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The risk of a cigarette smoking person developing cancer is much higher than a the risk of a non-smoker developing cancer (can I assume that this is common knowledge?) - this website claims that adding marijuana to the mix will reduce the risk due to the mechanism of how marijuana helps remove the cigarette toxins from the lungs. They don't mention by how much the risk is reduced, for which types of cancers it is beneficial and since they have financial interests, I'm skeptical about their claims.

In a Costa Rican study, it was found that chronic marijuana smokers who also smoked cigarettes were less likely to develop cancer than cigarette smokers who didn’t use marijuana. Since marijuana (smoking, as well as ingestion by other methods) dilates the alveoli, toxins are more easily eliminated with cannabis use regardless of its method of application. Nicotine, on the other hand, constricts the alveoli, so it is likely that the use of cannabis neutralizes, or even overwhelms the constriction, by its own tendency to dilation http://www.benefitsofmarijuana.com/benefits.php

Does cigarette smokers lower their risk of some cancers by also smoking cannabis?

  • I don't know where the exact study is online. – user15927 Oct 23 '13 at 22:49
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There is some truth to this claim although I would not rate the study which I source here as high-quality evidence.

Male and female [marijuana and tobacco smokers] had a lower probability of developing lung cancer than did [tobacco-only smokers].

It is a primary study using statistical modeling. It showed correlation, not causation.

Keep in mind that they only looked at lung cancer while smoking tobacco increases the risk for several other types of cancer.

For biological plausibility of these claim I might refer to:

Cannabis smoke contains cannabinoids whereas tobacco smoke contains nicotine. Available scientific data, that examines the carcinogenic properties of inhaling smoke and its biological consequences, suggests reasons why tobacco smoke, but not cannabis smoke, may result in lung cancer.

Although only a side-note, tobacco companies use low-quality fertilizer which may contain radioactive elements: http://www.sciencedirect.com/science/article/pii/S1350448708002060

Edit: I would like to add a quotation by a large meta analysis by Huang et al. 2015. After listing several reasons why smoking cannabis should cause cancer they state that:

In view of the above findings, a null association between marijuana use and lung cancer is somewhat surprising because marijuana smoke contains known carcinogens in amounts com-parable with those found in tobacco smoke (49). Although the generally smaller amounts of marijuana that are regularly smoked compared with tobacco might appear to explain the null association of marijuana with lung cancer, the absence of a dose–response relationship between marijuana use and lung cancer,in contrast to the strong dose–response relationship noted for tobacco (16), would argue against this explanation. A more likely explanation is a tumor-suppressant effect of THC and other cannabinoids evident in both cell culture systems and animal models of a variety of cancers, as reviewed by Bifulco and colleagues (57). These antitumoral effects (antimitogenic, proa-poptotic, and antiangiogenetic) could possibly counteract the tumor-initiating or tumor-promoting effects of the carcinogens within the smoke of cannabis.

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    Welcome to Skeptics! Great find. Nice first answer. Please stick around. – Oddthinking Apr 12 '15 at 15:22
  • Thx! There could also be a relationsship between pesitice residues in tobacco plant material and cancer rates. I will try to find a scientific reference as soon as I have more time.. – CuriousIndeed Apr 12 '15 at 15:38
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    Perhaps simply because the time spent smoking marijuana isn't spent smoking tobacco? – Clockwork-Muse Apr 14 '15 at 11:52

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