Marijuana smoking and the risk of lung cancer: time for pause

February 11, 2013 Eric Vallieres

While smoking marijuana may have recently been legalized, one shouldn’t conclude that it is necessarily healthy.

For individuals who treat lung cancer patients in the states of Washington and Colorado, the recently passed legislations raise serious concerns. Ironically, this happened on the eve of the 50th anniversary of the first US Surgeon General Report by Dr. Luther Terry in January 1964, which concluded that lung cancer is causally related to cigarette smoking. This report had followed decades of warnings by so-called common sense alarmists who had identified this as a major public health issue but who lacked the data to fully support their claims.

It is estimated that, in the US, we saw just over 225,000 new cases of lung cancer in 2012, 80 to 90% of these occurring in active or former cigarette smokers. That year, close to 160,000 American men and women died of their lung cancer. In the state of Washington alone, about 4,200 new cases were diagnosed. These numbers are quite disheartening considering the links that were clearly established and disseminated in the 1964 report and subsequently validated by a multitude of others since. This clearly shows that legal does not guarantee healthy.

Additionally, there is a 20-year lag in between the time when a population starts smoking and the time when the incidence of lung cancer begins to increase. In reverse, it takes 20 years before a decrease in consumption of cigarettes leads to a drop in the incidence of lung cancer.

Having recognized that clear link between tobacco and lung cancer, we are often asked if such a relationship exists with marijuana use as well.

The present medical literature on this issue is about as clear as were the medical and epidemiological literatures addressing lung cancer risks in cigarette smokers during the 1950s. Some population based (epidemiological) studies have concluded that smoking cannabis increases the risks, while others have suggested it doesn’t.

These studies, however, are limited by the fact that many marijuana smokers are also cigarette smokers, that some users mix tobacco and marijuana in their joint, and that the illegality of marijuana use may have influenced the willingness of participants to give honest answers regarding their use of marijuana, affecting the validity of the data used in these studies.

However, there are some facts that we should acknowledge. Laboratory work has demonstrated the occurrence of tissue, cellular and molecular pre-cancerous changes in the airways and lungs of cannabis users that are strikingly similar to those seen in cigarette smokers. Similarly, the carcinogenic effects of cannabis smoke have been demonstrated in both human and animal models: marijuana smoke contains 50 to 70% more carcinogenic hydrocarbons than tobacco smoke. Additional factors that may contribute to the carcinogenic potential of marijuana smoke are the tendency for marijuana to burn at a higher temperature and that marijuana smoke is typically inhaled deeper and held longer than tobacco smoke (two factors that promote prolonged contact duration of potential carcinogens on the lung lining).

Some argue that one or two joints per day of exposure to these carcinogens does not even come close to the 1 -2 packs per day contact a cigarette smoker experiences. While this may mathematically make sense, the fact is that we do not know of a safe level for such exposures. Some have estimated that one-joint-a-day is equivalent to a one-pack-a-day use of cigarettes. A 2008 study from New Zealand estimated that each joint-year* of cannabis exposure increases the risk of lung cancer by 8% after adjusting for confounding variables including tobacco smoking. Lastly, there are also some hints that these cancers may occur at a younger age.

In the same way that making it legal does not make it safe, a lack of data does not mean that there are no serious long-term consequences of smoking marijuana purely for recreational purposes. As a member of the medical community, my fear is that if legalization of marijuana leads to more recreational use by our children and adolescents, we may see an increase in the incidence of lung cancer in the 40 and 50 year old population… 20 years from now.

(*3 joint-years is either 1 joint a day for 3 years or 3 joints per day for 1 year)

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