"Some studies show that when people have smoked large amounts of marijuana for years, the drug takes its toll on mental functions. Heavy or daily use of marijuana affects the parts of the brain that control memory, attention, and learning. A working short-term memory is needed to learn and perform tasks that call for more than one or two steps.
Smoking marijuana causes some changes in the brain that are like those caused by cocaine, heroin, and alcohol. Some researchers believe that these changes may put a person more at risk of becoming addicted to other drugs, such as cocaine or heroin. Scientists are still learning about the many ways that marijuana could affect the brain."
"While not everyone who uses marijuana becomes addicted, when a user begins to seek out and take the drug compulsively, that person is said to be dependent or addicted to the drug. In 1995, 165,000 people entering drug treatment programs reported marijuana as their primary drug of abuse, showing they need help to stop using the drug
According to one study, marijuana use by teenagers who have prior serious antisocial problems can quicky lead to dependence on the drug.
Some frequent, heavy users of marijuana develop a tolerance for it. "Tolerance" means that the user needs larger doses of the drug to get the same desired results that he or she used to get from smaller amounts."
http://www.nida.nih.gov/MarijBroch/Marijteens.html
