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LETTERS: Roy Robertson Comparing cannabis with tobacco: Services are needed for acute and chronic effects of cannabis BMJ 2003; 327: 165-b [Full text]

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Carcinogenic Cannabis?

Cory Gordon   (26 February 2004)

Carcinogenic Cannabis? 26 February 2004
Cory Gordon, student 92119 Send response to journal: Re: Carcinogenic Cannabis?	To date, there have been 10 studies on the effects of cannabinoids on cancers in vivo. Eight of them demonstrated antineoplastic effects, especially on highly aggressive malignancies liable to be caused by polycyclic aromatic hydrocarbons (viz., lung adenocarcinoma, squamous cell carcinoma, and leukemia). These 8 studies were conducted in Spain, Italy, and the US. The remaining 2 studies were conducted at UCLA and demonstrated that cannabinoids promote tumor growth via immunosuppression. What is interesting about the UCLA studies is that they administered cannabinoids intravenously at inordinately high doses -- a systemic route and dose irrelevant to cannabis smokers. If the UCLA in vivo studies are relevant to anyone, it would be cancer patients who have repeatedly overdosed on IV marinol. If the primary concern is that the tissues exposed directly to cannabis smoke are vulnerable to malignant transformation, then relevant effects would be most evident from repeated applications of cannabinoids directly to pre-existing tumors (since there has never been an in vivo study to demonstrate cannabinoids or cannabis smoke induces cancer). As for evidence in humans, there have been 8 epidemiological studies conducted to date. The only prospective study showed an elevated risk in tobacco smokers but not in cannabis smokers even though a significant number of the cannabis smokers did not smoke tobacco. UCLA's much-vaunted proof of cannabis causing cancer at an early age derives from two epidemiological studies (one to be presented this summer), neither of which acknowledged the risk entailed by chronic exposure to paraquat among cannabis smokers in the southwest US, where, as opposed to other regions of the US, the available evidence suggests that heavier cannabis smokers were significantly more likely to be exposed to levels of paraquat too low to cause symptoms but high enough to be carcinogenic. That paraquat is carcinogenic in humans has been suggested by some, but not all, in vivo studies; however, the most convincing evidence that paraquat is a potent carcinogen in humans derives from a recent case-report of an agricultural worker who developed 100 squamous tumors on an area of his skin that was chronically exposed to paraquat but not exposed to excessive sunlight. Aside from the UCLA studies, 3 of the studies were conducted in Europe: two reported a significant risk, while one did not show a significant risk. The remaining 2 epidemiological studies were conducted in the US -- in Washington and in Maryland -- two regions where cannabis smokers would not face an elevated risk for paraquat exposure. Both of these studies were much larger than the UCLA study that has been widely reported in the popular media. Also, these studies used random-digit selection methodology and showed that tobacco -- not cannabis -- was a major risk factor. Competing interests: None declared