"Thanks for pointing out that study to me. I spent some time this afternoon reviewing what Dr. Tashkin has to say about his study. There are several rather severe problems with your statement, however, that marijuana has been shown to not produce cancer. You should note that Tashkin himself does not say that.
Firstly, Tashkin fully admits the results of other studies that have shown risk of heart attack to be as much as 4 times higher with marijuana smoking along with a greatly increased risk of lung infections. Marijuana contains four times as much tar and 50% to 70% more carcinogens than cigarette tobacco, the highest amounts to be found in "street marijuana" as opposed to government regulated and prescribed "medical marijuana". No one, certainly not Tashkin himself, is saying that we can relax now and expect that those will not harm, or even cause cancer in, marijuana smokers. Tashkin himself points out, in so many words, that we cannot say smoking marijuana doesn't cause cancer. What he claims is that the results of his study surprised him, and his colleagues in not showing as strong results of cancer as they expected based on the presence and concentration of carcinogens in the smoke.
In discussing possible reasons for this, Tashkin admits a problem with the study in that it does not properly represent the effects of regular smoking over long periods of time. It was difficult for them to find people who have been regular users for more than 30 years. He admits, himself, that the proven cancerous effects of tobacco relate to smoking over decades (30 to 50 years) and his study does not properly reflect such time periods. When you study someone who has been smoking regularly for fifteen years it doesn't mean that his smoking has not already given him cancer, it just means that he has not developed noticeable tumours yet. (Mom didn't get cancer, get rid of cancer and then get cancer again. She has had cancer since well before she noticed the first tumour, and still had it between the two bouts of tumour growth. Dad smoked for fifty years before he got lung cancer that was noticeable, but he may have had the cancer for years or even decades.) Tashkin also admits the presence of other scientific studies that have indicated higher risk of certain cancers with smoking marijuana than with smoking tobacco.
An additional point of interest from Tashkin's study, is that there may be some benefit from the THC in marijuana in reducing or inhibiting the growth of tumours. This would also help to explain the lower incidence of cancerous tumours observed and is an exciting possibility. (Could THC be directly administered to a tumour to reduce it or at least stunt its growth?). But do not make the mistake of equating the presence of a tumour with the presence of cancer. Just because no bomb has gone off yet does not mean there is no suicide bomber in the store. Tashkin also rightly points out that while tumour suppression MAY BE a potential side effect of THC we must not forget that it is a powerful immuno-suppressant and may lead to developing severe bacteriological infections, not only in the lungs, but elsewhere in the body. Additionally, marijuana has reliably been proven to greatly reduce the activity of our killer T-Cells that protect us from all manner of diseases. How happy would you feel NOT getting cancer from smoking marijuana but dying of bronchitis, pneumonia or some other bacteriological infection?
In his concluding remarks, Tashkin points out that, as a pulmonologist, he is absolutely against the practice of inhaling marijuana smoke into the lungs as a means of delivering the THC medically. He is in favour of the use of swallowed capsules and, possibly, vaporization. Both he and his colleage, Roth, say that smoking it is very bad for the health, despite the presence of THC. Also, both have stressed that their study DOES NOT show that smoking marijuana doesn't cause cancer. They say in no uncertain terms that the matter has not been proven and the presence of so many carcinogens suggests that smoking marijuana does cause cancer, although their study suggests that THC may help mitigate the incidence of cancer."
