Long-term cannabis use raises risk of lung cancer

Cannabis increases the risk of lung cancer and may cause 5 per cent of cases of the disease in people aged 55 and under, according to a new study being published later this year.

Researchers have found a five-fold increased risk of lung cancer in heavy users of cannabis. They calculate that the risk of developing lung cancer increased by 8 per cent a year for people whose cumulative exposure equated to smoking one joint a day. "Long-term cannabis use increases the risk of lung cancer in young adults, particularly in those who start smoking cannabis at a young age," according to researchers from the Medical Research Institute of New Zealand.

The increased risk is blamed on the tendency of cannabis smokers to inhale deeper and longer than with an ordinary cigarette.

The World Health Organisation in 1997 concluded that cannabis when smoked is twice as carcinogenic than tobacco. Some doctors claim it causes cancers of the lungs, larynx, mouth and oesophagus as well as other chronic lung diseases. Those smoking on a regular basis are thought to be most at risk.

The research adds to the existing evidence that cannabis use can increase the risk of emphysema, chronic bronchitis and lung cancer. This news comes as the debate over the dangers of cannabis - started by this newspaper two weeks ago - continues.