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 at the American Thoracic Society conference last week, reported that the largest study ever conducted to specifically test the hypothesis that marijuana use causes lung cancer has again found no support for this idea.

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. "What we found instead was no association at all, and even a suggestion of some protective effect."

Despite the existence of a body of epidemiologic evidence finding no major health problems due to marijuana 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 of the chemicals in cancer smoke will cause cancer when administered in massive doses to lab rats, he continues to argue, in effect, that smoking marijuana MUST cause cancer even if it doesn't.

The problem, Tashkin suggests, may be that in addition to all those purported carcinogens marijuana 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 marijuana possesses.