Smoking a joint is equivalent to 20 cigarettes in terms of lung cancer risk, say researchers, warning of an “epidemic” of lung cancers linked to cannabis.

Dr Richard Beasley, director of the Medical Research Institute of New Zealand, and colleagues report their findings on the cancer risk of pot smoking in the European Respiratory Journal.

Past studies on the link between smoking cannabis and lung cancer have produced mixed results, says Beasley, and a key problem has been separating out the contribution of tobacco, which is often mixed with cannabis.

He says the practice of mixing tobacco with cannabis does not tend to occur in New Zealand, unlike in other places such as the UK, and the study controlled for tobacco use in all forms.

Beasley and team interviewed 79 lung cancer patients and sought to identify the main risk factors for the disease, such as smoking, family history and occupation. They then asked the patients about alcohol and cannabis consumption.

The researchers found those who smoked more than a joint a day for 10 years, or two joints a day for 5 years were more than 5 times more likely to have lung cancer than those who didn’t smoke at all - after adjusting for other variables.

“While our study covers a relatively small group, it shows clearly that long-term cannabis smoking increases lung cancer risk,” says Beasley.

“Cannabis use could already be responsible for one in 20 lung cancers diagnosed in New Zealand,” he says.

“In the near future we may see an ‘epidemic’ of lung cancers connected with this new carcinogen,” says Beasley.

“And the future risk probably applies to many other countries, where increasing use of cannabis among young adults and adolescents is becoming a major public health problem,” he adds

Twice the carcinogens

Beasley says cannabis could be expected to harm the airways more than tobacco as its smoke contains twice the level of carcinogens, such as polyaromatic hydrocarbons, compared with tobacco cigarettes.

Smoking cannabis also increases the risk, since joints are typically smoked without a proper filter and almost to the very tip, which increases the amount of smoke inhaled, he says.

He says the cannabis smoker inhales more deeply and for longer, facilitating the deposition of carcinogens in the airways.

“Cannabis smokers end up with five times more carbon monoxide in their bloodstream (than tobacco smokers),” he says.

“What is intriguing to us is there is so little work done on cannabis when there is so much done on tobacco.”

Tan Ee Lyn
Reuters