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How does cannabis affect memory?

June 14, 2016

The stereotype of the scatterbrained stoner is one of the most common tropes associated with cannabis. While a forgetful pothead might make for a fun film character, impaired memory might not be so appealing in real life.

As our understanding of cannabis becomes more nuanced, there’s increasing evidence that cannabis does affect memory, however, it’s not all bad news. While certain forms of memory can be hindered by cannabis use, in other cases, cannabis may help with memory-related disorders. Let’s dive in.


What is memory?

Memory is a critical cognitive function that helps us make decisions, interact with others in meaningful ways, maintain a sense of self, and recall what we learn. 

The formation of memories is a dynamic process. The brain is constantly shimmering with activity, as different groups of neurons, responsible for creating thoughts and perceptions, drift in and out of action. 

When a specific group of neurons is activated for a second time, a memory forms. This reactivation strengthens the connections, known as synapses, between the neurons. When we recall a memory, we reactivate that specific group of neurons again. 

However, new memories are very fragile, and can become disrupted or corrupted by interferences such as intoxicating substances or a lack of sleep. Over time, however, the memory becomes stronger and more resilient, a process known as memory consolidation. As new memories accumulate, those that are not reactivated and retrieved eventually become lost. 

Memory can be categorized in different ways, but scientists generally agree that there are two main types of memory: short-term memory and long-term memory. Different parts of the brain are responsible for processing and storing each type. 

The hippocampus is mainly responsible for short-term memory and learning, while the prefrontal cortex plays a critical role in long-term memory. Nonetheless, there is a constant dance between these two regions of the brain, and they work in tandem when a memory is recalled. 

How can cannabis affect memory?

Most research suggests that the effects of cannabis on memory are primarily elicited through the hippocampus, a region of the brain that has abundant cannabinoid (CB1) receptors and plays a major role in memory and learning. However, the two main cannabinoids in cannabis, THC and CBD, can affect the hippocampus differently.

THC, the primary psychoactive cannabinoid in cannabis, has a strong affinity for CB1 receptors, and it binds to those receptors in the hippocampus when consumed. Upon binding, THC can then temporarily impair short-term memory and learning by making it difficult to concentrate, store, and recall information. 

On the other hand, CBD appears to have a different effect than THC. Recent research on human participants has shown that CBD can significantly increase blood flow to the hippocampus. While the implications of this are still unclear, higher resting hippocampal blood flow is associated with better memory performance. 

The researchers of the above study also speculate that increased cerebral blood flow may help treat individuals with cognitive decline, such as Alzheimer’s disease, in which there are defects in blood flow control.

Cannabis and short-term memory

Short-term memory is responsible for taking in and temporarily storing information that we have just been exposed to. Remembering someone’s name directly after an introduction is an example of short-term memory. Short-term memory can also include working memory, which manipulates and uses short-term memories. An example of working memory would be listening to a sequence of events in a story while making sense of what the story means. 

If there’s one thing nearly all researchers agree on, it’s that THC in cannabis can impair short-term memory.

With working memory, a 2020 study with human participants found that a 15 mg dose of THC could hinder it. Participants in the study were given a visual working memory task: They were exposed to an image of six colored blocks, and after a one-second break had to recall the location and color of each of the blocks. Participants who didn’t receive cannabis correctly identified the blocks more on average than those who did receive cannabis. 

It’s also worth noting that while a single 15 mg dose of THC impaired working memory, participants who were given a 7.5 mg dose did not experience a noticeable impact. 

The experiment also tested whether “mind-wandering” and “decreased monitoring of the task” affected the participants’ working memory. THC was found to strongly increase the likelihood of wandering off-task and zoning out. 

While it’s clear that short-term memory and working memory can be affected by THC, the authors of the study emphasize that THC affects the overall conscious experience. In other words, THC may impair our short-term memory because it also affects our ability to concentrate or be attentive. 

The implications of this are significant since short-term memory, focus, and attention play a critical role in learning. When it’s hard to sustain focus and concentration, the short-term storage of new information may also be compromised.

Cannabis and long-term memory

Long-term memory refers to the vast repository of knowledge and prior events that we remember for days or decades. Short-term memories ultimately become long-term memories, which we can then access at will. Our clearest long-term memories are those we contemplate or remember the most. 

While most researchers agree that cannabis use can impede short-term memory, less research has been conducted into the effects of cannabis on long-term memory.

A 2021 review suggests that heavy cannabis consumers experience long-term memory impairment. However, another innovative 2021 study exploring the effects of cannabis on visual episodic memory—a type of long-term memory—found that infrequent cannabis users did not show impaired long-term memory. The cannabis users in the second experiment had used cannabis at least once a month over a minimum two-year period. 

EEG brain recordings and analysis demonstrated that while the cannabis users processed their memories differently, the overall quality of their memories was good. They were also able to form long-term memories well.

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Did that really happen? Cannabis and false memories

Memory can be notoriously unreliable. The distortion or manipulation of memory can happen unconsciously. In some cases, it’s possible to create a recollection of an event that never took place, which is called a false memory. Several cannabis researchers have recently turned their attention to how cannabis increases susceptibility to false memory. 

In a 2020 experiment, 64 participants received an intoxicating dose of THC and were asked to complete several different recall activities. The researchers found that the cannabis users were more likely to have spontaneous false memories. For example, in one activity where participants were asked to remember words in a list, they recalled words that weren’t there. Interestingly, in the follow-up test a week later when participants were no longer intoxicated, some still experienced false memories in word recall tests. 

The participants were also more likely to experience false memories in response to misleading suggestions. In another activity, they witnessed or were the perpetrator of a computer-generated virtual reality crime. In an interview immediately after the event, participants were more likely to answer affirmatively to deliberately misleading questions. The researchers discovered that false memories were significantly more likely to occur while the participant was high. False memories disappeared, however, when participants were tested when sober a week later.

Some researchers have speculated that cannabis users may experience false memories because THC interferes with visual information processing. In turn, this can impair users’ visual memory of objects and increase the likelihood of false recognition. 

However, research into cannabis and false memory is still in its early stages. The 2021 study mentioned above was characterized by a small sample size, so it’s possible that larger, more robust studies in future may yield other findings.

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Can cannabis offer any benefits for memory?

Happily, there’s also a growing amount of data that emphasize the potential benefits of cannabinoids for memory disorders.

One of the more established beneficial effects is CBD’s ability to help fade fearful memories. CBD can soften fearful memories by reducing the anxiety associated with them. This is particularly helpful for individuals living with PTSD, whose harmful memories can be triggered by stressful situations. Researchers have also found that CBD may help to alter aversive memories in humans and prevent these memories from being reconsolidated. These benefits can significantly contribute to improvements in the symptoms associated with PTSD.

There’s also strong evidence that CBD can positively affect episodic memories. In a human study last year, participants received a single 12.5 mg dose of CBD e-liquid after learning 15 nouns. The participants were delayed for 20 minutes then asked to recall the nouns they had learned. CBD boosted their verbal episodic memory by 10% compared to those who had received a placebo. The study also found that CBD didn’t appear to negatively impact their attention or working memory.

Additionally, according to preliminary research, THC is considerably more effective than the approved drugs for Alzheimer’s disease in mice, a disease characterized by worsening memory loss. What’s more, CBD also appears to help with hippocampal neurogenesis. Several clinical trials exploring THC’s effects on Alzheimer’s patients are already underway. 

Emma Stone's Bio Image
Emma Stone

Emma Stone is a journalist based in New Zealand specializing in cannabis, health, and well-being. She has a Ph.D. in sociology and has worked as a researcher and lecturer, but loves being a writer most of all. She would happily spend her days writing, reading, wandering outdoors, eating and swimming.

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