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As documented in national surveys, for the past several years, marijuana has been the most commonly abused drug in the United States, with approximately 6% of the population 12 years and older having used the drug in the month prior to interview. The use of marijuana is not without significant health hazards. Marijuana is associated with effects on almost every organ system in the body, ranging from the central nervous system to the cardiovascular, endocrine, respiratory/pulmonary, and immune systems. Research presented in this special supplement will show that in addition to marijuana abuse/dependence, marijuana use is associated in some studies with impairment of cognitive function in the young and old, fetal and developmental consequences, cardiovascular effects (heart rate and blood pressure changes), respiratory/pulmonary complications such as chronic cough and emphysema, impaired immune function leading to vulnerability to and increased infections, and the risk of developing head, neck, and/or lung cancer. In general, acute effects are better studied than those of chronic use, and more studies are needed that focus on disentangling effects of marijuana from those of other drugs and adverse environmental conditions.