Evolution designed man to avoid, not seek, taking smoke into the lungs; thus, any smoked substance is likely to cause harm. In fact, marijuana users are more likely to have bronchitis (i.e., coughing and shortness of breath) and lung infections such as pneumonia (Hall W, Lancet 374, p 1383-1391, 2009). Most, but not all, studies suggest daily marijuana users have impaired lung function. Marijuana smoke includes many of the same cancer-causing chemicals as tobacco smoke and that marijuana use often involvesdeep inhalations; thus, one would think users would have more lung cancer. Although most studies do suggest marijuana can cause lung cancer, others have not. This may be because the studies had few participants who used marijuana several times a day. Many believe that given the logic that marijuana use should cause cancer, we should assume this is so, until proven otherwise.
–Dr. John R. Hughes