What Does Weed Do to Your Brain and Body?

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Ever wondered what’s really going on in your brain and body when you take a hit of marijuana or eat a pot brownie? The side effects are readily apparent, but what’s happening physiologically to create those sensations? And why do some people experience entirely different effects from weed, such as heightened anxiety or paranoia? What does weed really do to the brain? How does marijuana affect the body?

Curious about the acute impacts of cannabis for the occasional adult user, we asked two top researchers in the field to give us the blow-by-blow from inhalation (or ingestion) to intoxication to coming down from your high.

What Does Weed Do to the Brain?

The Cannabis sativa plant contains hundreds of different chemicals present in varying amounts, depending on the strain and how it’s grown. The compound that recreational marijuana users care about most is the psychoactive one: delta-9-tetrahydrocannabinol, or THC. Generally, the higher the THC content, the more potent and potentially intoxicating the weed. 

Because THC is the coveted compound, growers today breed plants to produce a lot of it, says Staci Gruber, Ph.D., associate professor of psychiatry at Harvard Medical School and director of the Marijuana Investigations for Neuroscientific Discovery program at McLean Hospital in Massachusetts. Nowadays, weed is more than 300 percent stronger than the stuff from the mid-1990s.

“THC content has gone through the roof while the amounts of CBD and other constituents that might mitigate the negative effects of THC have gone down,” she says. “The ratio of THC to other compounds went from 14 to 1 to 100 to 1.” This ratio matters because it dictates exactly how pot affects the brain and body. 

Marijuana works by interacting with the endocannabinoid system, which is composed of chemicals and receptors located all throughout the body. “There are endocannabinoid receptors in the liver, intestines, fat, vasculature, and every single cell and organ system,” says Daniele Piomelli, M.D., Ph.D., director of the Center for the Study of Cannabis at the University of California, Irvine. “But the greatest concentration is in the brain.”

THC attaches to and activates CB1 receptors, which influence everything from mood to metabolism to memory to movement. Most of marijuana’s acute impacts on the brain and body — the ones you can feel, anyway — are driven by the activation of CB1.

Smoking Marijuana and Effects on the Body

So you took a long draw off a blunt. Now what? The first thing to know is that the effects of smoking pot are a bit different than the effects of eating it. 

“When you take in cannabis by inhalation, the kinetics — the speed at which effects happen — are very fast,” Piomelli says. “It goes into the lungs, circulates into the heart, and goes up to the brain very quickly. But it also goes away more quickly than if you consume cannabis through other pathways.”

The first thing you’ll feel is the cardiovascular effect. “Blood pressure drops,” Piomelli says. “That causes an immediate reflex: The heart pumps more blood to restore normal pressure, and with that, heart rate increases. These effects are not pleasant, and they are the main reason why people who first try cannabis don’t like it. They feel lightheaded and scared because their heart is beating fast, although those feelings disappear very quickly.”

Next comes the high, as THC travels up to the brain and switches on the CB1 receptors. “When you have enough THC in the brain to activate enough endocannabinoid receptors, you get the reason why people use marijuana recreationally: that stoned sensation of euphoria and intoxication that is unique to cannabis,” Piomelli says. 

According to Gruber, this process activates the brain’s reward circuitry, creating a reinforcing effect: “You get a shot of dopamine that produces pleasure and makes you feel good.”

Edible Marijuana’s Effects on the Body

When cannabis is consumed as food, the chain of events is slightly different, and it typically takes 60 to 100 minutes to feel the effects versus just a few minutes for smoking, Gruber says. Obviously, that pot brownie bypasses the lungs, but it must be digested before the THC can be absorbed and delivered to the brain. 

Before THC goes to the brain, it must pass through the liver. There, it’s metabolized and converted into 11-hydroxy-THC, which is a more potent compound than THC, Piomelli says. Next, this more potent version travels to the brain.

Interestingly, cisgender women are much more efficient at making 11-hydroxy-THC than cis men are, which makes most women more sensitive to ingesting pot. “It is well known that men tend to like cannabis more than women, and it’s not because our brains are different,” Piomelli says. “It’s mostly because our livers are different, and women have more active metabolisms, so more 11-hydroxy-THC gets to the brain.”

THC is a fat-like molecule, so once it gets to the fatty brain, it sticks around for a while. “After several hours, the initial high starts dissipating, and one effect is feeling hungry or having the munchies,” Piomelli says. “But this isn’t real hunger — it’s more like an increased appreciation of the sensory properties of food. Once you start eating, it’s hard to stop because whatever you’re eating tastes amazing.”

The good news, he adds, is that when THC leaves the brain, it doesn’t have lingering effects. “When you start coming down from marijuana, it doesn’t leave you with a strong craving for more.”

The Short Term Effects of Marijuana Vary

Although the exact same biological processes occur in everyone who smokes or consumes cannabis, the way those processes make people feel varies wildly.

“Some people are incredibly sensitive to THC while others are less sensitive,” Gruber says. “Some get very paranoid; others have no problem. Some people say ‘oh my god, I got so hungry and ate everything in the house’ while others don’t get hungry at all. If someone hands you a bowl or a vape cartridge of Granddaddy Purple, you will very likely have a different experience than they do, even though you’re smoking the same stuff.”

There are many potential reasons for this. “So much of it depends on previous use history, whether you have some familiarity with THC and other substances,” Gruber says. “It also depends on your individual body chemistry and metabolism, as well as the product itself, whether it’s a cultivar that has high amounts of other cannabinoids that mitigate the effects of THC.” 

Another factor is how activated a person’s endocannabinoid system is before they use cannabis. “Let’s say 70 percent of your receptors are activated and only 20 percent of that person’s receptors are activated right before they use marijuana,” Piomelli says. “Because of this difference, you might become panicky after smoking while the other person feels more relaxed.” This also explains why the same person can feel different effects on different occasions — even if they consume the same product both times.

With so many unpredictable variables at play, Gruber advises any adult with limited marijuana experience, as well as more seasoned users trying out new products, to “start low and go slow” to see how they’ll react. “You can always add more, but you can never take it back,” she says. “You can’t un-inhale.”

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