Study Finds No Cancer-Marijuana Connection
By 
Marc Kaufman
Washington Post Staff Writer
Friday, May 26, 2006;  Page A03
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
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"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.
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."
Tashkin's group at the David Geffen School of
Medicine at UCLA had hypothesized that marijuana would raise the risk
of cancer on the basis of earlier small human studies, lab studies of
animals, and the fact that marijuana users inhale more deeply and
generally hold smoke in their lungs longer than tobacco smokers --
exposing them to the dangerous chemicals for a longer time. In
addition, Tashkin said, previous studies found that marijuana tar has
50 percent higher concentrations of chemicals linked to cancer than
tobacco cigarette tar.
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.
The study was limited to people
younger than 60 because those older than that were generally not
exposed to marijuana in their youth, when it is most often tried.