Long-term marijuana use can lead to addiction; that is, people have difficulty controlling their drug use and cannot stop even though it interferes with many aspects of their lives. It is estimated that 9 percent of people who use marijuana will become dependent on it. The number goes up to about 1 in 6 in those who start using young (in their teens) and to 25-50 percent among daily users. Moreover, a study of over 300 fraternal and identical twin pairs found that the twin who had used marijuana before the age of 17 had elevated rates of other drug use and drug problems later on, compared with their twin who did not use before age 17.

According to the 2008 NSDUH, marijuana accounted for 4.2 million of the estimated 7 million Americans dependent on or abusing illicit drugs. In 2008, approximately 15 percent of people entering drug abuse treatment programs reported marijuana as their primary drug of abuse; 61 percent of persons under 15 reported marijuana as their primary drug of abuse, as did 56 percent of those 15 to 19 years old.

Marijuana addiction is also linked to a withdrawal syndrome similar to that of nicotine withdrawal, which can make it hard to quit. People trying to quit report irritability, sleeping difficulties, craving, and anxiety. They also show increased aggression on psychological tests, peaking approximately 1 week after they last used the drug.