Marijuana users are often portrayed as absent-minded and forgetful. But is there any truth behind this belief?
It’s no secret that marijuana can affect a person’s memory. In fact, studies show that marijuana and memory are directly linked in many ways.
Using marijuana occasionally can impair your short-term memory as well as your ability to form new memories while high. But the effect is temporary, and memory function returns to normal after you sober up.
Long-term marijuana use may result in more lasting changes. However, it’s not clear whether the drug causes any permanent damage to memory.
The impact that marijuana has on memory also depends on genetic factors, THC/CBD levels of different strains, and how much you consume.
Let’s take a deeper look at what science says about marijuana and your memory.
What is Memory?
Memory is our ability to save information and knowledge so we can access and use it later. It’s a way of using past learning and experience to inform current behaviour.
Memory is the reason we can still picture the house we grew up in, and other experiences we had years ago.
In the brain, memory is complicated and involves many neurological mechanisms. In basic terms, when we access a memory, our neurons fire in ways that are similar to the original experience, to recreate the important aspects of that experience.
Types of Memory
Most people know the difference between short-term and long-term memory. However, there are many different ways of subdividing memory. You may have heard of working memory, verbal memory, or procedural memory.
According to the stage model of memory, memory can be roughly divided into 3 types: sensory, short-term, and long-term memory.
Sensory memory temporarily (~3 sec) holds information from your senses (sight, sound, smell, touch, taste) for processing in the brain.
When you don’t hear what someone said the first time, your brain can actually replay the past few seconds of sound from sensory memory. That’s why we often say pardon? and then remember what was said.
Short-term memory (working memory) is a kind of filter on sensory information. It essentially holds everything you’re currently paying attention to or thinking about, within 30 seconds or so.
You can think of working memory like a bottleneck. The human brain is limited in its capacity, so we simply can’t process everything that comes in through our senses. Working memory filters the unnecessary information out, and holds the information needed for current mental operations.
For an example of working memory in action, look no further than the sentence you are currently reading. When reading, your working memory stores the first half of the sentence while you finish reading it so you can grasp the complete thought.
Long-term memory is remembering something in a more permanent way. It includes memories of past events, facts, and procedures (know-how). An example would be remembering someone’s name.
Information in short-term memory can be moved into long-term memory in two main ways: emotion (caring about what’s happening), and attention (repeating it).
Memories fade, and can be forgotten if they are not maintained by rehearsal. For example, you can forget how to sing a song that you once knew all the words to.
Effects of Marijuana on Memory
When you smoke marijuana, THC and other chemicals enter your bloodstream and travel to the brain.
Once in the brain, these chemicals alter memory function by activating the endocannabinoid system. This biological system is intricately connected to parts of the brain associated with memory.
For example, CB1 receptors, which respond to THC, are found in a brain structure known as the hippocampus. The hippocampus is where most of your memories are processed and stored.
The endocannabinoid system is actually thought to function as an aid for extinguishing memories, helping us to forget bad memories and negative experiences.
Using marijuana has an immediate but temporary effect on your short-term and long-term memory.
Users under the influence show impairments in their short-term memory, and deficiencies in their ability to form new, long-term memories.
However, they do not have any trouble recalling existing memories, meaning marijuana cannot make you forget what you already know.
In other words, the impact of marijuana on long-term memory is limited to the formation of new memories. However, these impairments seem to disappear once a person sobers up.
This means frequent marijuana users have trouble holding information in their minds and recalling verbal information (words).
One important question is whether these effects are permanent or if memory function returns to normal over time. Unfortunately, studies aren’t very clear on the answer.
A 2008 review paper, which looked at past research, concluded that the effects of heavy cannabis use on memory may persist even after prolonged abstinence.
Yet a 2005 study found the opposite. Researchers compared long-term marijuana users who were still using heavily to those who were abstinent for 3 months.
They found that former heavy smokers showed no memory impairments, compared to significant impairments in those who were still smoking.
Experts suggest that the conflicting findings could reflect two subgroups: some people may more easily recover from the effects of cannabis on memory after quitting, while others don’t.
Negative Memories and PTSD
Negative memories are memories associated with negative feelings. Their function is to keep you from harm by training you to avoid something that harmed you in the past.
When people experience severe trauma, their negative memory system can become overactive, and they may develop post-traumatic stress disorder (PTSD).
PTSD involves flashback memories that cause symptoms of anxiety including hypervigilance, a feeling of panic, and distress.
Studies have shown that the endocannabinoid system is involved in memory extinction (i.e. forgetting bad memories). Activating the endocannabinoid system by smoking marijuana has been shown to facilitate this process.
Scientists have speculated that THC holds promise as a treatment for PTSD, by helping the sufferer to extinguish their troublesome memories.
Marijuana users can seem out of their element when sober if they’re used to being high. A possible explanation for this is based on the theory of context-dependent memory.
Studies have shown that it’s easier to access memories when your current environment shares features of the environment you were in when you formed the memory.
For example, students who listen to music while studying for an exam do better if they listen to music while writing the exam, too.
So if someone smokes a lot of marijuana, they will have trouble remembering things that happened while stoned if they are asked to remember them while sober, and vice versa. This could explain why stoners sometimes come across as forgetful or out of it.
Other Factors Involved
The link between marijuana and memory is anything but simple. Different strains, dosage, tolerance, and genetics are all thought to play a role in how marijuana affects memory.
Strains of marijuana vary in their levels of different cannabinoids (THC and CBD). Studies have suggested that CBD may protect memory function by reducing the effects of THC.
While still speculative, scientists are hopeful that high CBD strains could help marijuana users avoid the memory problems caused by THC.
The effects of marijuana on memory are also dose-dependent, meaning that memory impairments become more severe with larger doses. This means that people who consume larger amounts of cannabis are more susceptible to memory problems than those who use less.
On the other hand, long-term users may develop a tolerance to THC’s effects. Studies suggest marijuana has a stronger effect on memory in occasional users than it does in heavy users.
Finally, certain genetic factors have been found to predict a person’s response to THC and how much it impairs their memory. So, the impact of marijuana on a user’s memory may be partly due to genes as well.
Problems With Studies
Most studies on long-term marijuana use and memory have been observational, so their results must be interpreted with caution.
People who smoke marijuana may share other traits, such as a predisposition to memory problems. This means it’s hard to say whether it’s the marijuana, or another trait causing the effect.
Marijuana users may also be more prone to using other drugs, like alcohol, ecstasy, or cocaine. This makes it hard to separate the effects of marijuana on memory from the effects of other substances.
Finally, in some studies, marijuana users were given memory tests after being abstinent for only 12-24 hours. As anyone who’s experienced a pot “hangover” will know, there may be lingering effects in this timeframe.
Occasional users show temporary impairments in short-term memory and in long-term memory formation, but this effect goes away after the high wears off.
Frequent users show memory impairments even after they’ve been sober for a day, and sometimes longer.
Some studies suggest these effects could be permanent, but others show that users regain their memory after quitting marijuana.
To sum it up: you may have trouble doing mental math or following a conversation while high, and you may not store all the details of your experience.
But marijuana won’t make you forget anything you already know, and the jury is still out on whether the effects on memory are actually permanent.