Cannabis can produce some very different effects. One person feels relaxed and giggly, another gets hungry, loses track of time or finds ordinary sounds suddenly fascinating, and someone else becomes anxious, confused or paranoid. I wanted to know what is actually going on inside the brain to produce such a wide range of reactions.
The main answer is THC, the psychoactive compound in cannabis. It works by interfering with a signalling network that already exists in your body, the endocannabinoid system, and changing the way some of your brain cells talk to each other. That can affect mood, memory, appetite, coordination and the brain’s reward system all at once.
What is in cannabis?
Cannabis contains hundreds of chemical compounds, including a large group known as cannabinoids. The two best known are THC (delta-9-tetrahydrocannabinol), the main compound responsible for the high, and CBD (cannabidiol), which does not produce an intoxicating high. The plant also contains terpenes, which give different varieties their smell and may influence their effects.
Different products can contain very different amounts of THC. The National Drug and Alcohol Research Centre (NDARC) notes that THC potency varies widely between cannabis products, from dried flower to concentrates, which is one reason the effects can differ so much between products and between people.
Your body already makes its own cannabinoids
This is the part I find most fascinating. Your brain and body already produce chemicals that act on the same receptors THC targets. They are called endocannabinoids, and the two best known are anandamide and 2-AG. Together with the cannabinoid receptors and the enzymes that make and break them down, they form the endocannabinoid system. As the American Heart Association explains, these chemicals work through two receptors, CB1 and CB2.
The system helps regulate mood, memory, appetite, sleep and immune function. It is one of the many ways your body keeps its processes finely balanced, something I wrote about in my post on homeostasis. And it is exactly the system that THC hijacks.
How THC gets into your brain
When cannabis is smoked or vaporised, THC passes from the lungs straight into the bloodstream, so according to NDARC the effect is almost immediate. When it is eaten, THC has to be digested and pass through the liver before it reaches the bloodstream. NDARC says edibles can take between one and three hours to take effect, and because absorption from the gut is so variable, the effects are often unpredictable.
The liver also changes THC along the way, converting some of it into 11-hydroxy-THC, which is itself psychoactive. That is part of why an edible can feel different from inhaled cannabis and last considerably longer. The long delay is also a common trap: someone thinks it has not worked, takes more, and then experiences much stronger effects than they expected.
THC hijacks the brain’s cannabinoid receptors
Once THC reaches the brain, it activates CB1 receptors, which are especially important in the central nervous system. Normally, your own endocannabinoids switch these receptors on briefly and locally, when a brain cell needs to turn down the strength of an incoming signal. THC is different. It activates CB1 receptors throughout the brain, for far longer, and changes how much of various neurotransmitters nerve cells release. Rather than simply turning the endocannabinoid system on, it scrambles the normal signalling, and that disruption produces many of the effects of being high.
So where does the euphoria come from?
The pleasurable feeling involves dopamine, a neurotransmitter linked to reward, motivation and learning, but THC does not simply dump dopamine into the brain. CB1 receptors sit on neurons that normally keep dopamine-producing cells in check. When THC activates those receptors, it eases that brake, and research on the brain’s reward pathways, including this study in mice, points to the result being more dopamine signalling in the brain’s reward areas. That contributes to feelings of pleasure and euphoria.
It is only part of the story, though. The same broad disruption of normal signalling can produce effects people do not enjoy at all. NDARC lists mild euphoria and relaxation among the short-term effects, but also panic reactions, confusion and paranoia, which are mainly reported by people new to cannabis.
Why does cannabis affect memory?
Changes to short-term memory are one of the most noticeable effects, and they make sense once you know where CB1 receptors are. They are most densely packed in the hippocampus, the region that forms new memories, as well as the cerebellum and striatum. When THC activates them, it interferes with communication between the neurons involved in forming and retrieving memories. That does not wipe anyone’s memory, but it makes it harder to lay down new memories and to keep attention on what is happening, which is why someone who is high can start a sentence, get distracted and completely forget what they were saying.
Why does time seem to slow down?
A few minutes can feel far longer than they really are. The exact reason is not fully understood, but THC affects several brain systems involved in perception, attention, memory and processing sensory information, and together they distort how quickly time seems to pass. NDARC lists perceptual alterations, including time distortion and the intensification of ordinary experiences, among the short-term effects, which is why music can suddenly seem more interesting and a familiar room can feel strangely different.
Why does cannabis make you hungry?
The “munchies” are not just a stereotype. The endocannabinoid system helps regulate appetite, and THC can influence the brain pathways involved in hunger and food reward, so food becomes more appealing and its smell and taste can seem more intense. This effect has also helped researchers understand how cannabinoid signalling controls appetite in the first place.
Why does cannabis affect coordination?
THC changes signalling in the brain regions that coordinate movement and process sensory information, and NDARC lists dizziness with impaired balance and coordination among its short-term effects. That matters most on the road. Feeling capable of driving is not the same as having normal reaction times, driving after using cannabis can be dangerous, and in Australia it is illegal to drive with THC in your system, with separate rules in some states for people prescribed medicinal cannabis.
Why does cannabis affect people differently?
Give the same product to two people and they may have completely different experiences. The amount and potency of THC, whether it was inhaled or eaten, previous experience and tolerance, mood and expectations, and whether alcohol or other drugs were also used all play a part. NDARC specifically names mood, body weight, previous experience and the form of cannabis as influences. People who use cannabis regularly can also develop tolerance, and the Alcohol and Drug Foundation (ADF) notes that regular THC use can lead to both tolerance and dependence. Needing more to get the same effect does not automatically mean someone is addicted, but it can be a sign their relationship with the drug is changing.
What happens to THC after the high wears off?
The high fades, but the THC does not vanish. The body breaks it down into a number of metabolites, mainly in the liver, some inactive and some psychoactive. THC and its metabolites are also highly fat-soluble, so they can be stored in fatty tissue and released slowly back into the bloodstream. That is why cannabis can be detected in the body far longer than anyone actually feels high, and why a positive drug test does not necessarily mean a person is still intoxicated.
What does CBD do?
CBD is the other major cannabinoid in cannabis, and it behaves quite differently from THC. It does not produce an intoxicating high. The ADF notes that it is being researched for its potential to reduce anxiety, psychosis, nausea and seizures, although it is still not fully understood whether it is safe and useful for treating mental health conditions. The balance of CBD and THC in a product can make a real difference to its effects, and the ADF’s cannabis fact sheet suggests products with a balance of CBD may reduce the risk of negative effects compared with THC-only products.
Can cannabis be addictive?
Yes. Cannabis is sometimes described as a drug that cannot cause dependence, but that is not what the evidence shows. NDARC notes that cannabis dependence is now formally recognised in psychiatry’s diagnostic manual, the DSM-5. When someone who is dependent stops, they can experience withdrawal symptoms including insomnia, irritability, anxiety, low mood and reduced appetite, which typically peak in the first two to three days and can continue for weeks. The risk is not the same for everyone, and occasional use is very different from frequent or heavy use, but coming from a plant does not make cannabis risk-free.
What about psychosis and mental health?
This is one of the most important health issues with cannabis. THC can cause short-term paranoia and anxiety and, particularly at higher doses, hallucinations or other psychotic symptoms. Regular use is also associated with a higher risk of psychosis and schizophrenia-spectrum disorders. NDARC says people with an existing vulnerability to psychosis, including a family history, should be discouraged from using cannabis, and the ADF states that THC use can increase the risk of schizophrenia, bipolar disorder and psychosis.
Young people are a particular concern, because the brain keeps developing into the mid-twenties. The ADF’s guidance on drug-induced psychosis recommends delaying cannabis use until 25 to reduce that risk. That does not mean every young person who uses cannabis will develop a mental illness. As mental health charity SANE points out, genetics, early development and life experiences have a much bigger influence on someone’s overall chances, but cannabis use adds to the risk, particularly for people who are already vulnerable.
What about cannabis in Australia?
Cannabis is the most commonly used illicit drug in Australia. NDARC reports that 11.6% of Australians aged 18 and over said they had used it in the previous 12 months in 2022-23. The legal position can be confusing, because there is no single national law for recreational use, and in most of the country possession and use remain illegal.
The ACT is the exception. Since 2020, adults there can possess up to 50 grams of dried or 150 grams of freshly harvested cannabis and grow up to two plants each, with a maximum of four per household. But as the Alcohol and Drug Foundation explains, that is not the same as “legal weed”: selling or supplying cannabis is still illegal, using it in public is an offence, and cannabis remains illegal under Commonwealth law. The ACT Government also stresses that it is still illegal to drive with any level of illegal substance in your system.
Medicinal cannabis is a separate issue. NDARC notes that prescription cannabis products are approved through the Therapeutic Goods Administration and are prescribed case by case, typically as oral solutions or capsules with carefully controlled amounts of THC and CBD.
So how does cannabis get you high?
In short, THC changes the way your brain’s endocannabinoid system communicates. It reaches the brain and activates CB1 receptors that normally respond to your body’s own chemicals, but it does so more broadly and for much longer. That changes the release of neurotransmitters across several brain circuits, which can produce euphoria and relaxation, altered perception, a distorted sense of time, hunger, patchy short-term memory, slower coordination and, in some people, anxiety or paranoia. How strongly it hits depends on the dose, how it is taken, the person’s experience and tolerance, and a range of individual factors. Even after the high fades, THC is broken down into other compounds and can remain stored in fatty tissue long after the obvious effects have gone.
Cannabis is a fascinating example of a plant compound slotting into a system that already exists inside the human brain. It is also a good reminder that “natural” does not automatically mean harmless.
This article is for general information and is not medical or legal advice. If cannabis use is causing problems for you or someone you know, the National Alcohol and Other Drug Hotline is available on 1800 250 015. In an emergency, call 000.
Sources
- NDARC (UNSW): Cannabis fact sheet
- Alcohol and Drug Foundation: Cannabis (THC)
- Alcohol and Drug Foundation: Medical cannabis and mental illness
- Alcohol and Drug Foundation: What is drug-induced psychosis?
- Alcohol and Drug Foundation: Cannabis in the ACT, what you need to know
- ACT Government: Drug law reform
- SANE: Cannabis and psychosis
- American Heart Association: Use of marijuana, effect on brain health
- Study: CB1 receptor activation on glutamatergic neurons and THC-induced aversive effects in mice
- ClinicalTrials.gov: Cannabis effects on electroencephalography (CB1 receptor distribution)