The use of cannabis is embedded within many societies, mostly used by the young and widely perceived to be safe. Increasing concern regarding the potential for cannabis to cause mental health effects has dominated cannabis research and the potential adverse respiratory effects have received relatively little attention. Studies on cannabis are challenging and subject to confounding by concomitant use of tobacco and other social factors, and while many of the studies referred to in this review are beset by the difficulties inherent in undertaking epidemiological research of the effects of cannabis, there is an emerging concern among many chest physicians who would suggest that habitual smoking of cannabis may contribute to the development of chronic obstructive pulmonary disease, pneumothorax and respiratory infections, including tuberculosis. Special attention should be given to the risk of lung cancer, particularly as biological plausibility may precede epidemiology.