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Does smoking marijuana cause lung cancer?

Systematic review [1]
Results
Comment

One might well think so. We know that burning plant products produces smoke, which contains tars, carcinogens, and other unpleasant compounds. We know that breathing smoke gets these materials deep into our lungs - which is why, in part, inhaling drugs can be an important option for drug delivery. We know that breathing smoke regularly, as in cigarette and other tobacco smoking, leads to increased rates of lung cancer, in a dose dependent manner. OK, we don't have much in the way of randomised trials in humans, but we have mountains of observational data.

So our starting point is to expect marijuana smoking to cause cancer, by simple analogy. But some might want the evidence for this, which is where it gets complicated. A systematic review [1] helps, because it has looked for evidence from various sources to test the biological plausibility that smoking marijuana causes lung cancer.


Systematic review [1]

The review used a broad search strategy for experimental studies of any design published up to the end of 2005, but only in English. The 19 that were finally included were divided into which relationship was investigated between marijuana smoking and possible lung cancer.


Results

These are summarised in Table 1. Five general areas were examined, the relationship between marijuana smoking and exposure to tar, cytology of sputum, changes in alveolar macrophages and bronchial biopsies, and epidemiological studies examining lung cancer or related conditions.



Table 1: Summary of studies of association between marijuana smoking and lung cancer



Type of study Number Synopsis
Exposure to tar 4 experimental studies with 45 participants Smoking marijuana is associated with increased tar delivery to the lungs, to a greater extent than smoking cigarettes, perhaps by a factor of three
Cyomorphology of sputum samples 2 studies with 150 particiapnts Marihuana smokers who did not use tobacco had more metaplastic cells, pigmented macrophages and columnar cells than nonsmokers. Sysplasia in no non-smoker, but 12% and 4% of tobacco or marijuana smokers
Alveolar macrophage changes 3 studies with 128 participants Alveolar macrophages from marijuana smokers had less tunoricidal ability, enhanced oxidative stress, and a dose-dependent relationshiip between THC and reactive oxygen species. Possibly increased DNA damage
Histopathology and molecular alterations in bronchial biopsies 6 studies with 494 participants undergoing bronchoscopy Increase in abnormal and precancerous findings in marijuana smokers compared with tobacco smokers. Surrogate markers for lung malignancy more often found in marijuana smokers
Lung cancer or related outcomes 4 studies with almost 65,000 participants, predominantly in a single study No relationship between marijuana smoking and diagnosis of lung cancer. Limited evidence of higher use of marijuana in younger patients with lung cancer


Smoking marijuana led to increased tar delivery, more cellular changes in sputum, damage to alveolar macrophages, and increased abnormality in bronchial biopsies, with increased surrogate markers for lung cancer.

The epidemiological studies did not find any consistent relationship between marijuana smoking and cancer of all sites, though the bulk of the participants were from a retrospective review of a health plan from the early 1980s. Small studies of a few hundred patients indicated non-significant increases in lung cancer with marijuana smoking.


Comment

There is abundant circumstantial evidence pointing to a plausible biological relationship between marijuana smoking and lung cancer. We may be lacking the smoking gun, but few juries could fail to convict on this evidence.

We have the advantage of knowing that consistent smoke inhalation, whether tobacco, marijuana, or anything else, is a really bad thing. For instance, the high incidence of lung cancer among non-smoking Chinese women in Hong Kong has been linked, at least in part, to cooking fumes associated with frying [2]. Smoke in wildland fires in the USA has at least 15 carcinogens, and risk assessment for firefighters with chronic smoke exposure includes a significant excess of cancers [3]. This allows us to be even more certain of a probable link between marijuana smoking and lung cancer, and not to be too hung up about wanting more evidence.

References:

  1. R Mehra et al. The association between marijuana smoking and lung cancer. A systematic review. Archives of Internal Medicine 2006 166: 1359-1367.
  2. IT Yu et al. Dose-relationship between cooking fumes exposures and lung cancer among Chinese non-smoking women. Cancer Research 2006 66: 4961-4967.
  3. TF Booze et al. A screening-level assessment of the health risks of chronic smoke exposure for wildland firefighters. Journal of Occupational and Environmental Hygiene 2004 1: 296-305.

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