Science of THC: How Marijuana Affects the Brain & Body

Science of THC

THC is not a bad cannabinoid

It’s a myth outside of the cannabis industry that CBD is medicine and THC is not. THC has some incredible medicinal properties, from helping with sleep to killing cancer. 

Humans have a complex network of messengers and receptors, called the endocannabinoid system. These messengers and their receptors interact with cannabis in dozens of different ways to produce several medicinal benefits. Let’s take a deep dive into exactly how it works in the body to create different effects, and let’s take a look at what the research is saying about long-term use.

What is THC?

THC stands for delta-9 tetrahydrocannabinol, also known as delta-9 THC, delta-9, or d-9. It’s just a molecule made of carbon, hydrogen, and oxygen that looks like this:

THC Molecule, made of carbon, hydrogen, and oxygen.

THC is also referred to as a cannabinoid, just like CBD, CBG, or CBN. There are over 100 cannabinoids in the plant, and we know the most about THC and CBD at this time. Unlike CBD, THC causes euphoria or the feeling of being “high.” In addition to causing euphoria, THC has dozens of medicinal properties throughout the body.

Where is THC Found?

THC is the most abundant cannabinoid in the cannabis plant, and it’s stored in the Trichomes, or the little clear mushroom-looking part on the cannabis flower.

cannabis trichome

THC is housed in the trichome of the cannabis plant as THCa. THCa is later converted to THC when the harvested plant is exposed to air or heat. 

THC can also be fully synthetic, as in dronabinol (Marinol), a prescription medication. The THC is completely synthetic in this medication, meaning it’s made in a lab.

Benefits of Taking Marijuana

The primary psychoactive component of cannabis is THC. In addition to euphoria, it has many medicinal benefits.

THC acts on various receptors throughout the body. When it binds to the following receptors, it can have the following effects (to name just a few):

  • CB1: Affects cognition, mood, emotion, appetite, movement control, and memory.
  • CB2: Balances the immune system, decreases pain sensation, and stimulates bone growth.
  • GPR55: Involved with inflammatory and neuropathic pain, a potential target for cancer therapy.
  • TRPV: Involved with inflammation, neuropathic pain, cancer, and respiratory disorders.
  • PPAR: Neuroprotection, cardioprotection, possible anti-cancer effects. 
  • Opioid Receptors: Partially binds to opioid receptors, helping opioids to work better, so lower opioid doses are needed.
  • GPR18: Involved in intraocular pressure, and local inflammation.
  • GPR55: Involved in bone formation, development of cancer, pain sensation, and inflammation.

THC is therapeutic for so many reasons. It’s more than just the high!

THC receptors

Breathe easy, THC has medicinal properties and can be therapeutic.

Side Effects

Normal and expected side effects of THC include:

  • red eyes 
  • dry mouth 
  • increased appetite
  • short-term memory loss
  • increased heart rate
  • Euphoria

A side effect is not always a bad thing! Those having a lot of chest congestion (like hospice patients) benefit from dry mouth, because it also dries out the lungs, making it easier to breathe. 

The red eyes happen because the THC dilates the blood vessels in the eyes and decreases intraocular pressure in glaucoma patients.

The side effects of cannabis can be beneficial, especially for elderly people.

Cancer patients and those looking to gain weight or eat more benefit from an increased appetite.

Those with a lot of stress or anxiety benefit from feelings of euphoria and quieting of racing thoughts.

How Marijuana Get’s You High

The euphoria associated with THC is the result of how THC binds to receptor sites in the brain. We’re just starting to understand the complexities of all the brain changes that happen when you consume THC.

A recent MRI study found that patients receiving THC had more connectivity between two regions of the brain compared to the controls. The two areas that had increased connectivity were the Nucleus Accumbens, which is responsible for reward and motivation, and the medial prefrontal cortex, involved in long-term memory and decision-making.

The “feel good” sensation associated with euphoria is due to the release of dopamine from neurons in the brain.

THC also acts on receptors in the cerebellum and basal ganglia, which can impair motor function, balance, and reaction time.

The short-term memory loss you experience when consuming THC is a result of how THC binds to the receptors in the hippocampus, the area responsible for memory formation and information processing.

Ways that Marijuana Affects You Mentally

THC affects the brain in many ways. The most well-known way is how it causes euphoria or “high” when the THC binds to CB1 receptors in the brain, causing time distortion and a difficult-to-describe feeling of fogginess and clarity at the same time.

THC can cause short-term memory loss, euphoria, and impaired motor coordination, and that’s because of how it acts on different receptors in the brain. Its action on the hippocampus causes short-term memory loss. Because of short-term memory loss, we recommend writing about your session in a cannabis journal.

When THC binds to the CB1 receptors on the brain’s olfactory bulb, the part responsible for smell and taste, both the taste and smell of food is enhanced.

At low to moderate doses, cannabis helps to improve anxiety, irritability, insomnia, depression, and pain perception. At high doses, it can cause the same symptoms that you may be trying to manage, such as paranoia, anxiety, and nausea/vomiting.

Ways that Marijuana Affects Your Body

THC acts on various sites throughout the body. When it binds to the CB2 receptors that are primarily in the immune system, it helps regulate the immune response and decrease inflammation.

For example, a recent study found that hospitalized covid patients who were admitted to using cannabis in the month prior to hospitalization had lower inflammatory markers on admission and overall better outcomes.

When THC binds to receptors in the body, the following can happen:

  • Binding to the CB1 receptors causes an increased heart rate, but this diminishes with regular use.
  • Binding to CB2 receptors on an injured liver counteracts the progression of fibrosis.
  • Binding to CB2 receptors in the gut will decrease gastric secretions, and gut inflammation/pain, and reduce gut motility.
  • Binding to cannabinoid receptors on the skin can play a role in dealing with skin inflammation, acne, pain, and eczema. This is the principle behind topical products

One study shows how different cannabinoids, like THCa, CBDa, and CBGa bind to the spike protein on the coronavirus, stopping it from entering the cell.

Corona virus

Different cannabinoids have been found to bind to the spike protein on the coronavirus.

Long-Term Effects of Using THC

Research on the long-term effects of cannabis is limited and ongoing. Here’s what we have so far:

  1. A study of over 1000 people who were followed for 20 years found that the only health effect associated with cannabis use was poor dentition, likely to be related to smoke inhalation or prolonged dry mouth. They found no associations between cannabis and high blood pressure, high cholesterol, or overall poor physical health
  2. Another study looked at medical patients before they started and 3 months in. They found that participants had improvement in cognitive function and “reduced sleep disturbance, decreased symptoms of depression, attenuated impulsivity, and positive changes in some aspects of quality of life” as well as a “decrease in their use of conventional pharmaceutical agents from baseline.”
  3. Yet another study found that participants had a drop in IQ between adolescence and mid-life if they started using cannabis as a teen (assuming it was recreational). The drop in IQ increased as the amount of cannabis use increased.

The studies seem to show that heavy recreational use as a teen could be associated with negative psychological outcomes later in life. We at Trusted Canna Nurse do not endorse recreational use in teens. Long-term use as an adult isn’t really showing any long-term effects.

How Much THC is Too Much?

Tolerance to THC usually goes up over time, which is great for people using high amounts for cancer-related cases, but not so great for the recreational user on a budget.

Everyone’s endocannabinoid system is different, so there’s no recommended dose that’s considered “too much.” The mantra in using cannabis is to start low and go slow. “Low” usually means a 2-3mg edible or 1-2 puffs from a joint or less than 1 puff from a high-concentration vape for your first time. It’s completely ok if you don’t feel anything the first time!

If you’ve never had gummies, go easy the first few times until you know how you respond.

Too much THC is when you start having adverse side effects, such as nausea, paranoia, or anxiety. “Too much” is basically when you no longer feel good. Many people take an edible and if they don’t feel anything after half an hour, they take another one and find themselves having a rough time.

There are several speculations on what a lethal THC dose would be. It has been estimated that you may need to smoke an average of 2000 joints or consume 1500 pounds of marijuana to lethally overdose on the plant. The good news is that you cannot lethally overdose on THC. The slang term for too much THC is called greening out. This can be quite unpleasant, but it will resolve over time, and sometimes hot showers, broad-spectrum CBD, milk, or black peppercorns can help.

What are the signs You are greening out cannabis?

  1. Nausea
  2. Dizziness
  3. Paranoia 
  4. Anxiety 
  5. Increased heart rate 
  6. Drowsiness 
  7. Dry mouth 
  8. Vomiting 
  9. Hallucination 
  10. Panic attacks
  11. Breathing problems

Final Thoughts on THC

The science of how THC works in the body is quite complex, and we’re just starting to understand it.

THC does way more than just get you high. CBD and THC complement each other, but act in different ways and have different benefits. Side effects aren’t always a bad thing, and you can avoid adverse side effects if you start low and go slow. Happy medicating!


 M. H. Meier et al., (2016) Associations Between Cannabis Use and Physical Health Problems in Early Midlife. JAMA Psychiatry 73, no. 7 (pp. 731-40)

Last Updated on September 1, 2023 by Megan Mbengue, BSN, RN, CHPN

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