Marijuana is the most commonly used illicit drug around the world. It is most often consumed through smoking, placing the respiratory system in direct contact with the toxic constituents of the drug, which are similar to those of tobacco cigarettes. However, accurate study of the adverse effects of marijuana is difficult to perform, owing to marijuana's illegal status, variation in smoking technique, often short duration of use compared with tobacco and the frequently confounding factor of concomitant consumption of both marijuana and tobacco. Despite this, there is evidence to suggest that marijuana can impair lung function, damage large airway mucosa and possibly contribute to bullous disease, while its carcinogenic potential is controversial.