JAMA Watch Jan.11,2012: Moderate Marijauana Use is NOT Bad for Your Lungs
This weeks most interesting article is one in the research section about the effect---correction, the lack of effect that moderate marijuana smoking has on the lungs.
Before I get to the study results, a few words about how to evaluate research papers. If the study is a medical one, the most important thing is how was it conducted. The best studies are prospective, meaning that you take a group of people---the larger the better---and you follow them for a time----the longer the better---to see what happens to them. Along the way, you measure lots of things of interest. So, for example, you can measure cholesterol on day one and see if this seems to predict heart disease development over the course of the study. Much of what we know about chronic disease prevention comes from these types of research projects, of which the Framingham Heart Study is probably the best known.
A quicker, dirtier way to do the same type of research is a retrospective study. In this, a bunch of people who have a disease are questioned. A bunch of people who do not have the disease are also questioned. If a lot more of the people with the disease answer "yes" to the question, then you have an association. Remember, association is not causality. People with heart disease are also more likely to have had a stroke than folks without heart disease. That does not mean the stroke caused the heart disease. Retrospective studies are often flawed, because sick folks are more likely to remember things---like, say, the summer they worked with asbestos---than healthy folks are.
In the article "Association Between Marijuana Use and Pulmonary Function Over 20 Years" the researchers followed over 5000 men and women for two decades as part of the Coronary Artery Risk Development in Young Adults (CARDIA) project. Along the way, many pieces of information were collected, including how much tobacco was smoked and how much marijuana was smoked. The authors then compared this data to measures of lung function. Since many of those studied used both, the authors used statistical software that is able to take multiple possible factors and see how each is associated----or not associated---with different potential outcomes.
The results: tobacco cigarette smoking use was associated with reduced lung function. No surprise there. The more interesting finding: marijuana use was not associated with decreased lung function. Low level use (2 times a month or less) was actually associated with better lung function, while even heavy (i.e daily ) use did not significantly affect the lungs. However, heavy use was rare in the study, meaning that the findings here may not be valid.
Now, remember how I said that association does not equal causality. It is possible that low level use of marijuana strengthens the lungs in some way. But it is just as likely that other health or lifestyle differences associated with low level marijuana use (say, for instance, a healthy, more organic diet or living in a less polluted part of the country) could explain the lung findings.
So, don't walk away from this study thinking that two joints a month will give you the lungs of a marathon runner. On the other hand, if you need to use marijuana occasionally to control pain or glaucoma or nausea, you probably are not damaging your lungs.
Love the authors final paragraph, in which they admit that medical marijuana is beneficial for pain, appetite and other uses. The medical establishment is more than ready to start prescribing the drug. Now, if we could only convince government officials that the health and comfort of Americans is more important than the Prison Industrial Complex's ability to make money incarcerating low risk "offenders" whose only crime is smoking weed.