Updated August 20, 2015.
Written or reviewed by a board-certified physician. See About.com's Medical Review Board.
Question: Does Smoking Marijuana Cause Lung Cancer?
Answer:
The link between smoking tobacco and lung cancer is undeniable, but does smoking marijuana cause lung cancer, too? The short answer -- maybe.
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.
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.
Efffects of Marijuana on the Lungs
Researchers have found that regular use of marijuana causes injury to the airway that can be seen visibly as well as under the microscope. 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.
Why the Controversy about Cancer Risk?
Since marijuana is illegal, 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:
- 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, possibly predisposing individuals to cancer.
Bottom line on use for people without 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 most 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 it during pregnancy.
The Flip Side - Marijuana in Cancer Patients
When we talk about people who already have cancer, there are two different discussions. As noted above, results are still mixed, 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 cancer. According to the National Cancer Institute, "cannabinoids may have benefits in the treatment of 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. 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 increasing, this answer will become more clear in the future.
Secondhand Marijuana Smoke
A final concern about marijuana is the possible effects of marijuana on nearby non-users. Check out this article which discusses the latest findings on the effects of secondhand marijuana smoke on health and drug testing.
Further Reading:
This article talks about the recreational use of marijuana, but what about medical marijuana?
Sources:
Aldington, S. et al. Cannabis use and risk of lung cancer: a case-control study. The European Respiratory Journal. 2008. 31(2):280-6.
American Cancer Society. Lung Cancer (Small Cell). What are the risk factors for small cell lung cancer. Marijuana. Updated 07/28/10. http://www.cancer.org/Cancer/LungCancer-SmallCell/DetailedGuide/small-cell-lung-cancer-risk-factors.
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.
Hashibe, M. et al. Epidemiologic review of marijuana use and cancer risk. Alcohol. 2005. 35(3):265-75.
Mehra, R. et al. The association between marijuana smoking and lung cancer: a systematic review. Archives of Internal Medicine.
National Cancer Institute. Cannabis and Cannabinoids. Updated 08/07/15. http://www.cancer.gov/cancertopics/pdq/cam/cannabis/healthprofessional/page5
Sarafian, T. et al. Oxidative stress produced by marijuana smokers. An adverse effect enhanced by cannabinoids. American Journal of Respiratory Cell and Molecular Biology. 1999. 20(6):1286-93.
Taskin, D. Effects of marijuana smoking on the lung. Annuals of the American Thoracic Society. 2013. 10(3):239-47.
Tashkin, D. et al. Respiratory and immune consequences of marijuana smoking. Journal of Clinical Pharmacology. 2002. 42(11 Suppl):71S-81S.
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