabis is smoked together with tobacco then the effects are additive', it says.
Some studies suggest cannabis smoking may trigger chronic obstructive pulmonary disease which kills 32,000 people in Britain every year, the foundation's report adds.
'Research linking cannabis smoking to the development of respiratory cancer exists although there have also been conflicting findings.
'Not only does the tar in a cannabis cigarette contain many of the same carcinogens as tobacco smoke, but the concentrations of these are up to 50 per cent higher in the smoke of a cannabis cigarette,' it says.
Benzyprene, found in the tar of cannabis joints, can change the make-up of one of the genes which suppresses tumours and could therefore make cancer more likely for people who smoke joints.
There are also more than 75 case studies of young cannabis smokers with cancers of the throat and gullet - diseases usually rare in people under 60.
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