The data on marijuana's negative effects on the brain have been somewhat mixed...some studies showing effects, for example, on learning and some types of memory, and others show little lasting effects. There are other studies showing pretty clear effects on the body. For example, effects on the heart -- here are the results -- marijuana increases heart rate by 20-100 percent shortly after smoking; this effect can last up to 3 hours. In one study, it was estimated that marijuana users have a 4.8-fold increase in the risk of heart attack in the first hour after smoking the drug. This may be due to increased heart rate as well as the effects of marijuana on heart rhythms, causing palpitations and arrhythmias. This risk may be greater in aging populations or in those with cardiac vulnerabilities. Effects on the lungs -- studies have shown marijuana smoke to contain carcinogens and to be an irritant to the lungs. Marijuana smoke contains 50-70 percent more carcinogenic hydrocarbons than tobacco smoke. Marijuana users usually inhale more deeply and hold their breath longer than tobacco smokers do, which further increase the lungs' exposure to carcinogenic smoke. Marijuana smokers show disrupted growth of specific cells in their lung tissue, which could lead to cancer. Nonetheless, marijuana smokers can have many of the same respiratory problems as tobacco smokers, such as daily cough and phlegm production, more frequent acute chest illness, and a heightened risk of lung infections. 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. Finally, the effects on daily living -- research shows that marijuana has the potential to cause problems in daily life or make a person's existing problems worse. In one study, heavy marijuana abusers reported that the drug impaired several important measures of life achievement, including physical and mental health, cognitive abilities, social life, and career status. Other studies associate workers' marijuana smoking with increased absences, tardiness, accidents, workers' compensation claims, and job turnover. Hope this helps. For more information check out NIDA's web pages -- www.drugabuse.gov. Go to the drop down menu and select marijuana.