Study Shows Marijuana and Tobacco Pose Similar Cancer Risk

Marijuana was largely considered to be a mild intoxicant until the landmark study by Dr. Gabriel Nahas, published in book form in 1976 as Keep Off the Grass, informed the public of its myriad medical hazards. In the preface to the fifth edition, published in 1990, Dr. Nahas quoted the 1975 Helsinki Symposium, “The human pathology of marijuana smoking could only be written after several decades, when long-term studies of the marijuana-smoking population will have been completed.”

Since then, much of the research on marijuana use has focused on its negative psychotic effects, which are considerable. Other studies have clearly shown that negative cardiovascular effects from smoking or ingesting marijuana include increased heart rate and altered blood pressure with a higher risk of heart attacks and a broad assortment of other disorders. As time goes on, research continues to show that smoking above-ground parts of the hemp plant Cannabis sativa can lead to chronic bronchitis, emphysema and bullous lung disease. Now, a generation after the Helsinki Symposium, there is even more evidence of the dangers of marijuana use. The alarming results of a New Zealand study published in the February 1, 2008, issue of the European Respiratory Journal indicate that the carcinogenic properties of cannabis smoke may be greater than previously understood.

Although only the highest tertile of marijuana users showed a significant increase in lung cancer risk, the researchers noted that this does not indicate there is a particular threshold for adverse effect. Cigarette smoking is known to have a linear relationship to lung cancer risk, and every indication is that marijuana does too.

“The major finding from the present study was that for each joint-yr [the equivalent of one joint per day for one year] of cannabis exposure, the risk of lung cancer increased by 8%, after adjustment for confounding variables including tobacco smoking.”

Other studies have noted quantitative differences between marijuana and cigarettes for carbon monoxide levels and inhaled tar, but the study's authors point out that for cancer risk the dose ratio was much greater. “A major differential risk between cannabis and cigarette smoking was observed, with one joint of cannabis being similar to 20 cigarettes for risk of lung cancer.” 

What does this mean in practical terms? There are 20 cigarettes to the "pack," and the CDC classifies a "heavy smoker" as one who smokes 25 or more cigarettes a day. While there is little agreement as to what constitutes heavy marijuana use, some studies define heavy use as five joints or more per week. A marijuana smoker who smokes one joint per day could therefore likewise be considered a "heavy smoker" and would run the same cancer risks as a those who smoke more than a pack of cigarettes per day. 

Previous studies have sometimes shown varied results regarding lung cancer and marijuana use, but the New Zealand study used a population-based control group to avoid the inherent bias in hospital-based control groups. The authors caution that “recall of the amount of cannabis smoked over a long period of time may have been difficult for some subjects,” but note that this was likely to have been true of control groups as well. None of the subjects were aware of the study hypothesis.

The researchers conclude, “This issue is of major public health importance, due to the prevalent use of cannabis globally and lung cancer being responsible for over a million deaths in the world each year.” And on an individual level, this gives each person yet one more reason to avoid this harmful drug.