The largest study of its kind has unexpectedly concluded that smoking marijuana, even regularly and heavily, does not lead to lung cancer.
The new findings “were against our expectations,” said Donald Tashkin of the University of California at Los Angeles, a pulmonologist who has studied marijuana for 30 years.
“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.”
Federal health and drug enforcement officials have widely used Tashkin’s previous work on marijuana to make the case that the drug is dangerous. Tashkin said that while he still believes marijuana is potentially harmful, its cancer-causing effects appear to be of less concern than previously thought.
Earlier work established that marijuana does contain cancer-causing chemicals as potentially harmful as those in tobacco, he said. However, marijuana also contains the chemical THC, which he said may kill aging cells and keep them from becoming cancerous.
While no association between marijuana smoking and cancer was found, the study findings, presented to the American Thoracic Society International Conference this week, did find a 20-fold increase in lung cancer among people who smoked two or more packs of cigarettes a day.