During a recent walk with my 6-year-old, he told me he smelled marijuana. His comment speaks to its increased (and more open) use since legalization in our state. The macho, misguided part of my dad psyche was proud of his "street cred" but the thinking part of my brain was concerned. He seemed a little young for a talk about drugs.
I was able to provide a simple, watered-down list of reasons why he shouldn’t smoke marijuana or anything else. The "drugs are bad" aphorism sufficed for my 6-year-old but wasn’t worthy of an academic pulmonologist.
I retired from the military 2 years ago, so marijuana (I’m using the terms "marijuana" and "cannabis" interchangeably here) knowledge wasn’t required for regular practice. I recall one 60-year-old patient who reported smoking four joints a day for years. He had marked emphysema on CT, severe obstruction on spirometry, and he was functionally limited. Buttressed by scattered reports of acute lung injury due to dabbing or marijuana vaping, this anecdotal "n of 1" led to a predictably pedantic conclusion: Smoking marijuana is bad for the lungs and preaching cessation is worth my time and effort.
I now work in an inner-city hospital. My 6-year-old could identify the smell permeating the hallways and clinic rooms. I’ve reverted to counseling cessation using little more than my "drugs are bad" speech. When I came across a recent review in Seminars in Respiratory and Critical Care Medicine, I recognized the opportunity to read and do better. This summary is based heavily on that review.
Spoiler alert: The data aren’t great. By federal law, marijuana has been illegal in the United States since 1970, so neither funding nor recruitment has come easy. There’s lots of observational data that depend on self-report and are confounded by cigarette use. A lack of regulation results in variations in composition and concentration. In summary, though, smoking marijuana is associated with changes to the bronchial tree and respiratory symptoms, similar to those seen with chronic bronchitis. These symptoms improve with cessation.
The relationship between marijuana and airflow obstruction and lung function is complicated. A mix of contradictory data shows a reduction in the ratio of the forced expiratory volume in the first 1 second to the forced vital capacity (FEV1/FVC), an increase in FVC, and changes in conductance.
Biologic plausibility, essential to bolster causality but easy to manufacture, seems intuitive for the airway changes (decreased FEV1/FVC and conductance). The increase in FVC, explained by either the anti-inflammatory properties of delta-9-tetrahydrocannabinol (THC) or the impact from deep inhalations typical of marijuana use, is more difficult to understand. Regardless, I came away from the review less confident about marijuana’s impact on lung structure and function.
The Seminars review also explores marijuana’s association with lung cancer, emphysema, and other structural changes seen on CT of the chest. There’s certainly noise here but the data at present are underwhelming.
This all speaks to the general misconception I’ve had, perhaps shared by others, that the well-defined effects on the lung from tobacco abuse can be extrapolated to marijuana. In the past, I’d even gone so far as to equate a pack-year (smoking one pack of cigarettes per day for a year) to a joint-year (smoking one joint per day for a year), a rather dramatic oversimplification. While both are attempts to quantify exposure, the latter connotes far less information. The content of a joint can vary considerably in ways that the content of cigarettes does not, and there have been no formal studies of the comparative impact on the lung.
Final Thoughts
The nuance here matters for several reasons. Legalization means an increase in use and presumably more open reporting by patients. In a vacuum, it seems reasonable to council cessation to reduce symptoms and because additional harms can be assumed, given what we know about smoke inhalation in general. Will cessation drive patients to an increase in tobacco use where harm is better established?
Given its mixed effects on lung function, is it worth spending behavior change capital, the most precious of patient commodities, on marijuana counseling? Marijuana has numerous effects outside the lung that haven’t been touched on here. How should those be incorporated into our guidance? Legalization and regulation provide the opportunity to obtain the better data that are sorely needed.
Aaron B. Holley, MD, is a professor of medicine at Uniformed Services University in Bethesda, Maryland, and a pulmonary/sleep and critical care medicine physician at MedStar Washington Hospital Center in Washington, DC. He covers a wide range of topics in pulmonary, critical care, and sleep medicine.