Background: The prevalence of marijuana use is increasing in the United States. Marijuana smoking has been shown to impair the microbicidal activity of alveolar macrophages and decrease the number of ciliated epithelial cells in the bronchi with a parallel increase in the number of mucus-secreting surface epithelial cells, which may increase the risk of pneumonia. However, it remains unclear whether there is an association between smoking marijuana and pneumonia. Methods: Using data from the Multicenter AIDS Cohort Study (MACS), a long-term observational cohort study of men who have sex with men in the United States, we used Cox proportional hazards models to estimate the risk of pneumonia among HIV-infected (n = 2784) and HIV-uninfected (n = 2665) men from 1984 to 2013, adjusted for time-varying and fixed baseline covariates. Results: Weekly or daily marijuana use was not significantly associated with increased risk of pneumonia among HIV-uninfected men (adjusted hazard ratio; 95% confidence limits: 0.83, 0.56–1.23). In the disaggregated dose–response analysis, daily use (0.68, 0.34–1.35) was associated with a lower point estimate than weekly use [0.99, 0.79–1.25]. Conclusion: Marijuana smoking was not associated with a significant increase in risk of pneumonia among HIV-infected or HIV-uninfected men.
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