Table.

Strength of Evidence for Each Respiratory Symptom, Obstructive Lung Disease, Pulmonary Function, and Other Respiratory Outcomes

Outcome	Study Type	Strength of Evidence	Comments
Respiratory symptoms
Cough	4 prospective observational studies (2 low* and 2 moderate ROB) and 7 cross-sectional studies (3 low, 3 moderate, and 1 high ROB)	Low	All prospective studies rated as low or moderate ROB and the pooled cross-sectional studies found an association between marijuana use and cough.
Sputum production	4 prospective observational studies (2 low* and 2 moderate ROB) and 5 cross-sectional studies (2 low, 2 moderate, and 1 high ROB)	Low	Low- and moderate-ROB studies showed a consistent trend toward increased risk for sputum production with marijuana use. Pooled data showed that marijuana use was associated with an increased risk for sputum production.
Wheezing	4 prospective observational studies (2 low* and 2 moderate ROB) and 7 cross-sectional studies (3 low, 3 moderate, and 1 high ROB)	Low	Three prospective studies (1 low and 2 moderate ROB) showed that marijuana use was associated with increased risk for wheezing. Pooled analysis of 4 cross-sectional studies (1 low and 3 moderate ROB) showed that marijuana use was associated with wheezing, but heterogeneity was high. Low- and moderate-ROB studies showed a consistent trend toward an association between marijuana use and wheezing.
Dyspnea	2 prospective observational studies*(low ROB) and 5 cross-sectional studies (2 low, 2 moderate, and 1 high ROB)	Insufficient	The prospective studies were based on the same cohort, and the study with the longest follow-up found no effect on increased risk for dyspnea. This study was limited because the median marijuana exposure was not reported; however, the pooled analysis from cross-sectional studies showed that marijuana use was associated with increased risk for dyspnea.
Obstructive lung disease	1 prospective observational cohort study (moderate ROB) and 5 cross-sectional studies (2 low, 2 moderate, and 1 high ROB)	Insufficient	One prospective study found an increased risk for chronic bronchitis during a long follow-up. Pooled analysis of 3 cross-sectional studies (1 low and 2 moderate ROB) did not show that marijuana use was associated with increased risk for chronic bronchitis. All studies showed a trend toward increased risk for chronic bronchitis from marijuana use; however, the body of available evidence was limited.
Pulmonary function
FEV1	4 prospective observational cohort studies (3 low and 1 moderate ROB) and 6 cross-sectional studies (3 low, 2 moderate, and 1 high ROB)	Insufficient	The low-ROB prospective studies found no effect on decrease of FEV1; however, the studies were limited by minimal exposure to marijuana and the young age of participants. The cross-sectional studies were limited by small sample size, a young population, and variable rigor in analysis; the experimental study was limited by lack of adjustment for key confounders.
FVC	2 prospective observational cohort studies (low ROB) and 4 cross-sectional studies (3 low and 1 high ROB)	Insufficient	The prospective studies showed mixed results with marijuana use and an increase in FVC, and the 2 cross-sectional studies showed no effect of marijuana on FVC, but study quality and rigor in analysis varied.
FEV1-FVC ratio	4 prospective observational cohort studies (2 low* and 2 moderate ROB) and 6 cross-sectional studies (3 low and 3 moderate ROB)	Insufficient	All low-ROB prospective studies were based on the same cohort study. The studies with the longest follow-up did not report average marijuana exposure in the cohort. The cross-sectional studies were limited by few marijuana-only smokers, variable rigor in analysis, and mixed findings.
Airway resistance and specific conductance of airways	1 prospective cohort study (low ROB) and 3 cross-sectional studies (1 low, 1 moderate, and 1 high ROB)	Low	All relevant studies found that marijuana use was associated with increased airway resistance and decreased specific conductance of airways.
Other respiratory outcomes	2 prospective observational studies (1 low and 1 high ROB) and 2 cross-sectional studies (low ROB)	Insufficient	The prospective studies were limited by inadequate marijuana exposure, inadequate adjustment for confounders, and many other methodological flaws with mixed findings.

ROB = risk of bias.

^*Both studies were based on the same cohort.