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May 24 (foodconsumer.org) - Heavy use of marijuana does not appear to increase the risk of lung cancer and other cancers such as cancer of the head and neck, a new study has found.

Even in those heavy, long-term marijuana users, the risk of head and neck cancers including cancer of the tongue, mouth, throat and esophagus does not seem elevated compared with that in those who did not smoke.

The findings are surprising as the researchers expected the heavy marijuana users would have to face an increased risk of cancer after exposure to marijuana smoke.

Donald Tashkin, M.D., Professor of Medicine at the David Geffen School of Medicine at UCLA in Los Angeles and colleagues conducted the study of 611 participants with lung cancer and 601 with cancer of the head or neck regions, from Los Angeles County.

To do the marijuana and cancer study, 1,000 people without cancer who were matched on age, gender and neighborhood were used as controls.

All participants chosen were aged 60 or younger as those born before 1940 were unlikely to be exposed to marijuana in their teens and 20s when use of marijuana is peaked. This would allow the researchers to see the effect of marijuana on cancer if there is any.

The subjects were questioned about use of marijuana, tobacco, alcohol and other drugs. They were also surveyed for their diet, occupations, family history of cancer and socioeconomic status.

In the study, the heaviest smokers were those who had smoked more than 22,000 marijuana cigarettes, or joints. Moderately heavy smokers were those who had smoked between 11,000 and 22,000 joints.

The heavy marijuana smokers did not have an elevated risk of developing cancer. People who smoked more marijuana did not seem to have a higher risk than those who smoked less or none, the study found.

However, the study did find that 80% of lung cancer cases and 70% of patients with cancer of the head and neck had smoked tobacco, indicating smoking tobacco is linked with cancer risk.

People who smoked two or more packs of cigarettes a day faced a 20-fold increased risk of lung cancer, the researchers noted. There is also a dose-response correlation between tobacco smoking and risk of lung cancer and head and neck cancers.

The current findings regarding the effect of marijuana smoke on cancer is contradictory to previous studies in which other researchers found marijuana abuse has the potential to promote cancer of the lungs and other parts of the respiratory tract because it contains irritants and carcinogens.

The cancer-causing effect of marijuana is highly expected because marijuana smoke contains 50 to 70 percent more carcinogenic hydrocarbons than does tobacco smoke.

Marijuana smoke also induces high levels of an enzyme that converts certain hydrocarbons into their carcinogenic form—levels that may accelerate the changes that ultimately produce malignant cells.

An early study comparing 173 cancer patients and 176 healthy individuals produced evidence that marijuana smoking doubled or tripled the risk of developing cancer of the head or neck.

The current findings are surprising also because marijuana users usually inhale more deeply and hold their breath longer than tobacco smokers do, which increases the lungs' exposure to carcinogenic smoke.

Tashkin guesstimated that tetrahydrocannabinol (THC), an active substance in marijuana smoke, may promote earlier death of aging cells, preventing the injured cells from becoming cancer cells.

Although marijuana smoke appears free of cancer risk, experts warned that people stay away from marijuana because smoking marijuana has been well linked with other problems such as respiratory problems, increased heart rate, loss of motor skills, and increased heart rate.

One good thing about marijuana is that smoking marijuana can relieve severe p ain in people with cancer. But the federal government bans such a use.

In 2004, 14.6 million Americans age 12 and older used marijuana at least once in the month prior to being surveyed. About 6,000 people a day in 2004 used marijuana for the first time—2.1 million Americans, according to the National Institute on Dr ug Abuse.

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