Cannabis smoking may cause 5 per cent of lung cancer cases in people up to middle age, according to a New Zealand study which challenges international thinking on the drug.
Around 15 per cent of New Zealand adults under 46 use cannabis, drug-use surveys have found.
Researcher Dr Sarah Aldington, of the Medical Research Institute in Wellington, presented the new case-control study to the Thoracic Society conference in Auckland yesterday.
Cannabis users may have thought they were safe from lung cancer after a Californian study of more than 1600 people last year found no link between the disease and smoking the drug.
Dr Aldington said the evidence on cannabis and the risk of lung cancer was limited and conflicting.
Her study found the risk rose more than five-fold among the third of users smoking the most cannabis.
"In conclusion there is a relationship between cannabis smoking and lung cancer in this study," she said. "Approximately 5 per cent of lung cancer cases in those aged 55 and under may be attributable to cannabis..."
This equates to about 15 new cases a year - in 2002, 306 people aged 18-55 were diagnosed with lung cancer in New Zealand.
The study questioned about 60 people with lung cancer from eight health districts between Waikato and Canterbury and more than 200 "controls" - people randomly selected from electoral rolls in the same areas.
They were asked about risk factors, including cannabis and tobacco use.
The researchers calculated that the risk of developing lung cancer increased by about 8 per cent a year for people whose cumulative exposure equated to smoking one joint a day. This was about the same as the increase for someone with a one-pack-a-day tobacco habit.
The younger someone started smoking cannabis, the higher their risk of lung cancer.
"Long-term cannabis use increases the risk of lung cancer in young adults, particularly in those who start smoking cannabis at a young age," the researchers conclude.
Dr Aldington said cannabis was the most commonly used recreational drug in the world, used by 161 million people, and its use was increasing in many countries.
She said cannabis contained 50 per cent more cancer-causing chemicals than tobacco.
The study has found what the University of California researchers had expected to find but didn't.
A researchers from that study, Dr Donald Tashkin, said in the Washington Post his group had thought cannabis smokers' deeper inhalation and tendency to hold smoke in their lungs for longer than tobacco users would contribute to an increased cancer risk.
He said earlier work had shown cannabis contained cancer-causing chemicals as potentially harmful as those in tobacco. But cannabis also contained the chemical THC, which might kill ageing cells and keep them from becoming cancerous.
Middlemore Hospital clinical director of medicine Associate Professor Jeff Garrett, a leader of the Thoracic Society, said the Aldington study was "a good pilot study. It's early work, it's interesting, but there needs to be more work done."
Drug Foundation executive director Ross Bell was generally sceptical of the findings as they contradicted most cannabis research he had read, but he picked up on the increased risk found with starting young. "We need to be doing things that delay use," he said.
* Cannabis is used by 161 million people around the world
* In NZ 5 per cent of lung cancer cases in those aged under 55 may be due to cannabis smoking.