Recent findings from an American research group point toward lasting changes in memory function in teenagers who heavily consume marijuana for no more than three years. Current research indicates that teenagers who use marijuana, a drug largely viewed as safe or relatively harmless, can develop a range of serious problems, especially when they qualify as heavy or habitual consumers. In a study published in March 2015 in the journal Hippocampus, researchers from three U.S. institutions explored the consequences of heavy marijuana use during adolescence for long-term function in a brain area called the hippocampus, which plays a central role in normal memory processing. These researchers concluded that teenagers affected by diagnosable cannabis abuse\/addiction (i.e., cannabis use disorder) undergo memory-related structural and functional changes that persist into adulthood, unless they seek timely treatment for cannabis addiction. Teenagers, Marijuana and Cannabis Use Disorder Current figures from the National Institute on Drug Abuse and the University of Michigan indicate that marijuana ranks second only to alcohol as a recreational substance of choice among America\u2019s teenagers. Consumption of the drug actually fell slightly in 2014, although current lax attitudes toward the dangers of marijuana\/cannabis use point toward future increases in intake levels. Compared to the average adult, the average teenager has almost double the chance of developing cannabis use disorder (a modern diagnosis that incorporates the concept of marijuana\/cannabis addiction with the concept of damaging, non-addictive marijuana\/cannabis abuse). Risks for the disorder are drastically higher in teenagers (and adults) who qualify as heavy or habitual consumers by getting high every day of the week or nearly every day of the week. Marijuana is well known for its ability to produce short-term memory problems in users. The potential for harm is particularly high in teenagers, since they have yet to complete the growth and development process that gives the brain its full level of mature functionality. In addition to memory-related problems with normal growth and development, teen consumption of the drug can lead to problems with the ability to comprehend new information, think rationally or use the brain\u2019s higher-level mental functions in a coordinated and coherent manner. Memory and the Hippocampus The hippocampus is a seahorse-shaped structured buried deep within the brain. Humans rely on this structure to help form, store and recall both short-term memories and long-term memories. In addition, the hippocampus plays a critical role by passing information to a brain structure called the amygdala, which has responsibilities that include the regulation of strong emotional states. Problems with normal hippocampus function can lead to a range of serious memory deficits, including amnesia and Alzheimer\u2019s disease. In addition, problems with this brain area can lead to disorientation caused by a reduced ability to interpret spatial changes in one\u2019s surrounding environment. Impact of Teen Marijuana Use In the study published in Hippocampus, researchers from Northwestern University\u2019s Feinberg School of Medicine, the Warren Wright Adolescent Center and Harvard-affiliated Massachusetts General Hospital compared hippocampus shape and function in four groups of young adults. These groups were people who used marijuana for three years as teenagers and met the criteria for cannabis use disorder during adolescence, people with a current diagnosis of schizophrenia who qualified for a cannabis use disorder diagnosis during adolescence, people with a current schizophrenia diagnosis but no history of substance problems and generally healthy people with no history of substance problems or separate mental health problems. After reviewing maps of the hippocampus from the members of each group, the researchers concluded that the young adults with a teen history of cannabis use disorder showed clear signs of structural change in this brain area. This fact held true for the group affected by schizophrenia, as well as for the group unaffected by schizophrenia. In addition, the researchers concluded that the non-schizophrenic young adults with a teen history of cannabis use disorder had an average decline in long-term memory function (i.e., episodic memory function) of roughly 18 percent. In contrast, the schizophrenic young adults with a teen history of cannabis use disorder experienced longer-lasting diagnosable cannabis problems, as well as lower levels of success when trying to halt marijuana\/cannabis intake. The study\u2019s authors believe their findings may identify heavy marijuana intake in adolescence as a predictor of lingering memory problems in later life. However, they note that their findings may also merely call attention to marijuana users who have a pre-existing tendency toward memory problems. Future research will be needed to definitively resolve this issue.