Marijuana and Memory Loss: What Does the Research Say?

Popular culture has created an enduring trope of the ‘forgetful stoner’ character, one who is mellowed out to the point of confusion and forgetfulness. But are these tropes actually supported by science?

Cannabis is complex. There are hundreds of different biologically active compounds present in varying amounts in each different cannabis strain, and the drug’s effects can be highly dose-dependent.

Likewise, memory is a complicated concept, and not one that can be so easily measured. A person’s working memory, short-term memory, and long-term memory can all be altered independently by any number of external factors.

This complexity can make it particularly challenging to study the effects that cannabis consumption can have on a person’s memory. Still, the studies that have been done do appear to indicate that cannabis use can have a genuine impact on a person’s ability to form memories and recall information.

Marijuana's short-term effects

The acute effects of cannabis on memory function have generally been studied by having study participants consume a dose of the drug and complete some form of task designed to test a different aspect of their memory recall.

Delayed matching to sample tasks, for example, present participants with a complex visual pattern and then, after a short delay, require them to identify which of several similar patterns is an exact match. These tests can be used effectively to examine the effects of cannabis on short-term visual memory. From these and other similar experiments, scientists have come to the overall conclusion that acute consumption tetrahydrocannabinol (THC) does negatively disrupt the working and episodic memories of humans.

Acute cannabis use has also been linked to an increased susceptibility to forming false memories while under the influence. One study, published in the Proceedings of the National Academy of Sciences journal last month, found that intoxicated cannabis users were more susceptible to false-memory creation, which was assessed by using a virtual-reality eyewitness and perpetrator scenario. Intoxicated participants in this study also had higher rates of false-recognition in an associative word-list task both immediately, and when tested again on the information one week later.

While acute cannabis use does impair memory function, for the most part these impairments appear to be temporary. A 2002 clinical trial observed that THC did exhibit detrimental effects on episodic memory and learning during intoxication, but that by 24 to 48 hours post-consumption, this impairment had completely dissipated. Additionally, acute cannabis use doesn’t appear to affect the ability to recall existing memories.

Important factors to consider

There are several other important factors which can affect exactly how cannabis use might impact certain types of memory.

Scientists studying the relationship between genetics and cannabis use have found that a polymorphism in the catechol-O-methyltransferase (COMT) gene has the ability to moderate the effects of THC on verbal memory. This means that genetic variation could play a part in cannabis-related memory studies and could explain some amount of variation and inconsistency between different studies.

But an individual’s history of cannabis consumption may be a more important factor. One 2009 randomized controlled trial reported that when heavy cannabis users and occasional recreational users were given the same single dose of 500 mg/kg THC, the THC significantly impacted the performance of the occasional cannabis users in a number of important neurocognitive tasks. Conversely, the heavy cannabis users were almost completely unaffected by the same cannabis dose, only exhibiting slightly slower reaction times in one motor control task.

The notion that regular users may be able to build up some tolerance to the memory impairments caused by cannabis use could have important implications when considering the effects of long-term regular cannabis use.

Frequent cannabis use and memory recall

While long-term and heavy cannabis users may build up a tolerance to some of cannabis’ effects on memory function, there are still some concerns that come with long-term usage.

Higher doses of cannabis taken frequently, as is common in heavy long-term cannabis use, has been shown to have a negative effect on a person’s short-term verbal memory. One study on cannabis consumption habits, which followed nearly 3,400 Americans over a 25-year period, found that those who had smoked cannabis heavily for five years or more performed worse on tasks that involved verbal recall, even after controlling for other factors that are known to affect cognition, such as age, depression, and other substance use.

On the verbal recall tests, users who reported smoking cannabis every day were able to recall on overage 8.5 of the 15 words used in the test. By comparison, infrequent users and those reporting no cannabis use could recall nine words. While this difference may not appear major, the study authors suggested that this gulf in verbal recall could continue to grow the longer that cannabis was consumed chronically. As only eight percent of the study participants were classed as frequent users, it wasn’t possible to confirm this in this one study.

Notably, the frequent cannabis users in this study didn’t appear to be significantly impacted by their cannabis use on other measures of their memory and cognition, such as focus and processing speed.

CBD and memory loss

While it’s generally apparent that THC can impair a person’s memory, some studies suggest that cannabidiol (CBD), a major non-intoxicant in cannabis, may actually be effective in preventing cognitive decline and impairment.

A 2010 study comparing the effects of different cannabis strains and memory found individuals smoking strains low in CBD were impacted in their ability to recall prose. However, those who smoked cannabis strains with a high CBD content didn’t appear to show any memory impairments over the course of the study. The researchers concluded that CBD then may be able to attenuate the memory-impairing effects of THC, possibly through its antagonistic effects at the body’s CB1 cannabinoid receptor.

Additionally, CBD has shown potential as a neuroprotector, able to protect the brain against the memory and cognition problems developed by conditions such as alcohol-induced brain damage, Alzheimer’s, and dementia.

While THC and CBD have been investigated for their effects on memory and cognition, there is still much in the area to be understood, particularly regarding the effects of long-term cannabis use, and the cannabinoid profile of the cannabis being used.

As cannabis laws are reformed, it’s important that these effects of cannabis on memory and cognition are better understood, so that consumers and physicians are able to better assess the risks and benefits that the drug may hold.