Nature, a peer-reviewed scientific journal, stated in a Nov. 2, 2000 article titled "Bidirectional Control of Airway Responsiveness by Endogenous Cannabinoids" by Calignano et al.:

“Smoking marijuana or administration of its main active constituent, THC, may exert potent dilating effects on human airways. But the physiological significance of this observation and its potential therapeutic value are obscured by the fact that some asthmatic patients respond to these compounds with a paradoxical bronchospasm [constriction of the air passages of the lung].

The mechanisms underlying these contrasting responses remain unresolved. Here we show that endogenous cannabinoid anandamide exerts dual effects on bronchial responsiveness in rodents: it strongly inhibits bronchospasm and cough evoked by the chemical irritant, capsaicin, but causes bronchospasm when the constricting tone exerted by the vagus nerve is removed.”

Nov. 2, 2000