A systematic review assessing 19 studies that evaluated premalignant or malignant lung lesions in persons 18 years or older who inhaled marijuana concluded that observational studies failed to demonstrate statistically significant associations between marijuana inhalation and lung cancer after adjusting for tobacco use.
Epidemiologic studies examining one association of Cannabis use with head and neck squamous cell carcinomas have also been inconsistent in their findings. A pooled analysis of nine case-control studies from the U.S./Latin American International Head and Neck Cancer Epidemiology (INHANCE) Consortium included information from 1,921 oropharyngeal cases, 356 tongue cases, and 7,639 controls. Compared with those who never smoked Cannabis, Cannabis smokers had an elevated risk of oropharyngeal cancers and a reduced risk of tongue cancer. These study results both reflect the inconsistent effects of cannabinoids on cancer incidence noted in previous studies and suggest that more work needs to be done to understand the potential role of human papillomavirus infection.
With a hypothesis that chronic marijuana use produces adverse effects on the human endocrine and reproductive systems, the association between marijuana use and incidence of testicular germ cell tumors (TGCTs) has been examined.[10,11,12] Three population-based case-control studies report an association between marijuana use and elevated risk of TGCTs, especially nonseminoma or mixed-histology tumors.[10,11,12] However, the sample sizes in these studies were inadequate to address marijuana dose by addressing associations with respect to recency, frequency, and duration of use. These early reports of marijuana use and TGCTs establish the need for larger, well-powered, prospective studies, especially studies evaluating the role of endocannabinoid signaling and cannabinoid receptors in TGCTs.
A comprehensive Health Canada monograph on marijuana concluded that while there are many cellular and molecular studies that provide strong evidence that inhaled marijuana is carcinogenic, the epidemiologic evidence of a link between marijuana use and cancer is still inconclusive.
No clinical trials of Cannabis as a treatment for cancer in humans were identified in a PubMed search; however, a single, small study of intratumoral injection of delta-9-THC in patients with recurrent glioblastoma multiforme reported potential antitumoral activity.[14,15]