Kevin Hatfield's Blog

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 Dr. Donald Tashkin,
a UCLA 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.

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 lit 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 use in their youth, when it is
most frequently tried.