Some people think that marijuana -- by far the most popular illegal drug -- is harmless. Unfortunately, that's simply not true. While it may be less immediately dangerous or addictive than other drugs, it can still take a toll on your health, making your RealAge (physiologic age) older. Marijuana contains 50 percent more carcinogens and four times as much tar as cigarettes. Studies show that the heavy use of marijuana can cause residual neurologic effects that decrease cognitive (the learning, thought process part of mental) functioning. Heavy users actually experience aging less from the drug itself than from the behaviors it tends to induce, most notably a lack of motivation. For example, users are less likely to exercise, eat a healthy diet, or maintain the kinds of social networks that help protect against stress. They are also more likely to engage in risky behaviors, such as unsafe sex or driving under the influence of drugs or alcohol.