The New England Journal of Medicine, a peer-reviewed medical journal, published a 1973 study titled "Single-Dose Effect of Marihuana Smoke. Bronchial Dynamics and Respiratory-Center Sensitivity in Normal Subjects," by L. Vachon et al., that stated:

“Marihuana smoke, unlike cigarette smoke, causes bronchodilation [expansion of the air passages] rather than bronchoconstriction [narrowing of the air passages] and, unlike opiates, does not cause central respiratory depression.”