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.
Most of these deficits usually resolve with prolonged abstinence. However, in those who begin and continue to use marijuana regularly during their teenage years, memory and learning problems associated with marijuana use with may persist for years and may not be reversible even after stopping use. A recent longitudinal study showed that individuals who reported regular or heavy marijuana use as teenagers lost as much as eight points in IQ between ages 13 and age 38, even in those who stopped smoking marijuana as adults. The decline in IQ was not seen in individuals who started smoking marijuana as adults. Marijuana is thought to be "neurotoxic" to adolescent brain development because THC binds to cannabis (CB1) receptors and artificially disrupts the critical role that these receptors play in regulating the development of the pre-frontal cortex that occurs throughout adolescence into young adulthood. Other studies have shown that adolescent-onset marijuana use increases the risk of psychosis and may also increase the risk of becoming addicted to other drugs.
There is also evidence that prenatal marijuana exposure interferes with fetal brain development and is associated with lower birth weight. Approximately one third of the THC smoked by the mother during pregnancy crosses the placenta and affects the development of the endocannabinoid system, which is an important regulator of fetal brain development and the immune system. Prenatal marijuana exposure has been linked to inattention, hyperactivity, memory and learning problems in young children that persist at least into early adolescence. A longitudinal study reported that at age 14, the prenatally exposed youth continued to have learning and behavior problems and significantly lower academic achievement compared to those with no exposure or minimal prenatal exposure.
Taken together, current research on the health risks associated with marijuana use warrant heightened public awareness, concern and action if we are to reduce the public health impact of current national trends toward further increasing access, availability, and use of marijuana, nationwide. Especially urgent is the need for effective public education, legislation and policies that limit or prevent prenatal exposure and reduce youth access to marijuana.
Find out more about current research and get accurate, up-to-date scientific information about marijuana and other drugs on the NIH/NIDA's website.