A long-term study published in JAMA Network Open found that 10 years of marijuana use is not linked to most cancers; however, low-quality evidence suggests that it could be linked to testicular germ cell tumor (TGCT).
Researchers conducted a meta-analysis of PubMed, Embase, PsycINFO, MEDLINE, and the Cochrane Library to identify English-language studies published between January 1, 1973, and June 11, 2018, that assessed marijuana non-users and ever-users (defined as using one joint per day for one year). The final cohort included 25 studies: 19 were case-control, five were cohort, and one was cross-sectional; just two studies had a low risk of bias.
Marijuana use and risk of cancer
In a pooled analysis of case-control studies, marijuana ever-use was not associated with head and neck squamous cell carcinoma or oral cancer. However, in a pooled analysis of three case-control studies, more than 10 years of marijuana use (joint-years were not reported) was associated with TGCT (odds ratio [OR] = 1.36; 95% CI, 1.03-1.81; P=0.03) and non-seminoma TGCT (OR=1.85; 95% CI, 1.10-3.11; P=0.04).
Marijuana ever-use was generally not association with other cancers, but exposure levels were low and poorly defined. The researchers noted that the findings related to marijuana use and lung cancer were mixed, confounded by few marijuana-only smokers, poor exposure assessment, and inadequate adjustment.
“Long-term studies in marijuana-only smokers would improve [the] understanding of marijuana’s association with lung, oral, and other cancers,” the researchers concluded.