Paranoia tends to get included among the list of potential side effects caused by smoking marijuana, but any casual pot smoker will be quick to tell you that, if anything, a little cannabis is just what they need to mellow out. Researchers from the University of Oxford have concluded a study that suggests the mind-altering compound found in cannabis, tetrahydrocannabinol (THC), could explain short-term paranoia experienced by mistrustful people who smoke pot, but does it hold true for confident people?

“The study very convincingly shows that cannabis can cause short-term paranoia in some people,” lead researcher Professor Daniel Freeman said in a statement. “But more importantly it shines a light on the way our mind encourages paranoia. Paranoia is likely to occur when we are worried, think negatively about ourselves, and experience unsettling changes in our perceptions."

Freeman and his colleagues recruited 121 study participants between the age of 21 and 50 who reported mistrustful thinking and had smoked marijuana at least once in their lives. Two-thirds of the group was injected with THC extract from cannabis, a dosage equal to a “strong joint,” while the remaining one-third received a placebo. Researchers opted for injection over inhalation to control the amount of THC being introduced into each participant’s blood stream. Tests examining excessive suspiciousness included interviews, questionnaires, virtual reality stimulation, and real world social situations.

The effects of the THC dosage lasted an average of 90 minutes, in which time half of the participants receiving cannabis’ main ingredient experienced paranoid thoughts compared to 30 percent of participants in the placebo group. Other psychological side effects included anxiety, poor short-term memory, echoing thoughts, warped time perception, worrying, louder sounds and brighter colors, altered mood, and negative self-perception. Paranoia and other side effects deteriorated as THC exited the blood stream.

“The study identifies a number of highly plausible ways in which our mind promotes paranoid fears. Worry skews our view of the world and makes us focus on perceived threat,” Freeman explained. “Thinking we are inferior means we feel vulnerable to harm. Just small differences in our perception can make us feel that something strange and even frightening is going on.”

Clearly a sample size of 121 participants with a history of mistrust may not lead to the most reliable results. The findings could have also been skewed by the fact that half of the individuals participating in the study reported similar paranoid thoughts in the month prior to the start of the research. A more thorough analysis that includes confident people without a history of paranoia and mistrustful thinking may be needed to confirm the results of this study. A request to speak with the authors of the study was not answered by publish date of this article.

“The study provides a great deal more information about the immediate effects of cannabis, but it did not investigate clinically severe disorder,” Freeman added. “The results don’t necessarily have any implications for policing, the criminal justice system, or legislation. It tells us about the little discussed paranoid-type fears that run through the minds of so many people from time to time. The implication is that reducing time spent ruminating, being more confident in ourselves, and not catastrophizing when unusual perceptual disturbances occur will in all likelihood lessen paranoia.”

Source: Dunn G, Murray R, Freeman D, et al. How Cannabis Causes Paranoia: Using the Intravenous Administration of ∆9-Tetrahydrocannabinol (THC) to Identify Key Cognitive Mechanisms Leading to Paranoia. Schizophrenia Bulletin. 2014.