WASHINGTON – Regular cannabis smokers are no more likely to develop lung cancer than are people who indulge occasionally.
The finding of no significant increased risk held true whether the smokers imbibed once or twice – or more – each day, and regardless of how many years they had smoked, Dr. Li Rita Zhang reported at the annual meeting of the American Association of Cancer Research.
The study included data from six case-control studies conducted from 1999 to 2012 in the United States, Canada, the United Kingdom, and New Zealand, with a subject pool of 2,159 lung cancer cases and 2,985 controls. All of the studies were part of the International Lung Cancer Consortium (ILCCO), an international group of lung cancer researchers with the aim of sharing comparable data from ongoing and recently completed lung cancer studies from different geographical areas and ethnicities.
Dr. Zhang of the University of California, Los Angeles, performed two analyses. One compared all lung cancer cases and all controls, regardless of concurrent or past tobacco use. Then, to reduce confounding by tobacco, she restricted the analysis to those who had never smoked tobacco. That group comprised 370 cancer cases and 1,358 controls. The models were also adjusted for age, sex, sociodemographic factors, and tobacco pack-years. Habitual use was defined as one joint per day per year.
When compared with cannabis smokers who also used tobacco, habitual pot smokers had no significant increase in cancer risk.
In an analysis of marijuana smokers that excluded tobacco smokers, there were no significant differences in any of the comparisons, including habitual vs. nonhabitual use; number of joints smoked per day; duration of up to 20 years or duration of more than 20 years.
Other literature has shown a link between cannabis smoking and lung cancer, pulmonologist Michael Alberts said in an interview. However, he said, "The conventional wisdom is that cannabis smoking is not as dangerous as cigarette smoking."
The difference in risk is likely related to chemical additives in commercial cigarettes that aren’t present in most methods of inhaling marijuana smoke.
As a general recommendation, smoking anything isn’t good for the respiratory system, said Dr. Alberts, chief medical officer of the Moffitt Cancer Center, Tampa. But for patients using medical marijuana, the benefit could outweigh the risks.
"You can think of it as similar to a CT scan. Radiation isn’t good, but if the scan is something beneficial and the risk is low, you take it. If cannabis is indicated, and if it’s legal, and if there’s literature backing up the indication for use, then you weigh the risk of smoking and the benefit it could bring, and make the decision."
Dr. Zhang declined to comment on the study. In her poster presentation she noted that, "Our results cannot preclude the possibility that cannabis may exhibit an association with lung cancer risk at extremely high dosage over long periods of continued exposure."
Dr. Zhang did not disclosure any financial relationships. Dr. Alberts said he had no disclosures.