Cannabis is the most commonly smoked illicit substance in many countries including France. It can be smoked alone in plant form (marijuana) but in our country it is mainly smoked in the form of cannabis resin mixed with tobacco. The technique of inhaling cannabis differs from that of tobacco, increasing the time that the smoke spends in contact with the bronchial mucosal and its impact on respiratory function. One cigarette composed of cannabis and tobacco is much more harmful than a cigarette containing only tobacco. In cannabis smokers there is an increased incidence of respiratory symptoms and episodes of acute bronchitis. Cannabis produces a rapid bronchodilator effect; chronic use provokes a reduction in specific conductance and increase in airways resistance. Studies on the decline of Forced Expiratory Volume are discordant. Cannabis smoke and tetrahydrocannabinol irritate the bronchial tree. They bring about histological signs of airways inflammation and alter the fungicidal and antibacterial activity of alveolar macrophages. Inhalation of cannabis smoke is a risk factor for lung cancer. Stopping smoking cannabis will bring about important benefits for lung function. This should encourage clinicians to offer patients support in quitting smoking.