The Puzzle of Marijuana Use and Forced Vital Capacity.

In study after study, marijuana use has been found to be associated with increased forced vital capacity (FVC). This is puzzling, because marijuana is commonly consumed by inhalation of its smoke, and smoke exposure of any kind is not generally considered a cause of increased FVC. While this observation was first made decades ago, a satisfactory explanation remains elusive. This review surveys the evidence supporting the relationship between marijuana use and increased FVC, discusses potential threats to validity when inferring causation, and-presupposing a possible causal relationship-poses two key questions. Firstly, what are possible physiologic or pathophysiologic mechanisms by which marijuana use might increase FVC? Secondly, why might this effect be consistently observed with marijuana use but not with tobacco use? Explanations for the first question include lung and chest growth and remodeling from strenuous marijuana smoke inhalation and reductions in lung elastic recoil from marijuana smoke exposure. Explanations for the second include differences between marijuana and tobacco in smoke composition and patterns of consumption, such as smoking topography. Finally, the possibility that smoke-whether from marijuana or tobacco-might have non-monotonic effects on FVC depending upon the level of exposure is explored. In synthesizing a curated breadth of epidemiologic and physiologic science, this review leverages a perplexing observation to generate potential insights and avenues for further research into the biological effects of smoke, from marijuana or otherwise, on the respiratory system.