“…Scientists were therefore surprised to learn that a study of more than 2,000 people found no increase in the risk of developing lung cancer for marijuana smokers.

“We expected that we would find that a history of heavy marijuana use–more than 500 to 1,000 uses–would increase the risk of cancer from several years to decades after exposure to marijuana,” explains physician Donald Tashkin of the University of California, Los Angeles, and lead researcher on the project. But looking at residents of Los Angeles County, the scientists found that even those who smoked more than 20,000 joints in their life did not have an increased risk of lung cancer.

The study does not reveal how marijuana avoids causing cancer. Tashkin speculates that perhaps the THC chemical in marijuana smoke prompts aging cells to die before becoming cancerous. Tashkin and his colleagues presented the findings yesterday at a meeting of the American Thoracic Society in San Diego.”

More: http://www.scientificamerican.com/article.cfm?id=large-study-finds-no-link