Smoking a joint is equivalent to 20 cigarettes in terms of lung cancer risk, scientists in New Zealand have found, as they warned of an "epidemic" of lung cancers linked to cannabis.
Studies in the past have demonstrated that cannabis can cause cancer, but few have established a strong link between cannabis use and the actual incidence of lung cancer.
In an article published in the European Respiratory Journal, the scientists said cannabis could be expected to harm the airways more than tobacco as its smoke contained twice the level of carcinogens, such as polyaromatic hydrocarbons, compared with tobacco cigarettes.
The method of smoking 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. 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)," team leader Richard Beasley, at the Medical Research Institute of New Zealand, said in a telephone interview.
"There are higher concentrations of carcinogens in cannabis smoke ... what is intriguing to us is there is so little work done on cannabis when there is so much done on tobacco."
The researchers interviewed 79 lung cancer patients and sought to identify the main risk factors for the disease, such as smoking, family history and occupation. The patients were questioned about alcohol and cannabis consumption.
In this high-exposure group, lung cancer risk rose by 5.7 times for patients who smoked more than a joint a day for 10 years, or two joints a day for 5 years, after adjusting for other variables, including cigarette smoking.
"While our study covers a relatively small group, it shows clearly that long-term cannabis smoking increases lung cancer risk," wrote Beaseley.
"Cannabis use could already be responsible for one in 20 lung cancers diagnosed in New Zealand," he added.
"In the near future we may see an 'epidemic' of lung cancers connected with this new carcinogen. 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