Smoking marijuana does not cause cancer
In fact, it seems possible that it might even protect against cancer.
I’m blogging this because one of the claims opponents of Prop 7 in Nevada are making is that pot smoking causes cancer. But research shows that claim is false:
People who smoke marijuana do not appear to be at increased risk for developing, new research suggests.
While a clear increase inrisk was seen among cigarette smokers in the study, no such association was seen for regular cannabis users.
Even very heavy, long-term marijuana users who had smoked more than 22,000 joints over a lifetime seemed to have no greater risk than infrequent marijuana users or nonusers.
The findings surprised the study’s researchers, who expected to see an increase in cancer among people who smoked marijuana regularly in their youth.
“We know that there are as many or more carcinogens and co-carcinogens in marijuana smoke as in cigarettes,” researcher Donald Tashkin, MD, of UCLA’s David Geffen School of Medicine tells WebMD. “But we did not find any evidence for an increase in cancer risk for even heavy marijuana smoking.” Carcinogens are substances that cause cancer.
Tashkin presented the findings today at The American Thoracic Society’s 102nd International Conference, held in San Diego.
The study population was limited to people who were younger than 60 because people older than that would probably not have used marijuana in their teens and early adult years.“People who may have smoked marijuana in their youth are just now getting to the age when cancers are being seen,” Tashkin says.
A total of 611 lung cancer patients living in Los Angeles County, and 601 patients with other cancers of the head and neck were compared with 1,040 people without cancer matched for age, sex, and the neighborhood they lived in.
All the participants were asked about lifetime use of marijuana, tobacco, and alcohol, as well as other drugs, their diets, occupation, family history of lung cancer, and socioeconomic status.
The heaviest marijuana users in the study had smoked more than 22,000 joints, while moderately heavy smokers had smoked between 11,000 and 22,000 joints.
While two-pack-a-day or more cigarette smokers were found to have a 20-fold increase in lung cancer risk, no elevation in risk was seen for even the very heaviest marijuana smokers.
The more tobacco a person smoked, the greater their risk of developing lung cancer and other cancers of the head and neck. But people who smoked more marijuana were not at increased risk compared with people who smoked less and people who didn’t smoke at all.
Studies suggest that marijuana smoke contains 50% higher concentrations of chemicals linked to lung cancer than cigarette smoke. Marijuana smokers also tend to inhale deeper than cigarette smokers and hold the inhaled smoke in their lungs longer.
So why isn’t smoking marijuana as dangerous as smoking cigarettes in terms of cancer risk?
The answer isn’t clear, but the experts say it might have something to do with tetrahydrocannabinol, or THC, a chemical found in marijuana smoke.
Cellular studies and even some studies in animal models suggest that THC has antitumor properties, either by encouraging the death of genetically damaged cells that can become cancerous or by restricting the development of the blood supply that feeds tumors, Tashkin tells WebMD.
In a review of the research published last fall, University of Colorado molecular biologist Robert Melamede, PhD, concluded that the THC in cannabis seems to lessen the tumor-promoting properties of marijuana smoke.
The nicotine in tobacco has been shown to inhibit the destruction of cancer-causing cells, Melamede tells WebMD. THC does not appear to do this and may even do the opposite.
While there was a suggestion in the newly reported study that smoking marijuana is weakly protective against lung cancer, Tashkin says the very weak association was probably due to chance.
Cancer risk among cigarette smokers was not influenced by whether or not they also smoked marijuana.
“We saw no interaction between marijuana and tobacco, and we certainly would not recommend that people smoke marijuana to protect themselves against cancer,” he says.
This finding is particularly significant coming from Dr. Tashkin:
Marijuana smoking -“even heavy longterm use”- does not cause cancer of the lung, upper airwaves, or esophagus, Donald Tashkin reported at this year’s meeting of the International Cannabinoid Research Society. Coming from Tashkin, this conclusion had extra significance for the assembled drug-company and university-based scientists (most of whom get funding from the U.S. National Institute on Drug Abuse). Over the years, Tashkin’s lab at UCLA has produced irrefutable evidence of the damage that marijuana smoke wreaks on bronchial tissue. With NIDA’s support, Tashkin and colleagues have identified the potent carcinogens in marijuana smoke, biopsied and made photomicrographs of pre-malignant cells, and studied the molecular changes occurring within them. It is Tashkin’s research that the Drug Czar’s office cites in ads linking marijuana to lung cancer. Tashkin himself has long believed in a causal relationship, despite a study in which Stephen Sidney examined the files of 64,000 Kaiser patients and found that marijuana users didn’t develop lung cancer at a higher rate or die earlier than non-users. Of five smaller studies on the question, only two—involving a total of about 300 patients—concluded that marijuana smoking causes lung cancer. Tashkin decided to settle the question by conducting a large, prospectively designed, population-based, case-controlled study. “Our major hypothesis,” he told the ICRS, “was that heavy, longterm use of marijuana will increase the risk of lung and upper-airwaves cancers.”
The Los Angeles County Cancer Surveillance program provided Tashkin’s team with the names of 1,209 L.A. residents aged 59 or younger with cancer (611 lung, 403 oral/pharyngeal, 90 laryngeal, 108 esophageal). Interviewers collected extensive lifetime histories of marijuana, tobacco, alcohol and other drug use, and data on diet, occupational exposures, family history of cancer, and various “socio-demographic factors.” Exposure to marijuana was measured in joint years (joints per day x 365). Controls were found based on age, gender and neighborhood. Among them, 46% had never used marijuana, 31% had used less than one joint year, 12% had used 10-30 j-yrs, 2% had used 30-60 j-yrs, and 3% had used for more than 60 j-yrs. Tashkin controlled for tobacco use and calculated the relative risk of marijuana use resulting in lung and upper airwaves cancers. All the odds ratios turned out to be less than one (one being equal to the control group’s chances)! Compared with subjects who had used less than one joint year, the estimated odds ratios for lung cancer were .78; for 1-10 j-yrs, .74; for 10-30 j-yrs, .85 for 30-60 j-yrs; and 0.81 for more than 60 j-yrs. The estimated odds ratios for oral/pharyngeal cancers were 0.92 for 1-10 j-yrs; 0.89 for 10-30 j-yrs; 0.81 for 30-60 j-yrs; and 1.0 for more than 60 j-yrs. “Similar, though less precise results were obtained for the other cancer sites,” Tashkin reported. “We found absolutely no suggestion of a dose response.” The data on tobacco use, as expected, revealed “a very potent effect and a clear dose-response relationship -a 21-fold greater risk of developing lung cancer if you smoke more than two packs a day.” Similarly high odds obtained for oral/pharyngeal cancer, laryngeal cancer and esophageal cancer. “So, in summary” Tashkin concluded, “we failed to observe a positive association of marijuana use and other potential confounders.”
There was time for only one question, said the moderator, and San Francisco oncologist Donald Abrams, M.D., was already at the microphone: “You don’t see any positive correlation, but in at least one category [marijuana-only smokers and lung cancer], it almost looked like there was a negative correlation, i.e., a protective effect. Could you comment on that?”
“Yes,” said Tashkin. “The odds ratios are less than one almost consistently, and in one category that relationship was significant, but I think that it would be difficult to extract from these data the conclusion that marijuana is protective against lung cancer. But that is not an unreasonable hypothesis.”
Abrams had results of his own to report at the ICRS meeting. He and his colleagues at San Francisco General Hospital had conducted a randomized, placebo-controlled study involving 50 patients with HIV-related peripheral neuropathy. Over the course of five days, patients recorded their pain levels in a diary after smoking either NIDA-supplied marijuana cigarettes or cigarettes from which the THC had been extracted. About 25% didn’t know or guessed wrong as to whether they were smoking the placebos, which suggests that the blinding worked. Abrams requested that his results not be described in detail prior to publication in a peer-reviewed medical journal, but we can generalize: they exceeded expectations, and show marijuana providing pain relief comparable to Gabapentin, the most widely used treatment for a condition that afflicts some 30% of patients with HIV.
To a questioner who bemoaned the difficulty of “separating the high from the clinical benefits,” Abrams replied: “I’m an oncologist as well as an AIDS doctor and I don’t think that a drug that creates euphoria in patients with terminal diseases is having an adverse effect.” His study was funded by the University of California’s Center for Medicinal Cannabis Research.