Modest Marijuana Exposure Does Not Adversely Affect Pulmonary Function, but High Cumulative Exposure Does

Previous investigations have not shown a consistent impact of marijuana smoking on pulmonary function. Investigators conducted a longitudinal analysis of 5016 adults* enrolled in the CARDIA† study to assess the impact of marijuana exposure on pulmonary function (FEV1 and FVC). Marijuana exposure was assessed at 6 follow-up exams, permitting calculation of “joint-years” (where 365 joints=1 joint-year) and tobacco pack-years. Fifty-six percent of participants (n=2807) attended the 20-year follow-up examination without differential attrition by marijuana use.

• In adjusted analyses, the association between marijuana use and pulmonary function was nonlinear:
  • at low lifetime exposure, marijuana use was associated with increased FEV1 and FVC.
  • at >7 joint-years, the positive association shown between marijuana use and pulmonary function leveled off.
  • at >10 joint-years, there was a nonsignificant decline in FEV1.
  • at >20 episodes of marijuana use per month, the decline in FEV1 was significant.

Comments:

The longitudinal study design and analysis of linearity allowed the investigators to elucidate the complex impact of marijuana on pulmonary function. The positive effect of marijuana smoking on FVC (hypothesized to be a “stretching and training” effect) dominated marijuana’s impact on pulmonary function at lower exposures, whereas its negative effect on FEV1 seemed to dominate at higher exposures. The authors note that estimating the impact of heavy marijuana use is imprecise in this sample because of the low number of heavy users and few non-tobacco users among the heaviest marijuana users. Nevertheless, this study may help define thresholds for riskier marijuana use, similar to those defined for alcohol.

Hillary Kunins, MD, MPH, MS

Reference:

Pletcher MJ, Vittinghoff E, Kalhan R, et al. Association between marijuana exposure and pulmonary function over 20 years. JAMA. 2012;307(2):173–181.