Cannabis contains more than 400 different chemical compounds.
More than 100 of them are cannabinoids, the compounds that interact with the body to provide the effects we associate with weed.
But many people can only name one of them off the top of their heads: THC.
To be fair, the cannabinoid CBD has been making a name for itself in recent years, too. When people think about the substances in cannabis, though, THC is what automatically comes to mind.
THC is what gets you high when you smoke or vape flower, dab concentrates, or consume edibles. It’s also responsible for many of the important health and wellness benefits that medical marijuana provides. And on a less-pleasant note, THC (or one of its metabolites) is what causes positive weed tests.
You may have known all of that, but chances are good that you don’t know much more about THC. You should.
Let’s take care of that.
The Basics of THC
THC is shorthand for delta-9-tetrahydrocannabinol, sometimes just called tetrahydrocannabinol.
Most people know about THC because they believe it’s the most important chemical in the marijuana plant — but that’s not completely accurate.
THC is indeed the cannabinoid that contributes to almost all of the weed’s psychoactive effects. However, there’s virtually no THC in the cannabis plant. Most of the cannabinoids in raw weed are actually “precursor cannabinoid acids,” which only turn into the substances that cannabis is known for once they’ve been exposed to heat.
For example, raw marijuana contains THCA, not THC. It’s converted to THC when the flower is burned for smoking, heated for vaping, or decarbed in the oven to make edibles. The same is true for other major cannabinoids; raw weed contains CBDA and CBG, not CBD or CBG. For ease of discussion, though, most people talk about the THC in weed, not the THCA.
THC’s chemical structure is extremely similar to that of CBD, with the same number of carbon, oxygen, and hydrogen atoms. So why does THC get you high while CBD doesn’t? It’s only because the atoms are arranged slightly differently in the two cannabinoids. That slight difference is why THC and CBD work differently once they’re inside the body.
How does THC work in the body? Read on.
THC and the Endocannabinoid System
THC is characterized as a cannabinoid. The body produces chemical messengers, or neurotransmitters, called endocannabinoids. And it’s not a coincidence that the two substances have almost identical names.
Let’s back up for a minute and talk about endocannabinoids. “Endo” is short for “endogenous,” which means substances that are produced inside the body. They’re part of an enormous messaging system that runs throughout the body, called the endocannabinoid system (ECS).
The ECS controls a huge number of the body’s functions, using endocannabinoids to carry messages through the central nervous system to ECS receptors. Immunity, body temperature, inflammatory response, pain control, sleep, memory, and emotional processing are all regulated by the ECS.
Now back to THC, which is an exogenous cannabinoid (meaning it’s produced outside of the body). There’s something very interesting about THC: its molecular structure is virtually identical to the endocannabinoid anandamide. When THC enters the body, that similarity allows it to interact with the most important ECS receptors, known as the CB1 and CB2 receptors.
THC can attach to those receptors, blocking some ECS messages from being delivered and sending some of its own. That’s how THC is able to affect the way you feel, think, and act when you get high.
Importantly, the largest collection of ECS receptors is in the brain, where the majority of the psychoactive effects of weed are experienced. The high from THC can provide a wide variety of experiences, largely dependent on the amount of THC in a marijuana strain but also affected by the different terpenes in the strain, which enhance the effectiveness of cannabis.
Its effects can alter sensory and time perception, concentration, thought processes, coordination and movement — and of course, pleasure.
THC is broken down fairly rapidly in the body, which is why its effects only last a few hours; once it can no longer affect the ECS’s functions, those functions return to normal and the high wears off.
THC in the Body
Obviously, THC enters the body in weed smoke, vapor, or edibles. It can also be dripped under the tongue (sublingual administration) or used topically. The way that a weed user consumes THC plays a big role in how quickly it takes effect and how long it stays in your system.
The THC in smoke or vapor enters the lungs and is absorbed within seconds. It’s quickly moved into the bloodstream, where it can reach the brain and the rest of the body to attach to ECS receptors. Sublingual absorption is even faster.
When weed is in an edible, though, there’s a delay. The edible first has to be digested, and THC is then sent to the liver where it’s broken down into metabolites. One of those metabolites, 11-hydroxy-THC, is sent into the bloodstream (instead of THC) to bind to ECS receptors.
11-hydroxy-THC is more potent, so the high from edibles is stronger and lasts longer. However, all of that extra processing means that it doesn’t take effect for 30 minutes, an hour, or even longer after you consume an edible.
By contrast, topical products like oils and lotions that are infused with THC aren’t going to get you high (unless they’re transdermal patches). The THC in topicals doesn’t break through the skin barrier to reach the bloodstream; it simply interacts with ECS receptors at the skin level to provide localized pain relief.
Speaking of which…
Medical Benefits of THC
It’s easy to understand why medical marijuana has been legalized in well over two-thirds of U.S. states. Its medical benefits are impressive.
THC is an effective treatment for many types of chronic and severe pain, and it’s the #1 reason that doctors write prescriptions for weed. It’s also an accepted treatment for nausea and vomiting commonly caused by chemotherapy, as well as glaucoma and some types of epileptic seizures.
Research on the other medical benefits of THC isn’t quite as conclusive, but it appears to be effective for patients with a wide range of illnesses and diseases including muscle spasms, anxiety, insomnia, and low appetite. It’s also believed to provide neuroprotective benefits, have antioxidant and antimicrobial properties, and possibly even help fight some types of cancer.
All of these benefits are delivered in the same way that the cannabinoid provides its psychoactive effects: interaction with the ECS receptors in the brain and body. It’s a complicated dance, and science is still working to understand THC’s ability to have such an enormous effect on so many bodily functions.
What Is THC: FAQ
Q: How long can THC be detected by drug tests?
A: It depends on the type of test. Blood and saliva tests look for the presence of THC, and can usually only detect it for 1-2 days after use. Urine tests are much more commonly administered, and they search for the presence of the THC metabolite THC-COOH. That metabolite accumulates in body fat and is purged from the body slowly, so it can be detected in urine for a month or longer in heavy weed users, 3-4 weeks in frequent users, 1-2 weeks in moderate users, and a few days for occasional users.
Q: Are the medical benefits of THC and CBD the same, even if CBD doesn’t get you high?
A: They’re similar, but they act differently. THC attaches to the ECS receptors, while CBD just interferes with the messages coming and going; that affects the medical benefits they provide. CBD is government-approved for some forms of epilepsy, and is believed to help patients with a much larger assortment of medical issues ranging from diabetes and migraines to Crohn’s disease and inflammatory bowel disease. Some patients use weed strains that are high in THC, and others choose strains high in CBD, depending on the condition they’re treating.
- Atakan, Z. (2012). Cannabis, a complex plant: different compounds and different effects on individuals. Therapeutic advances in psychopharmacology, 2(6), 241-254. 
- Ng, T., & Gupta, V. (2021). Tetrahydrocannabinol (THC). In StatPearls [Internet]. StatPearls Publishing. 
- Zou, S., & Kumar, U. (2018). Cannabinoid receptors and the endocannabinoid system: signaling and function in the central nervous system. International journal of molecular sciences, 19(3), 833. 
- Sharma, P., Murthy, P., & Bharath, M. S. (2012). Chemistry, metabolism, and toxicology of cannabis: clinical implications. Iranian Journal of Psychiatry, 7(4), 149.
- Robson, P. (2001). Therapeutic aspects of cannabis and cannabinoids. The British Journal of Psychiatry, 178(2), 107-115.