Tashkin's study, funded by the National Institutes of Health's National Institute on Drug Abuse, involved 1,200 people in Los Angeles who had lung, neck or head cancer and an additional 1,040 people without cancer matched by age, sex and neighborhood.
They were all asked about their lifetime use of marijuana, tobacco and alcohol. The heaviest marijuana smokers had lighted up more than 22,000 times, while moderately heavy usage was defined as smoking 11,000 to 22,000 marijuana cigarettes. Tashkin found that even the very heavy marijuana smokers showed no increased incidence of the three cancers studied.
"This is the largest case-control study ever done, and everyone had to fill out a very extensive questionnaire about marijuana use," he said. "Bias can creep into any research, but we controlled for as many confounding factors as we could, and so I believe these results have real meaning."
(via Brian Carver)