- Popular claims that CBD is protective against the negative effects of THC on the brain have never been adequately tested.
- A recent study investigated the acute effects of cannabis products that contained four different ratios of CBD:THC.
- The results showed that co-administration of CBD with THC had no effect on the induction of either cognitive impairments or psychotic symptoms.
Several states in the U.S. have legalized cannabis for medicinal or recreational purposes. Nearly simultaneously, the concentration of the main psychoactive ingredient, Δ9-tetrahydrocannabinol (THC), has progressively increased. THC can cause acute impairments, especially in young adults, that include memory and attentional deficits and, in those who possess a genetic predisposition, psychotic symptoms. Long-term use of high-potency cannabis may increase the risk of cannabis use disorder. [A previous post discussed the age-dependent negative and positive effects of cannabis on the human brain.]
Cannabis also contains cannabidiol (CBD). CBD does not impair cognitive performance; it does not produce euphoria and may have antipsychotic properties. The native plant typically contains 2:1 ratio of THC to CBD. CBD competes for the same receptor binding sites as THC in the brain and reduces the ability of THC to activate these receptors. This pharmacological antagonism may explain why cannabis users who smoke products with a high CBD content may have a lower risk of cognitive impairments and psychotic symptoms. In addition, pre-treatment with CBD reduced THC-induced memory impairments and psychotic symptoms in some studies, but not others. Taken together, these results predict that cannabis products containing a relatively high CBD:THC ratio may be less likely to have adverse effects as compared to cannabis with a low CBD:THC ratio.
A recent study investigated the acute effects of cannabis products that contained four different ratios of CBD:THC (chosen because they are typically found in recreational cannabis) on cognitive performance and psychotic symptoms in healthy volunteers. The study tested the hypothesis that subjects who used cannabis products with a higher CBD to THC ratios would experience less memory impairment and fewer psychotic symptoms. The study was a randomized, double-blind, within-subjects study. The subjects were 46 healthy volunteers (age 21–50 years) who had used cannabis at least once in the past. The cannabis dose contained 10 mg of THC and either 0 mg, 10 mg, 20 mg, or 30 mg of CBD.
The study found the typical acute robust effects of THC on cognitive performance (standard memory and attention tasks) and psychotic symptoms. The results showed that co-administration of CBD (at all doses) with THC had no effect on the induction of either cognitive impairments or psychotic symptoms following cannabis use. CBD did not reduce the adverse effects of THC. Furthermore, CBD did not influence the subjective feelings or the pleasurable effects of THC.
The cannabis plant produces both THC and CBD from a precursor molecule named cannabigerolic acid. Therefore, if a genetically-modified plant produces a higher CBD:THC ratio, it will consequently not produce as much THC. Therefore, the results of the current study imply that the claimed reduced risk from using high CBD varieties of cannabis plants, such as on cognitive impairments or psychosis, is not due to the presence of a high CBD content, but rather may be due to a relatively low THC content.
Overall, the results of the current clinical study found that CBD does not protect against the acute negative effects of THC. It is currently unknown whether regular CBD use can protect against the long-term harms of cannabis use.
Englund A et al., (2023) Does cannabidiol make cannabis safer? A randomised, double-blind, cross-over trial of cannabis with four different CBD:THC ratios. Neuropsychopharmacology (2023) 48:869–876; https://doi.org/10.1038/s41386-022-01478-z
McGuire P et al., (2018) Cannabidiol (CBD) as an adjunctive therapy in schizophrenia: a multicenter randomized controlled trial. Am J Psychiatry. 175:225–31.
Wenk GL, Your Brain on Food, 3rd Edition. Oxford University Press.