How does cannabis affect memory?

Curation Sechat with information from Leafly

It is common for some patients and cannabis users to report memory deficits. However, although forgetting some facts may seem funny in fiction, these lapses may not be as appealing in real life.

As our understanding of cannabis and its derivatives becomes more comprehensive, evidence that the plant affects memory becomes understandable. However, not all news is bad.

While certain forms of memory may be impaired by the use of the plant, in other cases, its medicinal use can help with disorders related to its lack.

How can cannabis affect memory?

Most research suggests that the effects of cannabis on memory are primarily caused by the hippocampus, a region of the brain that has abundant cannabinoid receptors (CB1) and plays an important role in memories and learning. However, the two best-known cannabinoids in the plant, THC (tetrahydrocannabinol) and CBD (cannabidiol), act on the hippocampus in different ways.

THC, the psychoactive substance in cannabis, has a strong affinity for CB1 receptors and binds to these receptors in the hippocampus when consumed. After binding, the compound can temporarily impair short-term memory and learning, making concentration, storage, and information retrieval more difficult.

On the other hand, CBD seems to have a different effect from THC. Research in human participants has shown that cannabidiol can significantly increase blood flow to the hippocampus. Although the implications of this are still unclear, increased hippocampal blood flow at rest is associated with better memory performance.

The study's researchers also speculate that increased cerebral blood flow may help treat individuals with cognitive decline, such as Alzheimer's disease, which is often characterized by disorders in this blood flow in the brain.

Short-term memory

Short-term memory is responsible for receiving and temporarily storing information to which one has just been exposed. Remembering someone's name right after a presentation is an example of short-term memory. It may also include working memory, which manipulates and uses short-term memories. An example would be hearing a sequence of events in a story while understanding what the story means.

If there is one thing almost all researchers agree on, it is that cannabis THC can impair short-term memory.

In the case of working memory, a 2020 study with human participants found that a 15mg dose of THC could impair it. Study participants were given a visual working memory task: they were exposed to an image of six colored blocks and, after a one-second interval, had to remember the location and color of each block. Participants who did not receive cannabis correctly identified the blocks more on average than those who received the substance.

It is also important to note that, although a single 15mg dose of THC impaired working memory, participants who received a 7.5mg dose did not have a noticeable impact.

The experiment also tested whether “mind wandering” and “task monitoring decline” affected the participants' working memory. It was found that THC greatly increases the likelihood of deviating from the task and losing focus.

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

The implications of this are significant because short-term memory, focus, and attention play a critical role in learning. When it is difficult to maintain focus and concentration, the storage of new information can also be compromised.

Long-term memory

Long-term memory refers to the vast repository of knowledge and past events that we remember for days or decades. Short-term memories eventually become long-term memories, which we can access at will. These are the ones we contemplate or remember the most.

While most researchers agree that cannabis use can impair short-term memory, fewer studies have been conducted on the effects of cannabis on long-term memory.

A 2021 review suggests that heavy cannabis consumers suffer from impaired long-term memory. However, another groundbreaking study from the same year explored the effects of cannabis on visual episodic memory – a type of long-term memory – and found that infrequent substance users did not have impaired long-term memory. Cannabis users in the second experiment used derivatives at least once a month for a minimum of two years.

Brain recordings and analyses demonstrated that, although cannabis users processed their memories differently, the overall quality of their memories was good, being able to form long-term memories. We still have a long way to go to actually state whether substance “X” or “Y”, or even medication “A” or “B”, are good or bad for memory.

An example of this is the study from the Federal University of Latin American Integration (UNILA), which, by treating an Alzheimer's patient with a THC-rich cannabis extract, found that the patient showed an increase in cognitive and memory capacity. Therefore, the need for more research on the subject is necessary.