Cal NORML Testimony To OEHHA Re: Carcinogenicity Of "Marijuana Smoke"

                                                                    Oct 29th, 2007

Office of Environmental Health Hazard Assessment
Prop 65 Implementation

PO Box 4010
1001 I St., 19th Floor
Sacramento CA 95812-4010

      We are responding to OEHHA's request for scientific information regarding the carcinogenicity of marijuana smoke for hearings by the Carcinogen Identification Committee on November 19, 2007. 

      The best current evidence is that marijuana smoke does not cause cancer. Even though it can contain various carcinogenic pyrolytic compounds similar to those found in tobacco smoke, the best epidemiological studies have failed to detect elevated cancer rates in marijuana smokers.  One possible explanation is that the major psychoactive ingredient of marijuana, delta-9-tetrahydrocannabinol (THC) has anti-carcinogenic properties, whereas the major ingredient of tobacco, nicotine, is cancer promoting [Melamede[1]].

      A definitive judgment about the carcinogenicity of "marijuana smoke" is not possible due to the fact that its ingredients are not precisely defined.   The marijuana plant comes in multitudinous varieties; at least three subspecies and scores of varieties are known, all of potentially differing composition.  Composition of the plant may also be affected by different pesticides, fertilizers, growing method, and contaminants in the soil.  Different parts of the plant may be smoked:  the leaves and buds (which must be dried beforehand to burn properly), the resin scraped from the plant (known as hashish or kif), or even the seeds and stems, which are low in THC but often present in low-grade marijuana.  Each of these has distinctive burning properties that have not been scientifically explored. In general, domestic marijuana smoke studies have employed a single variety of marijuana from NIDA grown by the University of Mississippi, consisting of chopped leaves that have been freeze-dried.  This marijuana is of lower quality and potency than that that preferred by most users.  In recent years, higher-potency seedless "sinsemilla" cannabis buds have increasingly dominated the domestic market and are standard for medical users.   Because of DEA restrictions on research, there is a lack of information about the composition of sinsemilla smoke.

      Despite the lack of a well-defined scientific specification of the composition of marijuana smoke, it is reasonable to assume that it characteristically contains two major components:   (1) THC and other cannabinoids distinctive to the cannabis plant;  and (2) pyrolytic byproducts of leaf or bud combustion. (A third  component consisting of other medically active ingredients, such as terpenoids and flavonoids, may be distinguished, but these constitute a diminutive fraction of the smoke).   Such studies as have been conducted on burning marijuana leaf have found that its smoke contains many of the same pyrolytic byproducts of combustion as tobacco [IOM, Hoffman].[2] Among them are benzene and various polynuclear aromatic hydrocarbons (PAHs), which are carcinogenic and thought to be a significant factor in smoking-related cancers.   Unlike tobacco, however, marijuana smoke does not contain nicotine, which is a cancer promoter;  rather, it contains THC and other cannabinoids. 

      There is growing evidence that cannabinoids  inhibit certain cancers, including lung carcinomas, breast cancer and gliomas[Kogan[3]]. Therefore, it cannot be assumed that marijuana smoke is carcinogenic like tobacco smoke. A recent study by Harvard researchers found that THC inhibits lung cancer growth in mice and in vitro human cells.  [Preet[4]}.  Cannabinoids are even being investigated as potential anti-cancer agents [Guzman[5]].

         The most important evidence regarding carcinogenicity of marijuana smoke comes from epidemiological studies, which have failed to demonstrate a link between cannabis smoking and respiratory cancer.  The largest  and most definitive epidemiological study on this question was led by Dr. Donald Tashkin of U.C.LA., who examined 1,209 respiratory cancer patients against matched controls [Hashibe[6]  After adjusting for confounding variables, investigators found no increased risk for lung, oral, laryngeal, pharyngeal or esophageal cancers even for long-term heavy marijuana users.  Risk ratios for marijuana were below 1 in almost all categories.  There was even a statistically significant decrease in lung cancer risk for one subpopulation of subjects (those reporting 1 to 10 joint-years of use), suggesting a possible protective effect from marijuana.

         The largest epidemiological survey to date of marijuana users, encompassing 64,855 clients of Kaiser Permanente [Sidney[7] ], found no association between marijuana use and cancer of any kind; however, the subject population was under 50 and hence not old enough to manifest high cancer morbidity. While a couple of anecdotal reports have claimed to detect a relation between marijuana smoking and throat, neck and head cancers in younger users [Donald, Taylor][8] these have not been confirmed in larger studies.  In a case-control study at John Hopkins University, investigators found no relation between cannabis use and head, neck or lung cancer in 164 patients and 524 controls [Ford[9] ].Most recently, a population-based case-control study of 407 subjects with oral squamous cell carcinoma found no relation between marijuana use and SCC{Rosenblatt].[10]

      Whatever its physiological risk to those who consume it, there is no evidence that cannabis smoke presents an environmental hazard to others. Although there have been no studies on passive marijuana smoking, such a link appears unlikely in light of the facts that (1) direct exposure has so far produced no detectable cancer risk and (2) users typically generate much less smoke than tobacco smokers (1 gram of marijuana for a one-or-two joint per day smoker vs 20 grams of tobacco for a pack-a-day habit), thus exposing bystanders to a much lower volume of pollutants.

      Finally, it should be pointed out that noxious pyrolytic compounds in marijuana smoke can be virtually eliminated through a process known as vaporization, in which marijuana is heated to a temperature around 180 C, where THC vapors are generated without causing combustion[Gieringer[11] ]. Vaporized marijuana from such devices must therefore be distinguished from marijuana smoke. 


            Dale H. Gieringer, Ph.D.

               Director, California NORML


[1]  Melamede, R. " Cannabis and tobacco smoke are not equally carcinogenic,"

Harm Reduction Journal 2005,  2:21.

[2]     Institute of Medicine, "Marijuana and Health," (National Academy Press 1982), pp 13-7;   Hoffman, D. et al, "On the Carcinognicity of Marijuana Smoke," in Runeckles, VC (ed), Recent Advances in Phytochemistry (NY, Plenum Pub., 1975) pp. 63-81; 

[3]   Kogan, N. "Cannabinoids and Cancer," Mini Reviews in Medicinal Chemistry, 2005, 5 941-52.

[4]   Preet A, Ganju Rk, Groopman JE, "Delta(9)-Tetrahydrocannabinol inhibits epithelial growth factor-induced lung cancer cell migration in vitro as well as its growth and metastasis in vivo," Oncogene 2007 Jul 9th.

[5]   Guzman M, "Cannabinoids: Potential Anticancer Agents," Nature, Oct. 2003 Vol. 3, 745-55;

[6] Hashibe M, Morgenstern H,  Cui Y,  Tashkin DP,  Zhang ZF, Cozen W,  Mack T, and Greenland S, "Marijuana Use and the Risk of Lung and Upper Aerodigestive Tract Cancers: Results of a Population-Based Case-Control Study,"  Cancer Epidemiol Biomarkers Prev 2006;15(10):1829 34.

[7] Sidney S, Quesenberry CP Jr, Friedman GD, Tekawa IS, " Marijuana use and cancer incidence (California, United States)," Cancer, Causes, and Controls. 1997 Sep 8(5):722-8.

[8] Donald P, "Advanced malignancy in the young marijuana smoker," Adv Exp Med Biol 288:33-56 (1991); Taylor FM, "Marijuana as a potential respiratory tract carcinogen," South Med Journal 81:1213-6 (1988).

[9]  Ford D et al, "Marijuana Use is not Associated With Head, Neck or Lung Cancer in Adults Younger Than 55 Years:  Results of a Case Cohort Study," Society of General Internal Medicine 23rd Annual Meeting, Boston May 4-6, 2000 Abstracts:

[10] Rosenblatt KA, Daling JR, Chen C, Sherman KJ, Schwartz SM. "Marijuana use and risk of oral squamous cell carcinoma," Cancer Res. 2004 Jun 1;64(11):4049-54.

[11] Gieringer  D. & St Laurent J, "Cannabis Vaporizer Combines Efficient Delivery of THC With Effective Suppression Of Pyrolytic Compounds" Journal of Cannabis Therapeutics  Vol. 4 #1 (2004). Gieringer, D "Cannabis Vaporization: A Promising Strategy for Smoke Harm Reduction," Journal of Cannabis Therapeutics, Vol. 1 #3/4: 153-70 (2001).

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