Dhivya Ramalingam, Ph.D.​

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The problem: Tuberculosis (TB) is a highly contagious, infectious disease of the lungs that is caused by the bacterium mycobacterium tuberculosis (MTb). Although a quarter of the world’s population (1.7 billion people) may be infected with MTb, only about 5-15% of those infected with MTb will progress to active disease. It’s not known whether cannabis use—and especially inhaled use—increases or decreases the risk that someone who is exposed to MTb will develop an active infection.

 

Why do this study? Cannabis is a popular illegal substance in many countries and has been reported to be a risk factor in tuberculosis outbreaks.

 

The study: The authors conducted a systematic review of all published literature from various scientific resources (e.g. MedLine, Embase, WHO) until January 2018. All primary epidemiological studies involving adults over 16 years with history of cannabis use and having TB disease (active or latent) were eligible for inclusion. For studies that utilized a comparator group, the ROBINS-I tool was used to measure risk of bias. The goal was to identify a possible association between cannabis use and TB outbreaks.

 

Major results: The authors scanned 377 epidemiological studies, out of which 11 fit the inclusion criteria for this study. Of these, 6 had a relevant comparator group. Four of the six studies with relevant comparators indicated an association between cannabis use and TB infection. A US study by Morano et. al. identified an association between cannabis use and incident TB infection (adjusted odds ratio, aOR=1.57). A second study from the US showed that the risk of acquiring TB was 2.8 times greater when cannabis was smoked in confined spaces in a group.  In addition, two studies from Australia noted a higher incidence of TB disease among those who shared a water pipe. Neither of two studies that evaluated the association between cannabis use and active tuberculosis found an increased risk. For instance, in one of these, a matched case-control study of 562 TB-positive individuals and 1308 controls did not find a difference (aOR=1.64; p=0.21).

 

Conclusions: The authors only found a weak association between cannabis use and the development of active tuberculosis after exposure.

 

What does this study add? This study found an association between cannabis use and tuberculosis largely in situations in which cannabis users share the same inhalation device or use cannabis in a confined space.  There seems to be no indication that cannabis use per se increased the risk of tuberculosis.

 

Funder: National Institute for Health Research Health Protection Research Unit (NIHR HPRU) and Public Health England (PHE)

 

Author conflicts: None

 

Commentary: Although there might be theoretical reasons why cannabis could increase the risk of tuberculosis (e.g. through CBD-mediated immune modulation), this metaanalysis did not find a convincing association.  Overall, the studies reviewed here were difficult to interpret, since they used several very different methods and endpoints.  Still the authors did their best to try to make some sense of multiple approaches.  It seems safe to say that sharing an inhalation device with someone who has tuberculosis probably increases your risk of infection.  Otherwise, cannabis use doesn’t seem to have an effect.  It’s important to note, though, that this study only reviewed reports of recreational use.  It’s possible that an association might exist for medical cannabis users who are immunosuppressed (e.g. due to cancer chemotherapy).  So further research is needed to explore the possibility of an increased or decreased risk of tuberculosis in that population.