Does the regular smoking of marijuana cause lung cancer or in any way permanently injure the lungs?
General Reference (not clearly pro or con)
Mark J. Pletcher, MD, MPH, Associate Professor, In Residence, in the Departments of Epidemiology, Biostatistics, and Medicine at the University of California at San Francisco, stated the following in a press release titled "Marijuana Shown to Be Less Damaging to Lungs Than Tobacco," available on ucsf.edu:
"We found exactly what we thought we would find in relation to tobacco exposure: a consistent loss of lung function with increasing exposure. We were, however, surprised that we found such a different pattern of association with marijuana exposure...
An important factor that helps explain the difference in effects from these two substances is the amount of each that is typically smoked. Tobacco users typically smoke ten to 20 cigarettes/day, and some smoke much more than that. Marijuana users, on average, smoke only two to three times a month, so the typical exposure to marijuana is much lower than for tobacco...
Our findings suggest that occasional use of marijuana for these or other purposes may not be associated with adverse consequences on pulmonary function. On the other hand, our findings do suggest an accelerated decline in pulmonary function with heavier use – either very frequent use or frequent use over many years – and a resulting need for caution and moderation when marijuana use is considered...
I was surprised at these findings. I thought we were going to find some small negative affect on pulmonary function from marijuana use. But we didn't. At the level of marijuana that most marijuana users smoke, we did not find any appreciable negative consequence on pulmonary function on marijuana use."
The National Institute on Drug Abuse (NIDA) wrote in its webpage "Marijuana: Facts Parents Need to Know" on drugabuse.gov (accessed Oct. 18, 2011):
"Q. Does smoking marijuana cause lung cancer? A. We do not know yet. Studies have not found an increased risk of lung cancer in marijuana smokers, as compared with nonsmokers. However, marijuana smoke does irritate the lungs and increases the likelihood of other respiratory problems through exposure to carcinogens and other toxins. Repeated exposure to marijuana smoke can lead to daily cough and excess phlegm production, more frequent acute chest illnesses, and a greater risk of lung infections. Marijuana also affects the immune system, although the implications for cancer are unclear. Moreover, many people who smoke marijuana also smoke cigarettes, which do cause cancer, and quitting tobacco can be harder if the person uses marijuana as well.”
Does the regular smoking of marijuana cause lung cancer or in any way permanently injure the lungs?
Richard Beasley, MBChB, DM, FRACP, DSc, Director of the Medical Research Institute of New Zealand, in a Jan. 29, 2008 interview with Reuters explaining the results of his Feb. 2008 study "Cannabis Use and Risk of Lung Cancer: a Case–control Study" in the European Respiratory Journal:
"Cannabis smokers end up with five times more carbon monoxide in their bloodstream (than tobacco smokers)...
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."
Donald P. Tashkin, MD, Director of the Pulmonary Function Laboratories at the University of California, Los Angeles, wrote in his Mar. 1997 article for the Center for Substance Abuse Prevention titled "Effects of Marijuana on the Lung and Its Immune Defenses":
"Analysis of the smoke contents of marijuana and tobacco reveals much the same gas phase constituents, including chemicals known to be toxic to respiratory tissue...
With regard to the carcinogenic potential of marijuana, it is noteworthy that the tar phase of marijuana smoke contains many of the same carcinogenic compounds contained in tobacco smoke, including polycyclic aromatic hydrocarbons, such as benz[a]pyrene, which was recently identified as a key factor promoting human lung cancer...
Bronchial immunohistology revealed overexpression of genetic markers of lung tumor progression in smokers of marijuana.
Preliminary findings suggest that marijuana smoke activates cytochrome P4501A1, the enzyme that converts polycyclic hydrocarbons, such as benz[a]pyrene, into active carcinogens."
The American College of Physicians (ACP) wrote in its Feb. 15, 2008 position paper "Supporting Research into the Therapeutic Role of Marijuana":
"The chronic effects of smoked marijuana are of much greater concern, as its gas and tar phases contain many of the same compounds as tobacco smoke. Chronic use of smoked marijuana is associated with increased risk of cancer, lung damage, bacterial pneumonia, and poor pregnancy outcomes."
Jeanette M. Tetrault, MD, Clinical Epidemiologist at the Connecticut Department of Veterans Affairs, et al., stated in the article "Effects of Marijuana Smoking on Pulmonary Function and Respiratory Complications: A Systematic Review," published in the Feb. 12, 2007 issue of Archives of Internal Medicine:
"Conclusions: Short-term exposure to marijuana is associated with bronchodilation [opening of the air passages]. Physiologic data were inconclusive regarding an association between long-term marijuana smoking and airflow obstruction measures. Long-term marijuana smoking is associated with increased respiratory symptoms suggestive of obstructive lung disease."
Charles Ksir, PhD, Professor Emeritus of Psychopharmacology and Drug Policy at the University of Wyoming, and Oakley Ray, PhD, Emeritus Professor of Psychology and Pharmacology at Vanderbilt University, wrote in their 2004 textbook Drugs, Society and Human Behavior:
"Experiments have shown that chronic, daily smoking of marijuana impairs air flow in and out of the lungs. It is hard to tell yet whether years of such an effect results in permanent, major obstructive lung disease in the same way that smoking tobacco cigarettes does.
Also, there is no direct evidence linking marijuana smoking to lung cancer in humans. Remember that it took many years of cigarette smoking by millions of Americans before the links between tobacco and lung cancer and other lung diseases were shown...
Everyone suspects that marijuana smoking will eventually be shown to cause cancer, but how much of a problem this will be, compared with tobacco, is hard to say."
Ted Sarafian, PhD, Associate Research Scientist of Pulmonary and Critical Care Medicine in the Department of Medicine at the University of California at Los Angeles (UCLA) David Geffen School of Medicine, et al., wrote in their June 1999 study "Oxidative Stress Produced by Marijuana Smoke" in the American Journal of Respiratory Cell and Molecular Biology:
"Marijuana (MJ) smoking produces inflammation, edema, and cell injury in the tracheobronchial mucosa of smokers and may be a risk factor for lung cancer...
We conclude that MJ [marijuana] smoke containing Delta-9-THC is a potent source of cellular oxidative stress that could contribute significantly to cell injury and dysfunction in the lungs of smokers."
The British Lung Foundation noted in its Nov. 2002 report "A Smoking Gun" on lunguk.org:
"3-4 Cannabis cigarettes a day are associated with the same evidence of acute and chronic bronchitis and the same degree of damage to the bronchial mucosa as 20 or more tobacco cigarettes a day.
Cannabis smoking is likely to weaken the immune system. Infections of the lung are due to a combination of smoking-related damage to the cells lining the bronchial passage and impairment of the principal immune cells in the small air sacs caused by cannabis.
The evidence concerning a possible link between cannabis smoking and Chronic Obstructive Pulmonary Disease (COPD) has not yet been conclusively established. A number of studies indicate a causal relationship between the two whereas others contradict these findings.
Research linking cannabis smoking to the development of respiratory cancer exists although there have also been conflicting findings. Not only does the tar in a cannabis cigarette contain many of the same known carcinogens as tobacco smoke but the concentrations of these are up to 50% higher in the smoke of a cannabis cigarette. It also deposits four times as much tar on the respiratory tract as an unfiltered cigarette of the same weight. Smokers of cannabis and tobacco have shown a greater increase in cellular abnormalities indicating a cumulative effect of smoking both.
The THC in cannabis has been shown to have a short term bronchodilator effect. This has lead to suggestions that THC may have therapeutic benefits in asthma. However, the noxious gases, chronic airway irritation or malignancy after long term use associated with smoking would seem likely to negate these benefits."
Donald P. Tashkin, MD, Director of the Pulmonary Function Laboratories at the University of California, Los Angeles (UCLA), reported at the June 2005 meeting of the International Cannabinoid Research Society on his as-yet unpublished study of 1,209 Los Angeles residents aged 59 or younger with cancer:
"We found absolutely no suggestion of a dose response [i.e. marijuana smoking leads directly to lung cancer].
The data on tobacco use... revealed a very potent effect and clear dose-response relationship -- a 21-fold greater risk of developing lung cancer if you smoke more than two packs a day.
So, in summary, we failed to observe a positive association of marijuana use and other potential co-founders."
Donald P. Tashkin, MD, Director of the Pulmonary Function Laboratories at the University of California, Los Angeles (UCLA), responded when asked at the June 2005 meeting of the International Cannabinoid Research Society if "in at least one category [marijuana-only smokers and lung cancer], it almost looked like there was a negative correlation, i.e. a protective effect":
"Yes. The odds ratios are less than one almost consistently, and in one category that relationship was significant, but I think that it would be difficult to extract from these data the conclusion that marijuana is protective against lung cancer. But that is not an unreasonable hypothesis."
[Editor's Note:Prior to Dr. Tashkin’s June 25, 2008 Con position above, his position was Pro as indicated in his Mar. 1997 statement in the opposite column.]
Paul Armentano, Deputy Director of the National Organization for the Reform of Marijuana Laws (NORML) and the NORML Foundation, wrote in his May 5, 2006 report "Cannabis Smoke and Cancer: Assessing the Risk":
"Cannabis smoke contains many of the same carcinogens as tobacco smoke, including greater concentrations of certain aromatic hydrocarbons such as benzopyrene, prompting fears that chronic marijuana inhalation may be a risk factor for tobacco-use related cancers.
However, marijuana smoke also contains cannabinoids such as THC (delta-9-tetrahydrocannabinol) and CBD (cannabidiol), which are non-carcinogenic and demonstrate anti-cancer properties in vivo and in vitro...
Cannabis smoke -- unlike tobacco smoke -- has not been definitely linked to cancer in humans, including those cancers associated with tobacco use. However, certain cellular abnormalities in the lungs have been identified more frequently in long-term smokers of cannabis compared to non-smokers.
Chronic exposure to cannabis smoke has also been associated with the development of pre-cancerous changes in bronchial and epithelium cells in similar rates to tobacco smokers. Cellular abnormalities were most present in individuals who smoked both tobacco and marijuana, implying that cannabis and tobacco smoke may have an additive adverse effect on airway tissue...
However, to date, no epidemiologic studies of cannabis-only smokers have yet to reveal such a finding. Larger, better-controlled studies are warranted."
Colin Blakemore, PhD, ScD, Chair of the Department of Physiology at the University of Oxford, and Leslie Iversen, PhD, Professor of Pharmacology at the University of Oxford, wrote in their Aug. 6, 2001 op-ed in The Times (United Kingdom):
"It is claimed that cannabis smoke is more harmful to the lungs than tobacco smoke because it contains much the same mixture of noxious substances, and because cannabis users inhale more deeply and deposit more tar in their lungs. On the other hand, cannabis users do not smoke 20 to 40 times a day, as many cigarette smokers do. There may be a health risk, and it is compounded by the combination of cannabis with tobacco, but there is currently no indisputable evidence for a link with cancer.
The reports of cancers of the throat, mouth and larynx in cannabis users were based on small numbers and did not rule out effects of the concomitant use of tobacco. A much larger study in the United States monitored the health of a group of 65,000 men and women over a ten-year period. The 27,000 who admitted to having used cannabis showed no association between cannabis use and cancers, nor were there any other serious adverse effects on health."