Does Smoking Marijuana Cause Lung Cancer?

Effects of Marijuana on the Lungs

Hands pruning marijuana crop.
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The link between smoking tobacco and lung cancer is undeniable, but does smoking marijuana cause lung cancer, too? The short answer—maybe. Let's take a look at the long answer.

Studies Looking at Marijuana and Lung Cancer

In 2006, many of us in medicine were shocked when a review of research to date did not show an increase in lung cancer related to marijuana use. There was even a suggestion that marijuana had a protective effect against lung cancer.

More recent studies, in contrast, do appear to link smoking marijuana with lung cancer, although the results are mixed, and much uncertainty remains.

One study demonstrated a doubling in lung cancer for male marijuana smokers who also used tobacco (i.e., for men who smoked the same amount, the risk of lung cancer was twice as high for men who also used marijuana). Another study found that long-term use of marijuana increased the risk of lung cancer in young adults (55 and under), with the risk increasing in proportion to the amount of marijuana smoked.

A large international study conducted in 2015, in contrast, found little association between habitual and long-term use of cannabis and lung cancer. In this review, some association was found between cannabis use and lung adenocarcinoma but no association was found between cannabis use and squamous cell carcinoma of the lungs.

Effects of Marijuana on the Lungs

Researchers have found that regular use of marijuana causes injury to the airways that can be seen visibly as well as under the microscope.

There have also been reports of an increase in respiratory symptoms such as wheezing, shortness of breath, and persistent coughing in people who smoke pot. That said, regular smoking of marijuana does not seem to cause any significant changes in lung function, nor does it appear to increase the risk of COPD, and COPD is an independent risk factor for lung cancer.

The Controversy About Marijuana and Cancer Risk

Since marijuana is still illegal in the U.S. under federal law, it is hard to do the controlled studies that have been done with tobacco. Because of this, it helps to look at what we do know about marijuana that suggest it could increase lung cancer risk:

  • Many of the carcinogens and co-carcinogens present in tobacco smoke are also present in smoke from marijuana.
  • Marijuana smoking does cause inflammation and cell damage, and it has been associated with pre-cancerous changes in lung tissue.
  • Marijuana has been shown to cause immune system dysfunction, which could theoretically predispose individuals to cancer.

The bottom line on marijuana use and risk of cancer? Though marijuana most likely pales in cancer risk when compared to cigarette smoking, it's best to practice caution. There are reasons in addition to lung cancer risk (and the fact that it is illegal in many states) to avoid marijuana. Marijuana likely increases the risk of testicular cancer, prostate cancer, cervical cancer, a type of brain tumor, and the risk of leukemia in the offspring of women who use weed during pregnancy.

The Flip Side: Marijuana in Cancer Patients

When we talk about marijuana and cancer, there are generally two different discussions.

When talking about the cause of cancer, results are still mixed with some studies suggesting that marijuana smoking increases the risk of cancer and others saying that marijuana may instead protect against cancer.

What we do know, is that smoking marijuana may help some people cope with cancerAccording to the National Cancer Institute, "cannabinoids may have benefits in treating cancer-related side effects." Some of the side effects that may improve with use of weed include nausea, loss of appetite, pain, and sleep disturbances. And since cancer cachexia—a  combination of symptoms including loss of appetite, unintentional weight loss, and muscle wasting—is considered the direct cause of death in 20 percent of people with cancer, the use of cannabinoids by cancer patients deserves much further study.

As far as treatment, the difficulty in studying an illegal substance has limited research. One small study found that marijuana may have had a benefit in patients with a type of recurrent brain tumor. Hopefully, with legalization across the United States increasing, this answer will become clearer in the future. 

Secondhand Marijuana Smoke

A final concern about marijuana is the possible effects of marijuana on nearby nonusers. The effects of secondhand marijuana smoke on health and drug testing are being studied. Some studies have found marijuana smoke to be as much of a concern as tobacco smoke, so caution may be warranted.

A Word From Verywell

Until large studies can be performed, you can't be assured that smoking marijuana or being exposed to secondhand weed smoke has no health concerns. You have a variety of choices other than smoking for ways to use medical cannabis and recreational cannabis in states where it is legal. If you worry about your lung health and exposing nonsmokers, it may be best consider a different mode of delivery than smoking, such as edibles.

Sources:

Berthiller, J. et al. Cannabis smoking and risk of lung cancer in men: a pooled analysis of three studies in Maghreb. Journal of Thoracic Oncology. 2008. 3(12):1398-403.

Chen, A. et al. Hypothesizing that marijuana smokers are at a significantly lower risk of carcinogenicity relative to tobacco-non-marijuana smokers: evidence based on reevaluation of current literature. Journal of Psychoactive Drugs. 2008. 40(3):263-72.

Huang, Y., Zhang, Z., Tashkin, D. et al. An Epidemiological Review of Marijuana and Cancer: An Update. Cancer Epidemiology Biomarkers and Prevention. 2015. 24(1):15-31.

Martinasek, M., McGrogan, J., and A. Maysonet. A Systematic Review of the Respiratory Effects of Inhalational Marijuana. Respiratory Care. 2016 Aug 9. (Epub ahead of print).

Taskin, D. Effects of marijuana smoking on the lung. Annals of the American Thoracic Society. 2013. 10(3):239-47.

Zhang, L., Morgenstern, H., Greenland, S. et al. Cannabis Smoking and Lung Cancer Risk: Pooled Analysis in the International Lung Cancer Consortium. International Journal of Cancer. 2015. 136(4):894-903.

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