Marijuana-Induced Memory Loss: What the Research Shows

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.

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.

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 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.

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.

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.