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WELLINGTON, New Zealand, Jan. 29 -- Smoking a single marijuana joint may be as carcinogenic to the lung as 20 tobacco cigarettes, researchers here determined.

Those who smoked the equivalent of one joint a day for 10 years had a 5.7 times higher lung cancer risk than nonsmokers even after adjusting for tobacco use, reported Richard Beasley, M.B.Ch.B., of the Medical Research Institute of New Zealand here, and colleagues in the Feb. 1 issue of the European Respiratory Journal.

The effect on lung cancer risk in the population-based case-control study was even greater than the one joint to five cigarettes equivalency for lung damage previously reported by the research group. (See: One Cannabis Joint Equals Smoking Up to Five Cigarettes)

Smoke from cannabis contains up to twice as many carcinogenic polyaromatic hydrocarbons and tend to be smoked without filters while inhaling more deeply, leading to higher concentrations of smoke inhaled, the investigators noted.

Although cough, wheeze, and other respiratory effects expected with any type of smoking have been found with marijuana, the association with lung cancer has been inconclusive. (See: Marijuana Linked to Respiratory Complications)

However, the New Zealand findings provide sufficient evidence that some components of cannabis itself or cannabis smoke are real lung carcinogens, according to an accompanying editorial by Christian Brambilla, M.D., and Marc Colonna, Ph.D., both of the Institut Albert Bonniot in Grenoble, France.

"The prudence principle should be sufficient to convince everybody that lung cancer has to be added to the list of secondary effects of cannabis smoking, along with asthma and chronic obstructive pulmonary disease," they wrote.

Physicians should ask patients about smoking of both tobacco and cannabis in every day practice, they said.

But Norman H. Edelman, M.D., chief medical officer of the American Lung Association, was more cautious.

"Since there are some studies that reach other conclusions, we can't say that it nails down [the risk]," he said. "We need larger studies."

The researchers conducted in-home interviews on cancer risk factors including cannabis use among 79 lung cancer patients younger than 55 and 324 age-matched controls randomly selected from eight New Zealand health districts covering a population of 1.8 million.

Lung cancer patients were identified from hospital databases or the national cancer registers from 2001 through 2005. Most had non-small-cell lung cancer (80%) and none had lung metastasis from a distant primary.

The proportion of controls who had ever smoked cannabis was 36% after adjustment for the general population age distribution.

Overall, 26.6% of lung cancer patients in the study reported smoking at least 20 joints in their lifetime, whereas 12% of control participants had.

For every one joint-year -- the equivalent of one joint per day for one year -- smoked, the risk of lung cancer rose 8% (relative risk 1.08, 95% confidence interval 1.02 to 1.15).

The association between cannabis and lung cancer was strengthened with adjustment for the growth rate of lung cancer, by excluding exposure in the five years before baseline or diagnosis (RR 1.10, 95% CI 1.02 to 1.18), "as would be expected if a causal association existed."

The association was similar to the 7% risk seen for each pack-year of tobacco smoking (RR 1.07, 95% CI 1.05 to 1.09).

Participants who had smoked 20 or more joints over their lifetime were not at significantly higher risk than those who had smoked fewer (RR 1.2, 95% CI 0.5 to 2.6).

Only those in the highest use group with more than 10.5 joint-years of exposure were at significantly elevated lung cancer risk compared with nonsmokers (RR 5.7, 95% CI 1.5 to 21.6) after adjustment for tobacco exposure, age, sex, ethnicity, and family history of lung cancer.

Further adjustment excluding exposure over the prior five years, which would not have been expected to have contributed to the current diagnosis, showed a similar pattern, with 5.2-fold higher risk for those in the highest exposure tertile.

But this is not likely a threshold effect, Dr. Beasley and colleagues said.

Rather, the lack of association at lower intake levels could have been the result of the relatively small number of marijuana users in the study and the young age of the participants "reducing the time available for high numbers of joint-years to accumulate."

Even using the prevalence of the highest tertile of cannabis smoking among the control group, the researchers estimated that about 5% of lung cancer among those younger than 55 in New Zealand may be attributable to cannabis smoking.

"If any increased risk was maintained as these young people age," they said, "then a considerable burden from lung cancer due to cannabis smoking may occur in the future."

Although they said participants were unaware during their interview that the study was focused on cannabis use, they noted that epidemiologic research on cannabis use has been fraught with difficulties.

"While it is important to interpret the findings in the context of these limitations," the investigators concluded, "the balance of evidence would suggest a positive association between cannabis and lung cancer."

The researchers and editorialists provided no information on conflicts of interest or funding. Dr. Edelman reported no conflicts.

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