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Largest-ever federally-sponsored smoking study clears the air |
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Written by David F. Duncan, DrPH, FAAHB
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NIDA: Even very heavy marijuana smoking does not cause cancer
Opponents of the relegalization of marijuana frequently argue that marijuana smoke contains a number of carcinogenic chemicals and that therefore the prohibition of marijuana use must continue to protect the public from lung cancer. From the standpoint of an epidemiologist, these arguments have never been credible.
Persuasive evidence that marijuana use causes cancer or contributes to any serious health problems simply does not exist. A presentation in May at the American Thoracic Society conference, reported that the largest study ever conducted to specifically test the hypothesis that cannabis use causes lung cancer has again found no support for this idea.
 This study, funded by National Institute on Drug Abuse, was a case-control study that involved 1,200 patients in the Los Angeles area who had cancer of the lung, neck or head, compared to an additional 1,040 persons without cancer, matched for age, sex and neighborhood. Researchers found that even very heavy cannabis smokers showed no increased incidence of any of the three cancers studied. While no association between cannabis smoking and cancer was found, the study did find a 20-fold increase in lung cancer among people who smoked two or more packs of cigarettes daily.
The principal investigator of this new study was Dr. Donald Tashkin, of UCLA’s David Geffen School of Medicine, who has been conducting studies for three decades searching for respiratory damage due to marijuana use. In the past, his studies have been widely quoted by the federal government and other defenders of the war on drugs and have been harshly criticized by epidemiologists and statisticians. Dr. Tashkin acknowledges that the study’s findings were contrary to his biases. “We hypothesized that there would be a positive association between marijuana use and lung cancer, and that the association would be more positive with heavier use,” he said. {quotes}“What we found instead was no association at all, and even a suggestion of some protective effect.”{/quotes}
Despite the existence of a body of epidemiologic evidence finding no major health problems due to cannabis use, Tashkin has stated that he still believes marijuana is potentially harmful. While his study adds to the evidence of no cancer risk, he is only prepared to acknowledge that marijuana’s cancer causing effects appear to be of less concern than previously thought. Clinging to the fact that some chemicals in smoke will cause cancer when administered in massive doses to lab rats, he continues to argue, in effect, that smoking cannabis must cause cancer even if it doesn’t.
The problem, Tashkin suggests, may be that in addition to all those purported carcinogens, cannabis also contains the chemical THC, which he suggests may kill aging cells and keep them from becoming cancerous. In other words, the only reason they didn’t find any harm was that it is so harmful that one harm cancels out the other – a strikingly irrational argument. A far more logical suggestion would be that any carcinogenic effects are compensated for, not by some cell destruction but by the protective anti-cancer effect some studies seem to indicate that cannabis possesses.
* Published with permission from the National Association for Public Health Policy website, naphp.org, May 28, 2006. Oaksterdam News first reported on this study in October, 2005.
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Volume Two 
