Our brains are wired to ensure that we will repeat life-sustaining activities by associating those activities with pleasure or reward. Whenever this reward circuit is activated, the brain notes that something important is happening that needs to be remembered, and teaches us to do it again and again, without thinking about it. Because marijuana stimulates the same circuit, people learn to abuse it in the same way.
When some drugs of abuse are taken, they can release 2 to 10 times the amount of dopamine that natural rewards do. The effect of such a powerful reward strongly motivates people to take drugs again and again, leading to dependence.
Other criteria for substance dependence include tolerance (needing more of the substance to achieve the same effects, or diminished effect with the same amount of the substance); withdrawal symptoms; using a drug even in the presence of adverse effects; and giving up social, occupational, or recreational activities because of substance use.
According to the 2002 National Survey on Drug Use and Health, 4.3 million Americans were classified with dependence on or abuse of marijuana. That figure represents 1.8 percent of the total U.S. population and 60.3 percent of those classified as individuals who abuse or are dependent on illicit drugs
