Cannabis Cancer Risk Higher Than Cigarettes, Study Shows

New Zealand's cannabis abuse problem will eventually cause an "epidemic" of lung cancer, the Medical Research Institute of New Zealand is warning.

Team leader Richard Beasley said in an article published in the European Respiratory Journal that cannabis was potentially more harmful to the airways than tobacco, because cannabis smoke contains two times the amount of carcinogens.

"Cannabis smokers end up with five times more carbon monoxide in their bloodstream," he said.

The researchers interviewed 79 lung cancer patients and identified the risk factors that could have caused the disease, such as smoking, family history and occupation. The patients were questioned about alcohol and cannabis consumption.

In the high-exposure group, lung cancer risk was five times greater for patients who smoked more than a joint a day for 10 years, or two joints a day for five years.

Other studies have shown a possible link between cannabis and cancer, but few have looked at the actual incidence of lung cancer.

"Cannabis use could already be responsible for one in 20 lung cancers diagnosed in New Zealand," he said.

Foundation for Alcohol & Drug Education (FADE) managing director Colin Bramfitt said cannabis was used differently from tobacco and that is probably why it was more harmful.

When people smoke a cigarette it is normally filtered, whereas cannabis users do not normally use a filter.

"People need to understand that smoking anything, whether it be cannabis or cigarettes or anything, is one of the most invasive things you can do," he said.

He said people who smoke cannabis also tend to inhale more deeply and hold the smoke in their lungs. And cannabis burns at a higher temperature than tobacco.

NORML New Zealand Auckland representative Jonathan Rennie said recreational use of cannabis is most common in New Zealand, but people should acknowledge its merits in the treatment of chronic and terminal illnesses.

He said cannabis has been shown to be beneficial particularly in the treatment of neuropathic pain, arthritis and multiple sclerosis.

He said it was quite likely that long-term use of cannabis was harmful and could cause lung cancer.

An editorial in the European Respiratory Journal this year said that the use of cannabis has to be considered a societal problem.

"It would surely be interesting to find out the percentage of readers of our editorial who have been confronted by the fact that their children or their friends' children are cannabis smokers or have been approached by cannabis dealers.

"We are not facing the big tobacco companies with their huge amount of legal money, but an even more vicious underground enemy," the authors said.

A long-term University of Otago study shows high rates of both cannabis use and illicit drug use, with nearly 80 percent of participants using cannabis by age 25, and over 40 percent using other illicit drugs.

Principal investigator Professor David Fergusson said there has been long-standing debate over whether the use of cannabis increases the risk that young people will use other illicit drugs.

"Our research shows the regular use of cannabis increases the risks that young people will try other illicit drugs," he said.

He said cannabis use is illegal, which means users have to obtain supplies from the illegal market, exposing them to drug dealers.

"If the association arises because using cannabis increases contact with illegal drug markets, this is a ground for the decriminalisation or legalisation of cannabis.

"If, however, the association arises because using cannabis encourages young people to experiment with other illicit drugs the results could be seen as supporting the prohibition of cannabis use," Professor Fergusson said.

New Zealand Drug Foundation executive director Ross Bell said that alcohol and tobacco are still New Zealand's most problematic drugs.