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Cannabis and Cancer

Posted Jan 23 2010 6:14pm
 
Canada considers the case.

The link between cigarette smoking and respiratory disease is irrefutable. But what about pot smoking? A long history of contentious argument and clinical inference has left the picture as fuzzy as ever. Despite strenuous efforts to prove the case, the evidence remains ambiguous.

The Canadian Centre on Substance Abuse in Ottawa recently released an analysis of current evidence, “Clearing the Smoke on Cannabis: Respiratory Effects of Cannabis Smoking.” (PDF). In the report, prepared in 2009 by the Centre for Criminal Justice Research at the University of the Fraser Valley, Jordan Diplock and Darryl Plecas assess the argument that cannabis poses similar risks to the airways as tobacco, primarily due to the tar content of cannabis. It is sometimes argued that cannabis is even more dangerous than tobacco, due to the deeper inhalations and breath-holding manner of smoking typical of pot smokers. The well-known style of high, tight exhalations, while tightening the abdomen, is meant to increase the absorption of THC in the lungs. It is similar to the so-called Valsalva Maneuver, which increases thoracic cavity pressure through forcible exhalation against a closed airway, such as holding one’s nose and attempting to “pop” one’s ears.

Earlier studies by Moore and others had confirmed that “common self-reported respiratory problems include coughing on most days, wheezing, shortness of breath after exercise, nocturnal chest tightness, chest sounds without a cold, early morning phlegm and mucus, and acute and chronic bronchitis. These symptoms were associated to cannabis smoking even when gender, age, tobacco smoking, and asthma were controlled.” Nonetheless, the majority of cannabis smokers in such self-reported results were frequently cigarette smokers as well, making it difficult to assess the health risk such negative respiratory symptoms represent.

A study by Aldington and colleagues in New Zealand in 2008 reported that the risk of lung cancer “increased by 8% for each joint-year of cannabis smoking after adjusting for various confounding variables, including tobacco smoking.”  However, a significant degree of what researchers call “recall bias” may be at work in retrospective studies of this kind. Other studies that found connections have been hospital-based, which can introduce selection bias and other problems.

A 1997 retrospective study of more than 64,000 people in California  found exactly the reverse: “Current and ever-cannabis use (defined as use of cannabis six or more times over a lifetime) was not associated with an increased risk of cancer overall,” after adjusting (or attempting to adjust) for the usual factors like drinking and smoking.  The problem here is that there is no way of knowing whether these studies manage to capture a sufficient number of heavy, long-term marijuana smokers.

Smoking aside, what about the contention that THC in the lungs can damage respiratory tissue? The idea that THC causes immune system deficiencies, which, in turn, hinder the ability of the lungs to fight off pathogens, has been around for some time. But again, the evidence remains inconclusive. In fact, some evidence points in the other direction entirely. By curbing a substance called epidermal growth factor (EGF), THC may in fact confer a protective effect, inhibiting the growth of certain tumors. THC “seems to have a suppressive effect on certain lines of cancer cells,” according to a pulmonary specialist at New York’s Lenox Hill Hospital, quoted in a HealthDay article by reporter Amanda Gardner.

The Canadian authors caution that these inhibitory effects “have been demonstrated using THC (not cannabis smoke) in preclinical models, and do not necessarily imply that exposure to cannabis smoke can prevent cancer occurrence in humans.”

The problem is that, over the past ten years, these conflicting studies suggest either that: a) There is no association between cannabis smoking and an increased risk of chronic obstructive pulmonary disease (COPD), or b) There is a serious risk of COPD in people who smoke both marijuana and tobacco. Unfortunately, there is no c) There is (or is not) evidence of elevated COPD risk among people who smoke pot but not tobacco. And while there is always reason to speculate that sustained pot smoking could put users at risk for pulmonary problems, the authors of the Canadian report concede that the state of the research “is too limited to provide estimates of the prevalence of these and other serious health threats.”

So, the picture remains out of focus. Does pot smoking raise the risk of respiratory diseases, including lung cancer? We still don’t know.  The limited research literature remains wholly inconclusive, and the current connection between lung cancer and cannabis smoking remains weak at best.

Common sense suggests that inhaling hot vegetable matter that has been dried for smoking can’t be a terrific idea on the long run. The Canadian authors make a pitch for vaporizers as a harm-reduction  approach to marijuana smoking. Vaporizers heat the active cannabinoids enough to produce vapors but do not produce enough heat for combustion of the plant material.

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