Researchers from the Johns Hopkins University School of Medicine recruited 18 subjects who hadn't used cannabis recently to participate in three separate sessions. At each session, subjects ate either a brownie with 20mg of THC, a brownie with 20 mg of THC and 640 mg of CBD, or a placebo brownie with no cannabinoids at all. After giving some time for the effects to sink in, researchers monitored subjects' vital signs and asked them to perform standard cognitive assessment tests.
Subjects who ate the CBD-rich brownie showed significantly more impairment on tests of memory and attention span than those who ate the THC-only brownie or the placebo. Researchers also found that the CBD + THC brownie caused subjects' heart rates to increase by an average of 25 beats per minute, compared to around 10 bpm for the THC brownie and none for the placebo.
"Overall, we saw stronger subjective drug effects, greater impairment of cognitive [thinking] and psychomotor [moving] ability and greater increase in heart rate when the same dose of THC was given in a high CBD cannabis extract, compared with a high THC extract with no CBD," said lead author Austin Zamarripa, PhD, postdoctoral research fellow at the Johns Hopkins University School of Medicine, to News Medical.
Subjects also completed the Drug Effect Questionnaire (DEQ), a standardized tool used to assess the subjective experiences of consuming psychoactive drugs. Participants who ate the CBD-rich brownie rated themselves as significantly more stoned than those who ate THC alone. People who ate the combo brownie were also more likely to feel sick or to report unpleasant drug effects, dry or red eyes, and difficulty performing routine tasks than those who ate the THC brownie.
"We have demonstrated that with a relatively high oral dose of CBD [640 mg] there can be significant metabolic interactions between THC and CBD, such that the THC effects are stronger, longer-lasting, and tend to reflect an increase in unwanted adverse effects," said co-author Ryan Vandrey, PhD, professor of psychiatry and behavioral sciences at the Johns Hopkins University School of Medicine, to News Medical.
These findings partially contradict earlier research suggesting that CBD could reduce some of the negative side effects of THC. In 2017, researchers found that mice that were injected with both THC and CBD were less likely to experience cognitive impairment than those that received THC alone. A more recent study reported that people who smoked flower with equal amounts of THC and CBD were less likely to feel anxious or paranoid than those who smoked low-CBD bud. However, none of these earlier studies specifically focused on edibles.
These earlier studies convinced many cannabis producers to add CBD to THC edibles to help mellow out the high, but the present study suggests that this combo could potentially have the opposite effect. It's important to note that the brownies in this study had a 32:1 CBD to THC ratio, though, and most popular edibles offer only either a 1:1 or a 2:1 ratio. Without further research, it's impossible to conclude that lower ratios of CBD could also increase the side effects of THC.
The study does raise concerns for cannabis users who are already taking high doses of CBD. "Our new study suggests that it's important for folks to be aware that if they're going to take a high-dose CBD extract, they also need to be mindful about interactions with other medications," Vandrey explained. “Individuals should discuss with their doctor whether they should consider dose adjustments of THC and other medications if they're also taking CBD.”
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