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Cannabidiol—the ingredient of cannabis that doesn’t make you high, commonly called CBD—might be the angel to THC’s devil, a new study of people’s brains suggests. The research found that 17 people who smoked cannabis with mostly THC had worse brain function in certain regions than those who smoked cannabis with roughly equal levels of THC and CBD.


THC, or tetrahydrocannabinol, is the part of cannabis that’s responsible for its well-known, mind-altering high. Fun as using THC can be, though, some researchers have also worried about the harms of taking it in high doses or for long periods of time.



There’s some (conflicting) evidence, for instance, suggesting that chronic cannabis users, especially if they start at a young age, are more likely to become psychologically addicted to the drug or develop psychiatric disorders like schizophrenia. In places where recreational cannabis use has been legalized, more users are ending up in the emergency room with mental symptoms of anxiety and even hallucinations. There’s also evidence suggesting these health risks are directly tied to the growing potency of THC in cannabis.


“Over the last two decades, rates of addiction and psychosis linked to cannabis have been on the rise, while at the same time stronger strains of cannabis with more THC and less CBD have become increasingly common,” said lead author Matt Wall, a researcher at the University College London’s clinical psychopharmacology unit, in a statement.

Wall and his team recruited 17 volunteers who had used cannabis before, but not heavily . The subject inhaled either cannabis with very little CBD but 8 milligrams of THC (a strain apparently known as skunk in the UK); cannabis with 10 milligrams of CBD and 8 milligrams of THC; or placebo cannabis with neither THC nor CBD.

Here’s an amusing description from the paper of exactly how that went:

Doses were vaporized in a Volcano Medic Vaporizer... at 210°C, and the resulting vapour was collected in two balloons. These were inhaled sequentially at the participants’ own pace, with each inhalation held in the lungs for 8 seconds, until the balloons were empty.


Thirty minutes after inhaling the cannabis or placebo vapor, the volunteers had their brains scanned via fMRI. They were also asked about how they felt after taking the drug. These included questions about their general emotional state, like feeling alert or happy, as well as things we commonly associate with being “stoned,” such as whether they wanted to listen to music or eat more.

Relative to placebo, they found that after people used high-THC weed, neural connections were impaired in two brain networks, the default mode and salience network. The default mode is thought to help coordinate our sense of self and others, including memory and moral reasoning, while the salience network helps us focus and respond to external stimuli. THC especially affected one area of the default mode network, the posterior cingulate cortex (PCC), and users with a more impaired posterior cingulate cortex reported feeling more “stoned.”


But these same impairments were much less apparent in volunteers after they took the high-CBD strains.

“We have now found that CBD appears to buffer the user against some of the acute effects of THC on the brain,” Wall said.


The relationship between impairment in the PCC and feeling stoned was also blocked, but it’s tough to say whether CBD-heavy pot affected people’s subjective high, according to Wall.

“There were actually only very subtle (and not statistically reliable) differences between the two treatments in terms of people’s subjective ratings of the drug effects,” Wall told Gizmodo via email. “Anecdotally, people do say that a more balanced THC/CBD ratio feels like a somewhat different ‘high,’ but that wasn’t reflected in our ratings, possibly because they were insensitive for some reason, or we weren’t asking quite the right questions?”


The study’s sample size is obviously small, so the findings should be taken with some caution. But other studies have definitely suggested that CBD can cancel out the negative effects of THC or even boost its positive effects. And its protective effects on the salience network, which has been linked to addiction and psychosis, seen in this study lend more credence to the idea that CBD can help people with these disorders, too—a theory that some scientists are currently studying.

The study, if nothing else, is the latest to suggest that the growing number of regular and habitual weed users should start paying more mind to what exactly they’re using, including how much CBD and THC their cannabis contains.


“I think the best approach (in legal jurisdictions) is to a) educate consumers about these issues and the possible dangers of high-THC strains, and b) ensure that consumers have a choice of different strains available,” wrote Wall. “Consumers making an informed choice about what they’re using is ideal, but of course requires some kind of legal framework; users where it’s illegal rarely have a choice about what they’re supplied with, and it tends to be the high-THC stuff.”

The study was published in the Journal of Psychopharmacology.