Fact: Heavy use can be harmful. Since pot smoke is chemically very similar to tobacco smoke, heavy pot smokers are at risk for some of the same health effects as cigarette smokers, like bronchitis and other respiratory illnesses. Again, though, the risks are from smoking, which isn't the only way to use marijuana. Another hazard: car accidents caused by driving while high, though the risk is lower than from drunk driving.

Sources: NORML, National Institutes of Health, Harm Reduction Journal, Scientific American