The smoke from burning marijuana leaves contains several known
carcinogens and the tar it creates contains 50 percent more of some
of the chemicals linked to lung cancer than tobacco smoke. A
marijuana cigarette also deposits four times as much of that tar as
an equivalent tobacco one. 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 Uinversity 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 researchers interviewed 611 lung cancer patients and 1,040
healthy controls as well as 601 patients with cancer in the head or
neck region under the age of 60 to create the statistical analysis.
They found that 80 percent of those with lung cancer and 70 percent
of those with other cancers had smoked tobacco while only roughly
half of both groups had smoked marijuana. The more tobacco a person
smoked, the greater the risk of developing cancer, as other studies have shown.

But after controlling for tobacco, alcohol and other drug use as well
as matching patients and controls by age, gender and neighborhood,
marijuana did not seem to have an effect, despite its unhealthy
aspects. "Marijuana is packed more loosely than tobacco, so there's
less filtration through the rod of the cigarette, so more particles
will be inhaled," Tashkin says. "And marijuana smokers typically
smoke differently than tobacco smokers; they hold their breath about
four times longer allowing more time for extra fine particles to
deposit in the lungs."

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.