Researchers from three Canadian universities conducted this study to discover whether regular inhalation of cannabis smoke could increase cancer patients' risk of developing additional tumors. The researchers recruited 513 patients who had recently been diagnosed with cancer of the head or neck, 59 of whom were cannabis users. The subjects, who were all recruited from a single cancer center in Ontario, were tracked between 2011 and 2015, with a followup evaluation in November 2018.
Over the study period, researchers recorded how many subjects developed a second primary squamous cell carcinoma (SPC) and how many remained cancer-free. Only two of the 59 cannabis users (3.4 percent) developed a secondary primary cancer, compared to 23 of the 454 non-users (5.1 percent). “Our study did not find a significant association between cannabis use and developing an SPC,” the authors explained.
To review, primary cancers are cancers that initially form in human tissue. Let's say someone first receives a diagnosis for breast cancer, which would be a primary cancer. If that patient's breast cancer metastasizes, or spreads, to another part of the body — like the stomach — the new cancer is called a secondary cancer. Secondary cancers make up 15 to 20 percent of all cancer diagnoses.
When a patient develops a secondary cancer, it's always bad news. For instance, if lung cancer metastasizes and spreads to another part of the body, the patient's five-year survival rate drops from 56 percent to a dismal 5 percent.
The findings of this study are quite specific, being limited only to recurrences of head and neck cancers. The researchers had a good reason to narrow their focus, though. In the study, the authors wrote that “tobacco usage is a well-known risk factor for squamous cell carcinoma of the head and neck (HNC). This carcinogen leads to potential second primary squamous cell carcinoma (SPC) of the upper aerodigestive tract through the well-studied concept of field cancerization.”
Field cancerization is a biological process in which the body's respiratory and digestive tracts are exposed to carcinogens from smoke, which can cause cancer to develop in any of these affected areas. The authors further explain that cannabis smoke contains similar carcinogens to tobacco smoke, and inhaling pot smoke spreads these carcinogens to the same areas that are exposed to tobacco carcinogens.
But despite these potential risks, the researchers found that field exposure to cannabis smoke does not increase the risk of developing SPC in the same way that tobacco smoke does. These findings led the authors to conclude that their “results are consistent with the theory that cannabis is not carcinogenic and hence would not follow patterns of field cancerization.”
It's important to note that the present study does not suggest that cannabis can cure cancer, nor does it explore the relationship between pot smoke and primary cancers. Other researchers are continuing to study the interactions between weed and cancer, however. Several recent clinical studies have found that certain cannabinoids can effectively kill specific kinds of cancer cells, in both humans and animals.
And another recent study has found that regular pot users are actually less likely to get cancer than non-users, in addition to being healthier in general.