Smoking marijuana is far more dangerous than smoking cigarettes, according to a group of scientists in New Zealand.
The researchers found that smoking one joint is equivalent to 20 cigarettes in terms of lung cancer.
While studies in the past have shown that marijuana can cause cancer, few have actually established a strong link between marijuana use and the actual incidence of lung cancer.
For the study, researchers interviewed 79 lung cancer patients in an effort identify the main risk factors for the disease, such as smoking, family history and occupation. The patients were questioned about alcohol and marijuana consumption.
In the 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.
"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 news release.
The scientists also noted that marijuana could be expected to harm the airways more than tobacco since its smoke contained twice the level of carcinogens, such as polyaromatic hydrocarbons, compared with tobacco cigarettes.
"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,” said Beasley.
"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,” he added.
Study results appear in the in the European Respiratory Journal.