A study of 450 individuals found that people who smoke marijuana frequently but do not smoke tobacco have more health problems and miss more days of work than nonsmokers (9). Many of the extra sick days among the marijuana smokers in the study were for respiratory illnesses. Even infrequent use can cause burning and stinging of the mouth and throat, often accompanied by a heavy cough. Someone who smokes marijuana regularly may have many of the same respiratory problems that tobacco smokers do, such as daily cough and phlegm production, more frequent acute chest illness, a heightened risk of lung infections, and a greater tendency to obstructed airways (10). Smoking marijuana increases the likelihood of developing cancer of the head or neck, and the more marijuana smoked the greater the increase (11).
A study comparing 173 cancer patients and 176 healthy individuals produced strong evidence that marijuana smoking doubled or tripled the risk of these cancers. Marijuana use also has the potential to promote cancer of the lungs and other parts of the respiratory tract because it contains irritants and carcinogens (12, 13). In fact, marijuana smoke contains 50 to 70 percent more carcinogenic hydrocarbons than does tobacco smoke (14). It also produces high levels of an enzyme that converts certain hydrocarbons into their carcinogenic form-levels that may accelerate the changes that ultimately produce malignant cells (15). Marijuana users usually inhale more deeply and hold their breath longer than tobacco smokers do, which increases the lungs’ exposure to carcinogenic smoke. These facts suggest that, puff for puff, smoking marijuana may increase the risk of cancer more than smoking tobacco..