Marijuana is a product of the cannabis plant. According to the Substance Abuse and Mental Health Services Administration (SAMHSA), it remains one of the most commonly used and abused recreational drugs in the United States. Despite numerous states legalizing the use of cannabis products for medicinal purposes and even for recreational use, at the time of this writing, the United States Drug Enforcement Administration (DEA) continues to list all cannabis products in the Schedule I category. This classification is reserved for drugs that are believed to not have any significant medical uses, extremely dangerous to use even under the supervision of a physician, and highly prone to being abused or for developing physical dependence. Despite a body of research that indicates that cannabis products do have medicinal uses, the federal government remain steadfast on this position at the time of this writing.

Staunch proponents of legalization continue to claim that cannabis products are not addictive, and they can treat many different types of incurable conditions or diseases (e.g., cancer). Some proponents of legalization claim that the government is conspiring to withhold cannabis products from individuals who have the right to use them.

man experiencing memory loss from marijuana

Thus, cannabis products do have the potential to be used as legitimate medical treatments; however, like many drugs with significant medicinal uses, they also have a potential for abuse and to produce detrimental effects. Many people feel that making comparisons between cannabis products and substances that are legal to use for adults, such as alcohol and tobacco, does not make a solid case for the legalization of cannabis products, but instead makes a case for stronger control of tobacco and alcohol products. Instead, the strongest case for the legalization of cannabis products should be based on the relative benefits and dangers of their use. While it appears that the current classification of the drug by the DEA is extreme, it also appears that some level of control on the distribution and use of cannabis should be maintained.

A significant body of research has investigated the effects of cannabis use on memory functions. Overall, the research suggests that chronic use of cannabis products may result in significant issues with a person’s ability to learn and memorize new information. These effects appear to be more salient in younger individuals who begin to use cannabis products and in individuals of all ages who use cannabis chronically in moderate to high amounts.

Marijuana’s Effect on Memory Is Intensified in Adolescents


There are numerous psychoactive substances in marijuana that are referred to as cannabinoids. There are neurotransmitters in the brain that are similar in their structure to the cannabinoids in cannabis products. These endogenous neurotransmitters and the cannabinoids in cannabis products attach to receptors in the brain known as cannabinoid receptors. These natural cannabinoid-like neurotransmitters are implicated in numerous areas of cognitive functioning, including learning and memory. The cannabinoids in cannabis products bind to the same receptors in the brain to exert their effects, but they do not contribute to the development of the ability to learn and remember information more effectively.

The major psychoactive substance in cannabis that is studied by researchers regarding its psychoactive effects and other effects is often abbreviated as THC (for tetrahydrocannabinol although the formal term is delta-9-tetrahydrocannabinol). Research studies have indicated that THC exposure can have detrimental effects on learning and memory.

Research studies have also consistently demonstrated that moderate to heavy use of marijuana in adolescence is associated with:

  • Lower levels of education
  • Higher levels of psychiatric diagnoses
  • Lower levels of income
  • Higher rates of suicide attempts

adolescent male experiencing memory loss from marijuana

For example, animal studies have demonstrated that rodents exposed to THC in the womb display differential brain development that results in later deficits in learning and memory. Studies of rodents exposed to THC have also demonstrated that adolescent rats that are exposed to THC demonstrate different alterations in their brain, particularly with the development of white matter. White matter in the brain (myelin) is crucial to effective communication between neurons. Adolescent rats exposed to THC demonstrate decreased development of white matter in the brain that is believed to result in significant issues with performance on learning and memory tasks compared to rats that have not received this exposure.

According to sources such as the book The Everything Guide to the Human Brain, research studies using neuroimaging techniques have confirmed that adolescents who use cannabis products demonstrate similar deficits in the amount of white matter in their brain as adults. They also perform significantly worse on tasks of intellect, memory, and reasoning compared to adolescents not exposed to THC.

There have been some discrepancies in the research regarding the findings in humans. Some studies confirm the relationship between THC use in adolescents and significantly decreased white matter and other decreases in the volume of specific areas of the brain, whereas some studies find no differences between adolescents exposed to THC and those not exposed to the drug. However, larger research studies have indicated that the relationship between brain volume loss and marijuana use in adolescents may be dependent on the age of the person, especially the age when they started using the drug, and their amount and frequency of marijuana use.

The overall findings indicate the following:

  • The persistent use of marijuana starting in adolescence is associated with lower scores on IQ tests later in adulthood (a very mild decline of 6-8 IQ points).
  • Verbal memory scores were significantly lower in adolescents who used marijuana.
  • Other cognitive functions were not significantly affected.
  • People who begin using marijuana as adults did not demonstrate the same pattern.
  • The loss of IQ points and verbal memory functions did not return when individuals who had been smoking marijuana since adolescence stopped using the drug, suggesting that these neuroanatomical changes are permanent.

There are some important criticisms to consider regarding these findings. First, it could be argued that chronic users of marijuana had inherently lower intellectual abilities than nonusers, and this can account for the differences. However, when researchers adjusted for certain demographic factors, these results remained consistent. Secondly, the findings may be related to very chronic marijuana use and abuse in adolescents, and not to the type of marijuana use that occurs in the majority of adolescents who begin using the drug. Finally, these types of studies with humans cannot claim that marijuana use causes the decrease in white matter in the brain or a lower performance on tests of intellect and memory. The types of studies that investigate these relationships when performed on humans are referred to as correlational studies, and one of the limitations of this group of studies is that they are unable to make inferences about cause and effect. Nonetheless, the overall findings do suggest that beginning significant use of marijuana products at an early age is associated with a decline in memory functions and a small decline in performance on intellectual tests.

Neuroimaging research has suggested that there are high amounts of cannabinoids receptors distributed throughout the brain, including in an area of the brain known as the hippocampus. The hippocampus is a crucial area in developing new memories. It is one of the first areas affected by people who are diagnosed with Alzheimer’s disease. The degeneration of the area around the hippocampus results in the severe memory impairment that typically occurs with Alzheimer’s disease. Neuroimaging studies have suggested that use of THC from marijuana can alter the structure of the hippocampus, and this might account for issues with learning and memory in those exposed to THC in the womb and adolescence.

Again, it should be understood that this type of research cannot claim that the use of marijuana causes these later issues, but that there is an association that suggests a strong relationship between the use of marijuana in adolescence and these factors.

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Adults Are Also Affected


Adults are not immune to the effects of heavy cannabis use. Numerous investigations have determined that there is a significant association between heavy marijuana use in adults and the development of psychiatric disorders such as depression as well as poor performance on tests of learning and memory. Individuals with chronic depression often perform lower on tests of learning and memory than individuals who do not suffer from depression. In many cases, when the depression is treated, these individuals show improved memory performance; however, chronic depression is associated with lower overall performance on learning and memory tasks even when it is treated.

In addition, chronic and heavy use of marijuana in adults who did not display a history of chronic use of marijuana as adolescents is also associated with poor scores on learning and memory tests compared to adults who do not use marijuana products. Thus, although the primary effects of THC appear to occur when it is used early, there is research to suggest that chronic use of marijuana later in life can result in memory issues.

There is also research to suggest that THC use may affect the structure and functioning of certain cellular mechanisms in the brain that can result in cognitive problems, such as issues with learning and memory, even in adults. A recent study published in the journal Nature focused on the effects of THC on the cannabinoid receptor in the brain labeled CB1. This receptor is located in the mitochondria of the neurons. Mitochondria are the processing centers of nerves and responsible for converting glucose into energy to allow the cell to function. THC has been found to activate the CB1 receptors within the neurons in the hippocampus and results in the mitochondria in the hippocampus performing their functions inefficiently. The result of this is increased cell death in the hippocampus of rodents. When genetically modified rodents with no CB1 mitochondria receptors in the hippocampus were exposed to THC, they did not display issues with learning and memory. Thus, it is suggested that one potential explanation for poor scores on learning and memory in adolescents who had used marijuana and in heavy adult users may be related to this mechanism.

adult marijuana user experiencing memory loss

In order to provide relevance to the research findings, to acknowledge the argument made by many individuals who are staunchly in support of the legalization of marijuana, and to be fair, it is certainly true that marijuana use by adolescents and adults is obviously comparatively less damaging to them than the use of certain other types of drugs, such as phencyclidine (PCP), cocaine, methamphetamine, inhalants, etc. However, because there are other drugs of abuse that may be associated with significantly more dangerous effects than marijuana, this does not invalidate the research findings that demonstrate marijuana use in adolescence and adulthood may result in some detrimental effects. That comparison is very much like claiming that suffering brain damage as a result of playing football in high school is not as bad as the brain damage that can occur as a result of abusing PCP and therefore we should not be concerned about the effect of repeated concussions in high school athletes.

Brain damage is brain damage, and potential causes of brain damage should be a concern in every case.



Based on empirical evidence, we can conclude the following:

  • Use of marijuana can result in poor performance on tests of learning and memory.
  • The extent of the effects of marijuana use on memory is related to the amount of marijuana a person uses, the age that the person started using marijuana, and most likely other influences, such as heredity.
  • Women who are either pregnant or expect to get pregnant should not use marijuana products at all.
  • Adolescents should not use cannabis products at all unless they are prescribed by a physician.
  • The use of marijuana products in adults over the age of 21 can still result in significant issues with cognition depending on the extent of the person’s use.
  • The effects of THC exposure to the brain appear to be long-lasting and irreversible.

Note: This article is not meant to endorse a position on the legalization of marijuana in either direction. Instead, it is designed to present evidence that the use of marijuana does have some potential detrimental effects, and these effects should be considered by anyone who uses marijuana or is considering using marijuana, either for recreational or medicinal purposes. More extensive knowledge regarding the use of recreational drugs or even the use of drugs for medicinal purposes can allow individuals to make informed decisions about their use.

In addition, abuse of numerous substances, including medications with significant medical benefits, can lead to the development of a substance use disorder. Individuals who believe that they may suffer from a cannabis use disorder should discuss the situation with a qualified mental health professional.

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