Study finds no link between marijuana use and lung cancer

People who smoke marijuana–even heavy, long-term marijuana users–do not appear to be at increased risk of developing lung cancer, according to a study to be presented at the American Thoracic Society International Conference on May 23rd.

Marijuana smoking also did not appear to increase the risk of head and neck cancers, such as cancer of the tongue, mouth, throat, or esophagus, the study found.

The findings were a surprise to the researchers. “We expected that we would find that a history of heavy marijuana use–more than 500-1,000 uses–would increase the risk of cancer from several years to decades after exposure to marijuana,” said the senior researcher, Donald Tashkin, M.D., Professor of Medicine at the David Geffen School of Medicine at UCLA in Los Angeles.

The study looked at 611 people in Los Angeles County who developed lung cancer, 601 who developed cancer of the head or neck regions, and 1,040 people without cancer who were matched on age, gender and neighborhood. The researchers used the University of Southern California Tumor Registry, which is notified as soon as a patient in Los Angeles County receives a diagnosis of cancer.

They limited the study to people under age 60. “If you were born prior to 1940, you were unlikely to be exposed to marijuana use during your teens and 20s–the time of peak marijuana use,” Dr. Tashkin said. People who were exposed to marijuana use in their youth are just now getting to the age when cancer typically starts to develop, he added.

Subjects were asked about lifetime use of marijuana, tobacco and alcohol, as well as other drugs, their diet, occupation, family history of cancer and socioeconomic status. The subjects’ reported use of marijuana was similar to that found in other surveys, Dr. Tashkin noted.

The heaviest smokers in the study had smoked more than 22,000 marijuana cigarettes, or joints, while moderately heavy smokers had smoked between 11,000 to 22,000 joints. Even these smokers did not have an increased risk of developing cancer. People who smoked more marijuana were not at any increased risk compared with those who smoked less marijuana or none at all.

The study found that 80% of lung cancer patients and 70% of patients with head and neck cancer had smoked tobacco, while only about half of patients with both types of cancer smoked marijuana.

There was a clear association between smoking tobacco and cancer. The study found a 20-fold increased risk of lung cancer in people who smoked two or more packs of cigarettes a day. The more tobacco a person smoked, the greater the risk of developing both lung cancer and head and neck cancers, findings that were consistent with many previous studies.

The new findings are surprising for several reasons, Dr. Tashkin said. Previous studies have shown that marijuana tar contains about 50% higher concentrations of chemicals linked to lung cancer, compared with tobacco tar, he noted. Smoking a marijuana cigarette deposits four times more tar in the lungs than smoking an equivalent amount of tobacco. “Marijuana is packed more loosely than tobacco, so there’s less filtration through the rod of the cigarette, so more particles will be inhaled,” Dr. Tashkin said. “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 lung.”

One possible explanation for the new findings, he said, is that THC, a chemical in marijuana smoke, may encourage aging cells to die earlier and therefore be less likely to undergo cancerous transformation.

The next step, Dr. Tashkin says, is to study the DNA samples of the subjects, to see whether there are some heavy marijuana users who may be at increased risk of developing cancer if they have a genetic susceptibility for cancer.

From American Thoracic Society

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16 thoughts on “Study finds no link between marijuana use and lung cancer”

  1. The fact is that marijuana like any other drug has its cons but compared to the other illegal drugs as well as legal drugs marijuana has very little downsides. Alcohol and tobacco kill hundreds of thousands a year. Never have I found a death solely related to marijuana

  2. Just because someone prints something in a scientific journal it doesn’t mean that the data is good data. I haven’t seen the detailed methodology of it but from this brief abstract..

    a) Omitting patients over the age of 60 is a flaw as cancer increases in prevelance with age. So you would be missing out on a large group of people.

    b) the people conducting the study didn’t adjust for confounding factors. It could be any number of issues which increases a person’s susceptiblity of cancer. Unless you do a randomised control trial you can not prove causality

    c) 611 people in one trial is not enough evidence. 601 patients were in the head and neck region… so the other 10 were in the lungs (I guess). 10 patients are not enough to prove that marijuana use does not cause lung cancer.

    d) There is a problem with self reporting. Asking people how much they used to smoke is not always sufficient as people do lie.

    I’m not saying there is no link… i’m just saying that this isn’t enough evidence.

    • the man who wrote this reply to the page is obviously not older than 25. he has shown that using the word “good” is acceptable. he is obviously not ready to write a reply to this subject.

  3. There seems to be some confusion among previous commentators. As this article clearly states, the scientists were expecting to find that smoking weed increases one’s odds of developing lung cancer because they have long been aware of the carcinogens burning it generates.

    However, despite their expectations, no increased incidence of cancer was found even in the heaviest users.
    The researcher offered one hypothesis for why this might be, but further testing is required to answer, precisely, why the carcinogens do not lead to increased risk of cancer. However, we don’t have to know *why* it doesn’t to know that it doesn’t cause cancer.

    Generally speaking, though, smoking pot doesn’t cause cancer because carcinogens do not always cause cancer. We know that there is a link, but we do not its precise nature.
    It may interest you to know that oxygen (that stuff you inhale every few seconds) is also a carcinogen. That doesn’t mean you’re going to get cancer from breathing regular air. Likewise, just because marijuana smoke contains carcinogens does not mean you will get cancer from inhaling it.

    Already, THC has shown a variety of applications in fighting cancer. I eagerly await further studies into its interactions with cancerous cells, because it likely holds valuable clues to how we can cure Cancer once and for all.

  4. Um, if you could read and comprehend correctly, you would know that there were 611 patients of lung cancer in the trial, not 10. There were also 601 people with cancer in the head or neck regions, an then 1,040 other people. So that is 2,252 people, which seems appropriate, considering that the study was for the Los Angeles area.

    Also, it wouldn’t make sense to include a group of people that were less likely to be exposed to marijuana just because they are over 60 and more likely to have cancer. That wouldn’t help the study at all, because the study was to see if marijuana use led to cancer, not to see if people over 60 who didn’t use marijuana had cancer.

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