Marijuana is the most commonly used illegal drug in the U.S. In the past five years, rising rates of adult and adolescent marijuana use in the U.S. has been linked to the broad perception that marijuana is a relatively low-risk "recreational" drug and does not lead to addiction. A recent 2013 national Gallup poll reported that 58% of Americans currently support legalization of marijuana. Colorado and Washington have recently passed such legislation, legalizing the sale of "recreational" marijuana and a number of other states are currently considering similar legislation. In light of this growing national momentum, it is important for the public to have access to accurate scientific information and be informed of current research findings regarding the health risks and benefits of marijuana and the public health implications of increasing access and use of marijuana.
Research shows that individuals who occasionally use marijuana may suffer fewer negative consequences and have a lower risk of becoming addicted in comparison to those who use regularly. Regular use over long periods can cause changes in the brain and behavior similar to other addictive drugs and can lead to addiction. Tetrahydrocannabinol (THC) in smoked marijuana enters the bloodstream and arrives at the brain almost immediately, causing the user to feel euphoric or "high" by activating the brain's reward or pleasure center to release dopamine in the same way that many other drugs do. Research shows that approximately 1 in 11 (9%) people who use marijuana will become dependent on it. The risk increases to 1 in 6 in those who start smoking marijuana as teenagers. Individuals who become addicted to marijuana have difficulty controlling their use, even when marijuana begins to interfere with and cause problems in their lives. Chronic, heavier marijuana users often experience symptoms of cannabis withdrawal such as increased irritability, craving, anxiety and sleep problems that can make it difficult to quit. Withdrawal and continued use despite negative personal, social, legal or medical consequences are hallmarks of addiction and dependence.
The acute effects or "high" from THC generally last 1 to 3 hours and cause impaired attention and decision-making and slowed reaction time, often accompanied by poorer coordination and balance. Studies have shown that marijuana use interferes with learning, information processing, and a person's ability to form new memories. Because of marijuana's negative effects on attention, memory and learning, individuals who smoke marijuana daily may function at a reduced intellectual level some or all of the time. Chronic, heavy, marijuana users also report problems with memory and learning that last much longer than the acute "high," and which may persist for several weeks after cessation.