Smoking marijuana does not appear to increase the risk of lung cancer or head-and-neck malignancies, even among heavy users, researchers reported here.
“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, said Donald Tashkin, M.D., of the University of California in Los Angeles.
But in fact, they reported at the American Thoracic Society meeting here, marijuana use was associated with cancer risk ratios below 1.0, indicating that a history of pot smoking had no effect on the risk for respiratory cancers. Studies have shown that marijuana contains many compounds that when burned, produce about 50% higher concentrations of some carcinogenic chemicals than tobacco cigarettes.
In addition, heavy, habitual marijuana use can produce accelerated malignant change in lung explants, and evidence on bronchial biopsies of pre-malignant histopathologic and molecular changes, Dr. Tashkin said.
The investigators had also previously shown that smoking one marijuana cigarette leads to the deposition in the lungs of four times as much tar as smoking a tobacco cigarette containing the same amount of plant material. Marijuana cigarettes are not filtered and are more loosely packed than tobacco, so there’s less filtration of the tar. In addition, pot smokers hold the smoke in their lungs about four times longer than tobacco smokers do, Dr. Tashkin pointed out.
For the population-based case-control study, they identified cancer cases among people from the ages of 18 to 59, using the Los Angeles County Cancer Surveillance Program registry.
They identified 611 people with lung cancer, 601 with cancers of the head and neck, and 1,040 controls matched by age, gender and neighborhood (as a surrogate for socioeconomic status).
They conducted extensive personal interviews to determine lifetime marijuana use, measured in joint-years, with one joint-year equivalent to 365 marijuana cigarettes. The interviewers also asked participants about tobacco use, alcohol consumption, use of other drugs, socioeconomic status, diet, occupation, and family history of cancer.
The investigators also used logistic regression to estimate the effect of marijuana use on lung cancer risk, adjusting for age, gender, race/ethnicity, education, and cumulative tobacco smoking and alcohol use.
They found that the heaviest users in the study had smoked more than 60 joint years worth of marijuana, or more than 22,000 joints in their lifetime. Moderately heavy users smoked between 11,000 and 22,000 joints.
Despite the heavy use, “in no category was there any increased risk, nor was there any suggestion that smoking more led to a higher odds ratio,” he continued. “There was no dose-responsiveness,not even a suggestion of a dose response”and in all types of cancer except one, oral cancer, the odds ratios were less than one.”
In contrast, tobacco smoking was associated with increased risk for all cancers, and there was a “powerful” dose-response relationship. People who smoked more than two packs of cigarettes per day had a 21-fold risk for cancer, as opposed to a less than one-fold risk for marijuana, Dr. Tashkin said.
“When we restricted the analysis to those who didn’t smoke any tobacco we found the same results, and when we looked for interaction between tobacco and smoking ”would marijuana increase the risk, potentiate the carcinogenic effect of tobacco”we didn’t find that, nor did we find a protective effect against the effect of tobacco, which is very important, because the majority of marijuana smokers also smoke tobacco,” he commented.
It’s possible that tetrahydrocannabinol (THC) in marijuana smoke may encourage apoptosis, or programmed cell death, causing cells to die off before they have a chance to undergo malignant transformation, he said.
Note: Reviewed by Robert Jasmer, MD; Associate Clinical Professor of Medicine, University of California, San Francisco.