Again with the Marijuana

Blogging on Peer-Reviewed Research

What is it about reporting on pot that makes people so Puritanical? Today I read in the Guardian Cannabis joints damage lungs more than tobacco – study.

A single cannabis joint may cause as much damage to the lungs as five chain-smoked cigarettes, research has found.

Is that so? Let’s take a look at the data.

The authors of the article compared smokers to fairly heavy marijuana users – based on the mean smoking exposure of the groups (54.2 joint years compared to 23 pack-years for the smokers both with a mean age ~42-46) the group was clearly smoking multiple joints a day over decades while the smokers – based on mean age and use – were probably smoking a little less than a pack a day on average. This first figure shows the many variables they examined using their method of high-resolution CT scanning that allowed them to quantify changes observed in the lungs, as well as some more classic techniques.


It’s been a while since I’ve taken respiratory physiology, but I think we can make it through this figure OK. The first set of numbers are area measurements from CT slices of the patients lungs, and an evaluation of whether or not there are macroscopic changes in the lungs consistent with emphysema. It shows that tobacco smokers get emphysema at a rate about 17 times that of marijuana smokers, and that’s if you consider that 1.3 % to be statistically significant, which it isn’t.

The second set of data comes from plethysmography (great word huh?). You have the patient blow air into a tube and measure their ability to push air out of their lungs, as well as the volume of air they are capable of moving in and out. In emphysema for instance, you have a large lung volume, but because of the loss of elasticity of your lungs you have little ability to push it in and out of your body. For the most part, the tobacco smokers did the worst in these tests, although the pot smokers showed some decreased lung function.

The third set is mostly subjective data about symptoms of coughing, wheezing etc., that the patients experience. Again, in most measures it was the cigarette smokers that consistently performed worse across the board.

Now this is interesting. The Guardian article makes a big deal about how smoking a joint is 2.5-5x worse for your lungs – based solely on changes in some of the lung function tests – however only the smokers show signs of emphysema. Is this really a sign that pot is worse than cigarettes?

Surely, if you smoked pot like cigarettes – 20-25 times a day – the data suggest that you might experience more significant airflow changes. However, no one smokes that much pot. It’s a very different drug from cigarettes and people don’t smoke all day – while driving, while working, while eating etc. – which allows the tobacco habit to become so dangerous. Further, no one has been able to link marijuana smoking to emphysema, COPD, or lung cancer (even in very heavy users) while cigarettes show a striking increase in risk for many smokers.

It is biologically plausible to think that smoking marijuana is ultimately a risk factor for lung cancer – albeit much smaller than for cigarettes – and I don’t think it can be ruled out. Studies like this one and others do show there are negative health consequences to smoking marijuana – bronchitis, wheezing, cough etc. Putting smoke in your lungs, any smoke, is worse than no smoke at all of course.

But to make these comparisons that pot is somehow “worse” than cigarettes simply can not be justified. Marijuana is not physiologically addictive (it doesn’t cause serious withdrawal) like cigarettes and other drugs, it does not have an established cancer risk and even the heavy users studied did not show signs of emphysema and COPD. Basically they showed some changes in lung function associated with their habit of inhaling marijuana smoke, but it wasn’t as bad as being a smoker, and doesn’t seem to cause emphysema like smoking does. Pointing out that the changes in one ratio of lung function tests and calling pot 2.5-5x worse seems misleading and unnecessarily alarmist.

What is it about covering the marijuana beat that makes people lose their objectivity I wonder? Rather than spending their time studying the health effects of marijuana, which have been persistently shown to be slight, maybe they should study the real risks of the drug – listening to bad jam bands and being socially boring.

1. Aldington, Sarah, Williams, Mathew, Nowitz, Mike, Weatherall, Mark, Pritchard, Alison, McNaughton, Amanda, Robinson, Geoffrey, Beasley, Richard
Thorax 2007 0: thx.2006.077081