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Nearly half of adults have used marijuana at some point in their lives, and rates are increasing; among young adults, marijuana usage doubled from 10.5% in 2002 to 21.2% in 2014. Given this widespread usage, the question of marijuana-associated cancer risk is an important one, and it is the subject of a systematic review and meta-analysis now published in JAMA Network Open.
Marijuana smoke and tobacco smoke share a number of carcinogens. In addition, tetrahydrocannabinol, marijuana's primary psychoactive ingredient, may have cancer-associated adverse immunomodulatory effects, and smokers who smoke only marijuana have increased expression of two proto-oncogenes in the bronchial epithelium compared with those who only smoke tobacco. On the other hand, tetrahydrocannabinol and other cannabinoids are capable of restraining proliferation of some types of cancer cells. They can also prevent angiogenesis––the development of new blood vessels––in cell cultures, and they can reduce cancer growth in some animal models. So what exactly is the association between marijuana and cancer risk?
The meta-analysis included English-language studies that reported on the development of cancer and involved adult participants with at least 1 joint-year exposure––the equivalent of 1 joint per day for 1 year––of cumulative marijuana use. Review articles, commentaries, case reports, case series, studies with under 10 marijuana users, studies reporting outcomes only after short-term exposure in a laboratory setting, and studies that did not primarily evaluate marijuana exposure were excluded. Twenty-five studies met criteria to be included in the analysis.
In a pooled analysis of 3 case-control studies, over 10 years' worth of marijuana usage was associated with the development of testicular germ cell tumor (TGCT), with an odds ratio of 1.36, and with nonseminoma TGCT, with an odds ratio of 1.85. In a pooled analysis of case control studies, ever use of marijuana was not associated with head and neck squamous cell carcinoma or oral cancer. In general, evaluations of ever use found no association with cancers; however, exposure levels were low and were not well defined.
For lung cancer, the association was unclear. Of the 8 studies examining the association between lung cancer and marijuana use, all had a moderate to high risk of bias, and most were limited by the small number of marijuana-only smokers, as most marijuana smokers also smoked tobacco. In addition, they were characterized by minimal exposure to marijuana, poorly described assessment of use, and inadequate adjustment for confounding variables. As a result, the study authors concluded that the evidence was insufficient to evaluate the association.
"Low-strength evidence suggests that smoking marijuana is associated with developing TGCT; its association with other cancers and the consequences of higher levels of use are unclear," state the researchers, led by first author Mehrnaz Ghasemiesfe, MD, of the Northern California Institute of Research and Education, located in San Francisco.
"With increasing marijuana use and the high number of cancer-related deaths, understanding the association between marijuana use and cancer incidence is important," write Dr. Ghasemiesfe and colleagues. "Large-scale longitudinal studies with representative samples of marijuana-only smokers are needed to better understand the association of marijuana use with the development of lung, oral, and other cancers. In the meantime, clinicians should discuss marijuana use with patients to raise awareness of the lack of clarity on potential clinically important harms and to debunk beliefs in unproven benefits."
For More Information
Ghasemiesfe M, Barrow B, Leonard S, et al (2019). Association between marijuana use and risk of cancer: a systematic review and meta-analysis. JAMA Netw Open. [Epub ahead of print] DOI:10.1001/jamanetworkopen.2019.16318
Image credit: Cannabis Pictures. Licensed under CC BY 2.0